Performance of Concrete: Resistance of Concrete to Sulphate and Other Environmental Conditions; A Symposium in Honour of Thorbergur Thorvaldson 9781487584092

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PERFORMANCE OF CONCRETE

CANADIAN BUILDING SERIES Sponsored by the Division of Building Research National Research Council, Canada 1. Canada Builds, by T. Ritchie

2. Performance of Concrete, edited by E.G. Swenson

UNIVERSITY OF TORONTO PRESS

Performanc e of Concrete RESISTANCE OF CONCRETE TO SULPHATE AND OTHER ENVIRONMENTAL CONDITIONS

A Symposium in Honour of Thorbergur Thorvaldson

E. G. Swenson TECHNICAL EDITOR

Copyright Canada 1968 by University of Toronto Press Printed in Canada Reprinted in 2018 ISBN 978-1-4875-7361-4 (paper)

Foreword

ROBERT F. LEGGET DR. THORBERGUR THORVALDSON was an internationally recognized cement chemist, an outstanding leader in Canada's educational community, and a highly respected teacher, associate, counsellor, and friend. His studies on the chemistry of cement were carried out at the University of Saskatchewan where he was Assistant Professor of Chemistry, then Head of the Chemistry Department, from 1919 to 1946, when he was appointed first Dean of Graduate Studies. Shortly after joining the University, Dr. Thorvaldson directed his research interests toward the chemical aspects of the serious regional problem of the deterioration of concrete structures in the ground. With the assistance of successive generations of graduate students whom he directed and inspired, he was successful in establishing the nature of this chemical attack and his work led directly to the development of sulphate-resistant cement. As these studies were extended, Dr. Thorvaldson became increasingly well known for his contributions on the nature of the compounds in portland cement and the reactions they undergo. His interests were not narrow for he found time from teaching, research, and administration within the University to participate in the work of the Canadian Institute of Chemistry, the National Research Council, and other national and international scientific bodies. Many were the honours given to Dr. Thorvaldson during his lifetime. He was an Honorary Fellow of the Canadian Institute of Chemistry, receiving the first CIC Medal in 1951 and the Montreal Medal in 1959. In 1951 he was awarded the Henry Marshall Tory Medal of the Royal Society of Canada. The Government of Iceland (in which country Dr. Thorvaldson was born) made him a member of the Order of the Falcon, with Knight's Cross, in 1939 and a Commander of the Cross in 1956. After his retirement, Dr. Thorvaldson was appointed to the Board of Governors of the University he had served so well. The Thorvaldson Scholarship was established by his former students at the University; the enlarged Chemistry Building on the campus has been renamed after him. Though so honoured in recognition of his scientific achievements, Dr. Thorvaldson remained the most modest of men. He is remembered by his associates and students for his high standards of thoroughness, accuracy, and rigorous honesty

THORBERGUR THORVALDSON 1883-1965

FOREWORD

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in scientific work, and for his special personal qualities that were a lasting inspiration to all. His gentle nature, his unquestioned integrity, and his deep knowledge which he so freely shared won him esteem and reverence throughout the scientific world. Dr. Thorvaldson's work, honoured and distinguished though it was, was not widely known in Canada. Some of his long-time associates considered how a special Canadian recognition of his contributions to the development of sulphateresistant cement and concrete could best be given and plans were made to hold a symposium in his honour. The Division of Building Research of the National Research Council of Canada was delighted to organize and subsequently publish the symposium record. The original plan to hold the symposium in a western Canadian city with Dr. Thorvaldson present had to be revised because of his death in 1965. Fortuitous circumstances then made it possible to organize a memorial symposium in association with the annual meeting of the American Concrete Institute in Toronto in April of 1967. The Division records its indebtedness to the Institute and to its officers for making this happy joint event possible and to the Southern Ontario Chapter of the Institute, which arranged the meeting and whose efforts resulted in a record attendance. The subject matter of the symposium was first to have been limited to the sulphate resistance of cement and concrete. It was later decided, however, to include some papers on other aspects of the durability of concrete since Dr. Thorvaldson's interests had not been confined to the sulphate resistance of cements. It was considered appropriate that Canadians prominent in research on the durability of concrete in general should be represented in any such Canadian symposium as was planned. Space and time limited individual requests for papers, and several prominent workers who had been invited were not able to participate. Contributors other than Canadian were also invited. It was again necessary to limit invitations; again some who were asked were not able to participate because of lack of available material. Contributors of papers were asked to emphasize field performance. The literature on sulphate resistance is almost entirely based on results of laboratory experiments and on the nature of the destructive action. Concrete performance in highly sulphated soils has received relatively little attention. Such affected areas as the western prairies, both north and south of the international boundary, have literally hundreds of examples of concrete exhibiting varying degrees of resistance. It has become increasingly evident that the establishment of good concrete specifications and recommended practices requires interpretation of field performance as well as of laboratory results. It was also considered desirable to include an up-to-date review of the nature of sulphate attack, and this was achieved through the paper of Dr. W. C. Hansen. The papers dealing with durability aspects of concrete other than sulphate resistance are pertinent and timely, particularly as they also concern field performance. The

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FOREWORD

organizers of the symposium record their appreciation to all the authors who so willingly contributed to the symposium and who by their presence joined in this tribute to Dr. Thorvaldson. Any consideration of the contributions of Dr. Thorvaldson to research on sulphate resistance of cement and concrete must take into account the role of Dr. C. J. Mackenzie, so happily a co-author of the initial historical paper. Dr. Mackenzie was mainly responsible for the early recognition of the seriousness of the sulphate attack on concrete experienced in the western provinces, and for the first organized efforts to solve the problem. As Professor of Civil Engineering in the Engineering Department of the University of Saskatchewan in Saskatoon he began, in 1919, to expose concrete test specimens in sulphated soils. He discovered a mutual interest in Dr. Thorvaldson of the Chemistry Department, who undertook analyses of ground waters and started an investigation of his own. Later, as chairman of the committee of the Engineering Institute of Canada which was responsible for developing initial research on the problem, Dr. Mackenzie made possible continuing support for Dr. Thorvaldson's studies. Dr. Mackenzie's first proposals for a basic, chemical approach and Dr. Thorvaldson's plan to study the problem through experiments with the pure constituents of portland cement were considered at the time to be radical in the extreme. These two men, who were lifelong friends, displayed exceptional courage as well as foresight. The historical paper places in perspective the pioneering work done by Dr. Thorvaldson, particularly on the sulphate problem, and traces the events that led to the development of sulphate-resistant portland cement. It was a very special pleasure for all who attended the meeting in Toronto to hear Dr. Mackenzie himself deliver the opening paper with his accustomed vigour. Many contributed to the success of the symposium and to the development of this memorial volume. Special acknowledgement must go to Messrs. R. Peterson and G. C. Price of the Prairie Farm Rehabilitation Administration, Mr. W. D. Hurst, City Engineer of Winnipeg, and Mr. Peter Smith of the Ontario Department of Highways. Mr. E. G. Swenson carried the main burden of the arrangements for the Division of Building Research of the National Research Council, assisted by Mr. L. P. Ruddy in arrangements for the symposium and by Miss M.A. Gerard and Mrs. L. A. Graham in preparing this volume for the press. The attention devoted to the production of this volume by the staff of the University of Toronto Press is well shown by its finished form. DBR/ NRC is pleased to have been able to arrange with the Press for the establishment of the Canadian Building Series. It is particularly fitting that this first technical volume in this new series is one in honour of Dr. Thorvaldson whose kindly interest and wise counsel meant so much to the senior members of the staff of the Division in the earlier years of its work. Ottawa

ROBERT F. LEGGET Director, DBR/ NRC

Contents

FOREWORD

1 2 3 4 5 6 7 8 9

Robert F. Legget

V

Contributions of Thorbergur Thorvaldson to Cement and Concrete Research E.G. Swenson and C. J. Mackenzie

3

The Chemistry of Sulphate-resisting Portland Cements W. C. Hansen

18

Some Studies on the Performance of Concrete Structures in Sulphate-bearing Environments

F. M. Lea

56

Field and Laboratory Studies of the Sulphate Resistance of Concrete Bryant Mather

66

Combating Sulphate Attack on Concrete on Bureau of Reclamation Projects Bernard P. Bellport

77

Experience with Concrete in Sulphate Environments in Western Canada G. C. Price and R. Peterson

93

Field and Laboratory Studies of the Sulphate Resistance of Concrete G. J. Verbeck

113

Experience in the Winnipeg Area with Sulphate-resisting Cement Concrete W. D. Hurst

125

The Performance of Ordinary Portland Cement Concrete in Prairie Soils of High Sulphate Content J. J. Hamilton and G. 0. Handegord

135

10 Performance of Concrete in Sea-Water: Some Examples from Halifax, N.S. D. C. Tibbetts

159

11 Case Histories of Poor Concrete Durability in Ontario Highway Structures J. Ryell and P. Smith

181

12 Observations of Sidewalk Concrete during Fifteen Years' Exposure W. S. Weaver and H. L. Isabelle

205

13 Scaling of Concrete by Frost Action

J. Hode Keyser

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PERFORMANCE OF CONCRETE

1

I

Contributions of Thorbergur Thorvaldson to Cement and Concrete Research

A Historical Review E. G. SWENSON AND C. J. MACKENZIE*

"THERE ARE, occasionally, in all branches of endeavour, men who become great. They become the savants whom we look back to with respect and love. Such men were Le Chatelier and Michaelis. Professor Thorvaldson has given his life to the study of cement and concrete; he is better known than anyone else in the world for his work on the durability of concrete, particularly in sulphate waters. . . ." This was the tribute paid to Dr. Thorvaldson by his contemporary, Dr. R. H. Bogue, at the 1952 London Symposium on the Chemistry of Cement. He went on to say that Dr. Thorvaldson's personality was such that no one who came in contact with him could fail to love him. Dr. Bogue is himself one of the most eminent of scientists in the field of cement chemistry. Dr. Thorvaldson was appointed Assistant Professor of Chemistry at the University of Saskatchewan in 1914, and became Head of the Chemistry Department in 1919, holding this position until his retirement in 1949. In 1946 he also became the first Dean of the College of Graduate Studies. During this period, and up to his death in 1965, he published over 80 scientific papers, most of which were in the field of cement chemistry. These described his investigations into the sulphate resistance of cement and concrete, and, deriving from this, his studies of the properties and reactions of the pure compounds of cement. Dr. Thorvaldson's interests and activities were wide. He was an educator of the highest stature; he was associated with important scientific societies; he provided continuing public service to those concerned with cement and concrete; and he readily investigated problems in areas other than cement and concrete if this was required or requested. Reports of only a few of these investigations were published ( 1-3). He was consulted extensively, for example, on the early problems connected with the development of the potash industry in Saskatchewan.

*E. G. Swenson, Senior Research Officer, Division of Building Research, National Research Council of Canada. Following studies in cement chemistry under the late Dr. Thorvaldson, he has engaged in research on concrete materials. C. J. Mackenzie, President of the National Research Council, 1944 to 1952 (Acting President 1939-1944); Dean of the Faculty of Engineering, University of Saskatchewan, 1921 to 1939; President of Atomic Energy Control Board, 1948 to 1961; Chancellor of Carleton University since 1954.

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Dr. Thorvaldson's life's work was, however, the study of the properties and processes of cement and its compounds, his greatest achievement being the experimental demonstration of concepts which led directly to the development of a sulphate-resistant cement. This work has been referred to and reviewed elsewhere ( 4) ; the present paper is a historical review which is primarily intended to place Dr. Thorvaldson's great contributions in their proper perspective. In addition to a reference list of published papers, there is appended to this paper a list of documents which were sources of historical and technical information. These documents, referred to as D- throughout the paper, form part of a collection that is readily accessible for confirmation of statements made in this paper. EARLY RECOGNITION OF THE SULPHATE PROBLEM IN WESTERN CANADA By 1918 in Western Canada, the destructive action of alkali ground waters on concrete structures was generally recognized by engineers to be a problem of major importance. As early as 1908 there were reports from Winnipeg of destructive action on concrete of salts present in the ground water (D 1). Little was then known about the scientific design of concrete mixes, and the few cases of deterioration reported were usually ascribed to poor concrete. Ten years later, however, when expensive and well-designed structures in the area became affected, engineers became alarmed. The Engineering Institute of Canada (EiC) had branches in each of the three Prairie Provinces, which early became active in the study of local problems connected with sulphate attack. At a meeting of the EiC in Saskatoon in 1918, reports were made by these local branches on "decay of concrete in alkali soils" ( 5). The following resolution was adopted: "Resolved that the Council [of the EiC] be requested to appoint a committee on the action of the alkali salts on concrete, to carry out such experiments and investigations as they consider desirable, with power to coUect funds in the name of the committee to enable them to carry out the work" (D2). Between 1918 and 1920 no comprehensive plan was developed to carry out organized research on the problem. In Saskatoon in 1919, Dr. C. J. Mackenzie of the Engineering Department of the University of Saskatchewan began some field exposure testing. The analyses of the ground waters for the site were undertaken by Dr. T. Thorvaldson, the newly appointed Head of the Chemistry Department. Through this association Dr. Thorvaldson became interested in the broader problem and started an investigation of his own. In January 1920, Dr. Mackenzie presented a paper to an EiC meeting which described this programme of tests, together with background history of what was

CONTRIBUTIONS OF THORBERGUR THORVALDSON TO RESEARCH

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then known of the sulphate problem ( 57). At an EIC meeting in August of 1920, papers on the problem were presented by members of the local provincial branches, and a report of the Saskatoon work was published (D2). At that meeting the important decision was made to implement the 1918 resolution to form a joint group, raise money, and hire experts for a concentrated effort. At this time the results of laboratory experiments, field tests, and field observations had led to a common conclusion that "the best of concrete will be acted upon chemically and be deteriorated in time if subjected to a sufficiently strong alkali ground water" (D2) . In field testing, for example, attempts had been made to determine the effects of various concrete mixes, admixtures, and surface coatings. In the laboratory, ground waters and affected concretes had been analysed for sulphate content ( 5) . In these EIC groups there was an understanding of the European as well as the American literature, and the theories of mechanism of the reaction were under review ( 5). The formation and possible destructive effect of calcium sulphoaluminate were recognized, but explanations ranged from crystallization of salts in pores, and formation of products of increased volume, to leaching of salt products of the reaction. APPOINTMENT OF DR. THORV ALDSON TO HEAD CHEMICAL RESEARCH In early 1921 Dr. C. J. Mackenzie became chairman of a Joint EIC Committee on Deterioration of Concrete in Alkali Soils. The secretary was Mr. G. M. Williams who had been with the U.S. Bureau of Standards and who had been recently appointed by Dr. Mackenzie to his staff in the Engineering Department at the University of Saskatchewan. Mr. Williams, who was considered to be a leading authority on the sulphate problem at that time, was placed in charge of implementing the field test programme (D2) . Dr. Mackenzie was eventually able to raise over $47,000, which was to cover a three-year period. The contributors were the National Research Council, the Canada Cement Company, the provincial governments of Saskatchewan and Alberta, the Canadian Pacific Railway, and the City of Winnipeg. His plan for the investigation was two-pronged : the major emphasis was to be placed on chemical research, but the field test programme directed by Mr. Williams was intended to correlate the findings of the chemical work (D2) . At an earlier meeting of the Joint Committee, a proposal was made to appoint a chemist to direct the chemical investigations (D3) . Several persons were considered and a report of the executive committee was issued, stating that Dr. Thorvaldson, Professor of Chemistry, University of Saskatchewan, had been interviewed "after having ascertained that he had been working on certain phases of the same problem for a period of about three years, and that he has at the present

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PERFORMANCE OF CONCRETE

time men working under him on such work" (D4). The executive committee recommended that "this work be undertaken by Dr. Thorvaldson at a remuneration not to exceed $3000 per annum for a year at least" (D4). In September 1921, Dr. Thorvaldson was formally appointed director of the chemical investigation for one year, having obtained leave from his teaching duties at the university. In September 1922, he was given an extension of another year. The following is the story of the investigations of Dr. Thorvaldson on the sulphate problem and an outline of his basic research on the chemistry of portland cement. EARLY IDEAS ON ATTACKING THE PROBLEM As a first action in September 1921, Dr. Thorvaldson visited the U.S. National Bureau of Standards and the Geophysical Laboratory in Washington. He was informed that "no work bearing directly on the problem we are interested in is being conducted or planned at these laboratories" (DS). Upon his return, and after a thorough study of the literature as well as consultation with others, he outlined his plan of attacking the problem in an Interim Report to the Committee (DS). In a recent summary of his early investigations, Dr. Thorvaldson stated that "the basic plan of the chemical investigation from the beginning of my work in 1919 was to study the action of alkali waters on the chemical components of portland cement, as far as these were known through the work of Rankin and Wright, and others" (D6). Three years passed before he was able to begin this important phase of his work. The plan he presented to the Committee was: (a) Preparation in the pure state of all the distinct chemical substances which are known to exist in portland cement and the study of the chemical reactions these undergo in water and in solutions of alkali salts; (b) A study of the physico-chemical properties of waters from affected areas. A chemical study of these waters has been carried out during the last three years in our laboratory; ( c) A study of the direct action of water and solutions of alkali salts on set portland cement; ( d) A study of the colloidal matter in sound and deteriorated concrete; ( e) The testing out in a practical way of any remedies suggested by the above work.

Subsequent developments have demonstrated that Dr. Thorvaldson's recognition of the need for a basic approach, and of the importance of preparing and studying the role of the pure compounds of portland cement, was a major turning-point in research on the sulphate problem. Of equal significance was his recognition at this early date that a cure might be effected through a modification of the chemical composition of portland cement. In the same Interim Report of May 1922 (DS) he stated that "burning of cement on a fairly large experimental scale will be desirable," and that "we hope that arrangements may be made for trial burns with

CONTRIBUTIONS OF THORBERGUR THORVALDSON TO RESEARCH

7

some of the Canada Cement companies." It should be noted that this was recommended eleven years before these "burns" were made. It is a matter of record that top men in concrete technology at that time believed that the cure lay in the improvement of the concrete mixes, and that portland cement approached an ideal composition that would not need any radical change. This belief has unfortunately persisted in varying degrees to this day. CONTEMPORARY RESEARCH In his report to the EIC Council (D2), Dr. C. J. Mackenzie noted that the Joint Committee kept itself fully informed on research on the sulphate problem and the related one of sea-water attack. Literature published up to that time dealt mainly with observations made in France, but also included observations made by the Institution of Civil Engineers of Great Britain, the drainage investigation of the U.S. Bureau of Public Roads, and the large-scale exposure tests of the Portland Cement Association carried out under the well-known Duff Abrams. The previous work of Miller and Manson and other early references were known to the Committee. These have been reviewed by Hansen ( 4). Dr. Thorvaldson at this time also visited the Director of Marine Investigations in New York. The work of this organization was sponsored by the U.S. National Research Council. His various reports indicate a special interest in the several hypotheses that attempt to account for the destructive action of sulphate on concrete. Mr. Williams prepared concrete specimens for exposure tests and these were placed in 1921 at three sites: Cassils near Calgary, Grandora near Saskatoon, and Deacon near Winnipeg. Several types of cement were used, as well as a number of admixtures and surface treatments. Dr. Mackenzie outlined the history of the work already done on the sulphate problem by the EIC Joint Committee and detailed plans for future studies ( 58, D2). After six years of exposure, the conclusions drawn from these tests, and from similar results obtained and reported by the U.S. National Bureau of Standards, were given in Dr. Mackenzie's Annual Report to the EIC of 28 December 1927 (D7). It was found that Na2SO 4 and MgSO4 differed in their effects on concrete, and that different cements showed varying degrees of resistance to attack. Other factors that produced different effects were changes in concentration of salts and richness of mix. Of special significance, then and now, was the finding that a concentration of soil or soil water alkalis was not always a measure of the degree of deterioration to be expected, but that capillary action and subsequent evaporation were major factors. It was further concluded that "great care must be used in evaluating field exposure results in order to avoid erroneous interpretation." It was found that "no integral compound used appreciably prolonged the life of the concrete beyond that found for untreated concrete of the same mix, and in some

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cases the use of such a compound seemed to accelerate the disintegration. Surface coatings of the bituminous types tend to retard the disintegration during the early exposure periods but none employed in this work have proven to be permanent" (D8) . The early chemical work done by Dr. Thorvaldson involving tests of soils and ground waters, and observations on associated field exposure tests, led him to three main conclusions: (a) The concentration of sulphate in the ground water at times varied greatly within very small areas. In a Winnipeg plot measuring 50 by 50 feet, test results varied from 3200 to 17,570 ppm (D9). This could have been a major factor in the variability noted in some field exposure tests. (b) It was found that ground waters in Alberta were generally higher in Na 2 SO4 than in MgSO 4 , those in Manitoba were higher in MgSO 4 , and those in Saskatchewan tended to fall between these two. Dr. Thorvaldson suggested that this might be because MgSO 4 is more soluble than Na2 SO4 , particularly at low temperatures, and because the general flow of rivers and streams was from higher ground in Alberta to lower ground in Manitoba (D 1). ( c) Lean concretes were found to disintegrate rapidly when surrounded by disintegrated concrete and pure water. This indicated the presence in affected concrete of excessive amounts of available sulphates. INITIAL STUDIES OF PURE COMPOUNDS OF PORTLAND CEMENT Dr. Thorvaldson had been trying to obtain a first-rate petrographer, to prepare the pure minerals present in portland cement clinker. In this he was successful in June 1922 when Dr. G. R. Shelton became his assistant. Dr. Shelton and Dr. Thorvald son prepared the pure compounds then known, tricalcium silicate ( C 3 S), dicalcium silicate (C 2 S), and tricalcium aluminate (C 3 A) . These were characterized and their reactions in pure water and in sulphate solutions were studied, mainly by microscopic methods. Three papers were published on this preliminary work (6-8). The main conclusions were: (a) The formation of calcium sulphoaluminate is characteristic of the reactions between hydrated C3 A and solutions of Na2SO4 in all concentrations and MgSO 4 in low concentrations. (b) With more concentrated solutions of MgSO 4 , Mg(OHh was present and this tended to retard the formation of calcium sulphoaluminate. (c) MgSO 4 appeared to have a generally more destructive effect than Na 2 SO4 on the hydrated C3A and also on portland cement. It was recognized that the precipitation of the hydroxide ion by the magnesium ion upset the equilibrium required for the stability of the calcium silicate hydrates, thus producing the second destructive action recognized today. Drs. Thorvaldson

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and Shelton had noted more than once that sulphates tended to increase the rate of liberation of lime from the calcium silicates. In 1925, Dr. Mackenzie presented a paper to the EIC in which he reviewed the work of Dr. Thorvaldson (59). He had found that the Committee had some difficulty in following Dr. Thorvaldson's scientific explanations and deemed it necessary to summarize the work in simpler language. A joint paper by these two colleagues won the Plumer Medal in 1927 ( 56). DEVELOPMENT OF THE LEAN MORTAR BAR TEST From the beginning of his work Dr. Thorvaldson recognized the need to develop a method "for a more direct correlation of the chemical and microscopic results with the behaviour of concrete when exposed to sulphate solutions" (D6). He began with strength tests (9) but in 1924 began to use the mortar bar for length change measurements with time. The early development and later refinement of this method, so well known today, were made in connection with studies of the behaviour of mortars and pastes in sulphate solutions. These are well documented (10, 55, D7, DlO). It now became possible, by the measurement of expansion of pastes and mortars, to determine quantitatively the contributions of the constituents of portland cement and the presence of any other substance. It is significant that this technique of following a reaction by volume change measurement of mortar bars, one of the major achievements of Dr. Thorvaldson, has become common to many other areas of research. Today this method, or modifications of it, is used in tests involving such phenomena as sulphate resistance, alkali-aggregate reactions, and drying and carbonation shrinkage. It has the supreme advantage of being a direct measure of the course of reactions involving volume change. The lean mortar bar test was for years the standard method of determining resistance of cements to sulphate attack specified by the Canadian Standards Association, Specification A-5. It was later supplanted by a modification based on integrally added sulphate. The American Society for Testing and Materials ( ASTM) Committee C-1, Cement, thoroughly investigated the lean mortar bar test ( 11), with favourable conclusions. The simpler mortar bar method based on integrally added sulphate was adopted, however, and is currently in use. DIRECT EVIDENCE FOR THE CONTRIBUTIONS OF THE PURE COMPOUNDS Up to 1926, there had been general agreement that the formation of sulphoaluminate from gypsum and CsA produced an increase in volume sufficient to cause a breakdown of concrete. There had been no way, however, to determine what

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amounts of sulphoaluminate were deleterious and what contributions, if any, were made by the other mineral compounds. Dr. Thorvaldson was now ready to provide the answers with the mortar bar test method. The preparation of the pure compounds, C 3S, C 2 S, and CaA, in sufficiently large quantities, and the preparation of mortar bars with these compounds and combinations of them, were begun in 1924. The results of the first series of mortar bar tests were published in 192 7 ( 12). Dr. Thorvaldson proved through length change measurements of mortar bars that C 3 A was the component in portland cement that produced excessive expansion of mortars and resultant breakdown. He also introduced for the first time muchneeded quantitative relations. A main conclusion was that mortar bars made from C 3S or C 2 S to which C 3 A had been added disintegrated rapidly in solutions of Na 2SO 4 and MgSO 4 • He showed that mortars made from pure CnS and pure BetaC3S do not disintegrate in solutions of Na 2SO4, but expand and disintegrate in solutions of MgSO 4. It was recognized that the formation of Mg(OHh in the latter case removed enough lime to destroy the stability of the hydrates of the silicates. Dr. Thorvaldson also showed that composite cements made from these pure compounds, in the normal proportions found in portland cement, had the same strength and the same response to treatment with sulphate solutions as had portland cement. These preparations appear to have been the first "artificial cements" made with pure compounds. An important observation was that the relative expansion at which mortar bars broke down physically was about the same for all samples. The significance of the length change results and the reproducibility of the method were established. In 1928, Hansen, Brownmiller, and Bogue published an important paper in which they established tetracalcium aluminum ferrite, C 4 AF, as the significant iron phase in portland cement clinker ( 13). Dr. Thorvaldson immediately prepared this pure component and introduced it in an expanded mortar bar series which also included combinations of the other pure compounds. Significant results were obtained from this second mortar bar series as early as the autumn of 1929 and these were made known to the Joint Committee (D6). The paper describing the final results of the study was published in 1932 (14). The important finding was that the performance of mortar bars made with C4AF was excellent as compared with the poor performance of those made with C:iA, when these were immersed in solutions of sulphate. Transforming C 3 A to an equivalent amount of C4AF greatly increased the volume stability of mortar bars. Dr. Thorvaldson was at last in a strong position to propose the composition changes in portland cement clinker that he had years before anticipated as necessary. It was simply a matter of adding an iron constituent to the raw mix to transform the major part of the C 3 A to C 4AF in the clinker. It is perhaps not out of context here to note a quality of Dr. Thorvaldson which

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permeated his relations with everyone and affected the tenor of his reports. This was his inherent sense of the rightness of things, a quality apparent in his selfquestioning about the results described in the above paper (15, D6). Despite the clearness of the evidence, even as judged today, he was concerned about such questions as these: (a) his pure compound mixtures might not behave the same as the combination occurring in portland cement; (b) the C3A and the C4AF in portland cement are largely in solid solution, whereas no liquidus phase was present in the preparation of the pure compounds; (c) differences in particle size might be of importance; and ( d) what is the true role of CaO and how is it affected by the efficiency of burning? THE CEMENT PLANT "BURNS" Since the start of Dr. Thorvaldson's research on portland cement, the Canada Cement Company had been very helpful in supplying him with samples of cement and clinker, as well as making several financial contributions to the budget of the Joint Committee. In September 1922, at a meeting of the EiC Committee it was agreed that Mr. AG. Fleming, Chief Chemist of the Company, should be invited to become a member of the Committee (D 11). This he did shortly thereafter. In line with the ideas on plant "bums" which he had as early as 1922, and because his work so far indicated the remedial action to follow ( 14), Dr. Thorvaldson suggested to the Canada Cement Company that two types of "bums" should be tried on a plant scale: "reduction of AlzO:/' in order to reduce C 3 A, and reduction of C 3A by "substituting in part some other fluxing material" (D6). The first method was tried out at the Exshaw plant in Alberta but the results were disappointing to Dr. Thorvaldson. He attributed this to insufficient burning (D6). From the analyses of cements from various plants of the Canada Cement Company, Dr. Thorvaldson had noticed that the product from the Hull plant in Quebec had an unusually high Fe2O 3 content. In a letter of 16 November 1927 he made a second request for a sample "because of the fact that it contains a low percentage of alumina but a relatively high percentage of iron, and it will therefore differ materially in its composition from any of the cements which I have experimented with." This cement was tested by Dr. Thorvaldson and showed fairly high sulphate resistance (D6). Dr. Thorvaldson had followed closely the work at the U.S. National Bureau of Standards that led to the first paper by Hansen, Brownmiller, and Bogue in 1926 on phase studies of the system CaO-Fe2O3-SiO2 (16), and his plans for the cement bums were being made shortly after the publication in 1928 of the second paper by the same authors on C4AF ( 13). These plans became firm when the results on the mortar bars made clear the relative sulphate resistance of C 3A and C4AF referred to above.

12

PERFORMANCE OF CONCRETE

Mr. Fleming wrote to Dr. Thorvaldson on 21 February 1928: "it is very interesting for us to note that the sample of cement received from Hull seems to have a fairly high sulphate resistance, and from what you have previously said I presume that you attribute this partly to the high iron content as well as to the fairly well-balanced combined silica" (D6) . This statement is significant as the ideas in it embody the principles used today in the manufacture of sulphate-resistant cement. In early 1930 Mr. Fleming visited Dr. Thorvaldson's laboratory in Saskatoon and, as a member of the Joint Committee, was briefed on the results of the mortar bar studies which involved the C 3 A and C 4 AF. In the same year Mr. Fleming made his first plant "burn" using an increased Fe20a content. Samples of the cement made from this clinker were tested in Dr. Thorvaldson's laboratory, and these showed greatly improved resistance to sulphate attack (D6). On his return from an extended study tour in Europe, Dr. Thorvaldson in the summer of 1930 undertook to design raw mixes for proposed plant "bums" at the Exshaw plant. He was present at the burning of these mixes in July 1931. Cements from these clinkers were tested by Dr. Thorvaldson and were also found to have improved sulphate resistance. They also passed specifications for portland cement although strengths were on the low side (D6). The notable success of these modified cements was considered by Dr. Thorvaldson to end this phase of his work. The refinements in composition and burning he left to the manufacturer. His last major act in this regard was to help Mr. Fleming set up facilities for making and testing mortar bars according to the method developed in Saskatoon (D7, D12) . The Canada Cement Company began producing a sulphate-resistant cement, based on the above ideas, which they patented and called Kalicrete. Mr. Fleming published the results of his work in 1933 (17) . Dr. Thorvaldson continued to observe the performance of modified cements and concretes made with them. His main effort, however, was now directed toward fundamental studies of the pure compounds. In 1927 Dr. Mackenzie had arranged for the National Research Council to assume entire support for Dr. Thorvaldson's continuing work (60) and the Joint Committee was disbanded in 1928. Dr. Thorvaldson continued to apply for and receive direct grants from the National Research Council (DlO, D14-Dl 7). During the years following the development of sulphate-resistant portland cement, Dr. Thorvaldson's technical advice was in great demand, and he gave generously of his time to help with practical problems. During the last four years of his life he acted as consultant to the Prairie Fann Rehabilitation Administration in its investigation of concrete materials for the South Saskatchewan River Dam. "His advice and guidance proved invaluable in the development and testing phases of a research programme designed to study sulphate resistance and cement aggregate reaction in concretes made with a variety of cements, alone and in combination with pozzolans" (D18). This programme has been described elsewhere (40).

CONTRIBUTIONS OF THORBERGUR THORVALDSON TO RESEARCH

13

CONTINUING RESEARCH IN CEMENT CHEMISTRY In addition to the investigations already described, Dr. Thorvaldson had been following other ideas, with particular attention to possible remedial measures to guard against deterioration of concrete by ground waters high in sulphates. The main studies involved: (a) carbonation; (b) hydrated silica as an admixture (18); (c) effect of salts other than alkali sulphates; (d) influence of solubilities; (e) resistance of high alumina cements (14, D13); (f) slag and natural cements; (g) steam curing and autoclaving. Dr. Thorvaldson's studies of the effects of curing at elevated temperatures, with and without pressure, were extensive. He had found quite early that steam curing at and above 100°C sharply increased sulphate resistance, but some loss in tensile strength was noted ( 19, 20). He found this increase in sulphate resistance to be due to a "new crystalline substance" (21), later identified as a form of hydrated tricalcium aluminate, which was also produced by autoclaving (22). Dr. Thorvaldson made extensive studies of the effects of autoclaving on the iron-bearing components of portland cement (23) and on composite mortars made with the pure silicate and aluminate components (24), confirming his idea that sulphate resistance was associated with the stabilization of the aluminate to C 3 A.6H 2O. His interest in the effect of steam curing extended to the influence of the chemical nature of the aggregate (25). Dr. Thorvaldson's pioneering work in the use of the pure compounds of portland cement in the study of sulphate resistance required extensive studies of the other properties of these substances, particularly their hydration characteristics. The great contribution that these studies made to the basic science of cement chemistry has been recognized internationally. An example of the significance of this work is to be found in one of his earliest papers, published in 1928, on the action of water on CaS and Beta-C 2S (26) . One conclusion reads : "The results indicate that the product of hydrolysis highest in lime, which can exist in equilibrium with a solution of calcium hydroxide below saturation, has a lime-silica ratio of approximately 3:2." Today, after years of extensive research by many eminent cement chemists, it is generally recognized that the high-lime, main hydration product of the calcium silicates has the formula C:{,S2.3H2O, referred to as tobermorite. Other studies of the silicates of lime were reported (27-29) . Dr. Thorvaldson's various aluminate preparations and their hydration products were studied in great detail and reported in many papers (21, 22, 30-36). Heat of solution and heat of hydration studies were also carried out (37-39). Dr. Thorvaldson collaborated with Dr. J. W. T. Spinks, then Head of the Chemistry Department and now President of the University of Saskatchewan, in developing the radioactive tracer technique in the study of chemical reactions and

14

PERFORMANCE OF CONCRETE

processes of portland cement and the pure compounds ( 41 ) . This method made it possible, for example, to clarify important questions regarding the mechanism of hydration of C 3 S and Beta-C2S ( 42), of plaster of Paris ( 43), and of calcium oxide ( 44). A paper describing the exchange of calcium between hydrated C3 S and solutions of Ca(OH)z was presented by Dr. Thorvaldson at the 1960 Washington Symposium ( 45) . Most of this work was done by Dr. Thorvaldson after his retirement. Studies were made of the significance and scope of the alcohol-glycerol method for extracting and determining the quantity of free lime in portland cement and its pure compounds ( 46, 4 7). Also investigated was the detection of lignosulphonate admixtures in cement pastes using an ultra-violet absorbance method ( 48) . Dr. Thorvaldson has contributed a major scientific paper to each of the International Symposia on the Chemistry of Cement and Concrete, held in Stockholm in 1938 (49) , London in 1952 (15), and Washington in 1960 (45). He made a lecture tour in 1941-42 on behalf of the Canadian Institute of Chemistry (CIC), speaking about the silicates ( 50) and aluminates ( 51) of calcium. In 1942 he delivered his Presidential Address to the Royal Society of Canada on "Reactions in the Solid State" (52). In 1949 he gave the fourth Westman Memorial Lecture at the CIC Annual Meeting, on "Some Aspects of the Chemistry of Portland Cement" (53) . To the CIC meeting in 1951 he delivered the Medal Address on "The Training of Chemists for Industry" ( 54). SUMMARY

The major scientific achievements of Dr. Thorvaldson related to the sulphate problem are as follows. Early in his research he recognized the need for the basic approach and began studying the behaviour of the pure compounds of portland cement. In addition, he early recognized that the remedy would probably lie in a modification of the chemical composition of cement. He developed the lean mortar bar test and, by means of it, demonstrated directly the role of C3A among the pure compounds in producing excessive expansions in mortars and concretes. His conclusion that the C 3 A could be reduced by formation of C4AF was put to test in the plant "burns" that resulted in the modern sulphate-resistant cement. These major accomplishments were accompanied by equally important research on the properties and reactions of the pure compounds of portland cement. All these won him international recognition. Space does not permit a listing of the many research assistants who came and went over the years, and who played a large role in Dr. Thorvaldson's work. These names can be found in the references as co-authors of papers. Whether they were assistants, colleagues, or students, those who were associated with Dr. Thorvaldson over the years were impressed by his fine intelligence, his original approach to scientific problems, and his practical sense and judgment.

CONTRIBUTIONS OF THORBERGUR THORVALDSON TO RESEARCH

15

"T.T.," as he was affectionately called by his graduate students and colleagues, was universally respected for his ability, loved for his kindliness, and revered for the firmness with which he upheld his high standards of scientific honesty and personal integrity. DOCUMENTS ON RECORD Dl. D2. D3. 04. 05. 06. 07. 08. D9. 010. Dll. 012. 013. 014. 015. 016. 017. 018.

Extract from Eng. J. (Oct. 1922). Discussion of work of EiC Committee. Report of Committee on Deterioration of Concrete in Alkali Soils, Oct. 1922. Also published in Eng. J. ( Feb. 1923) . Letter to Dr. Thorvaldson from Chairman of the EiC Committee, B. Stuart McKenzie, Winnipeg, 12 July 1920. Minutes of Meeting of Committee on Deterioration of Concrete in Alkali Soils, EiC, Saskatoon, 9-11 Aug. 1921. Interim Report by Dr. Thorvaldson to Committee on Deterioration of Concrete in Alkali Soils, Saskatoon, 31 May 1922. Notes on Investigations of Deterioration of Concrete in Sulphate Waters, by Dr. Thorvaldson, 19 July 1965. Report of EiC Alkali Committee, by C. J. Mackenzie to the President and Council, EiC, 28 Dec. 1927. Report of EiC Committee on Deterioration of Concrete in Alkali Soils, by C. J. Mackenzie, to the EiC President and Council, Dec. 1923. Report by G. M. William5 on field tests to the Committee on Deterioration of Concrete in Alkali Soils, 31 May 1922. Interim Report on Chemical Investigation of the Deterioration of Concrete in Alkali Soils, by T . Thorvaldson, 7 March 1927, and addressed to President H. M. Tory of the National Research Council. Minutes of General Meeting of the Committee on Deterioration of Concrete in Alkali Soils of the EiC, Winnipeg, 4 Sept. 1922. Progress Report on the Investigation of the Action of Alkali on Cement and Concrete during the Year April 1930 to March 1931 Inclusive, by T. Thorvaldson, 11 April 1931 , addressed to Secretary S. P. Eagleson of the NRC. Letter and Report to C. J. Mackenzie from T . Thorvaldson, 13 March 1925. Interim Report on Chemical Investigation of the Deterioration of Concrete in Alkali Soils, by T. Thorvaldson, 17 March 1927, with covering letter to President H. M. Tory of the NRC. Progress Letter by T. Thorvaldson to President H . M. Tory of the NRC, 19 March 1928. Report on the Investigation on the Action of Alkali on Cement and Concrete, with covering letter to President H. M. Tory of the NRC, 23 March 1929. Letter to F. E. Lathe of the NRC from T. Thorvaldson, 19 June 1929. Letter to E.G. Swenson from G. C . Price, 27 Feb. 1967.

REFERENCES 1. T . THORVALDSON. The Selenium Problem in the Great Plains Region of North America.

Can. Chem. and Process Ind. 33, 1047 (1949). 2. G . T . E. GRAHAM and T. THORVALDSON. The Removal of Copper and Cadmium in the Metallurgy of Zinc. Can. J. Res. B20, 93 (1942). 3. T. THORVALDSON and L. R. JOHNSON. The Selenium Content of Saskatchewan Wheat. Can. J. Res. B18, 138 (1940). 4. W. C. HANSEN. Attack on Portland Cement Concrete by Alkali Soils and Waters: A

16 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

PERFORMANCE OF CONCRETE

Critical Review. Highway Res. Bd. Highway Res. Record no. 113, 1335 (Washington, 1966). A. G . BLACKIE. Causes of Disintegration of Concrete. Eng. J. 1, no. 4 (1918). G. R. SHELTON. Action of Sodium and Magnesium Sulphates on Constituents of Portland Cement. Ind. and Eng. Chem. 17, no. 6,589 (1925). G. R. SHELTON. Action of Sodium and Magnesium Sulphates on Calcium Aluminates. Ind. and Eng. Chem. 17, no. 12, 1267 (1925). G. R. SHELTON. Action of Sodium and Magnesium Sulphates on Portland Cement. Ind. and Eng. Chem. 18, no. 8, 854 (1926). T . THORVALDSON, R. H. HARRIS, and D. WOLOCHOW. Disintegration of Portland Cement in Sulphate Waters. Ind. and Eng. Chem. 17, no. 5, 467 (1925). T . THORVALDSON, R. K. LARMOUR, and V. A. VIGFUSSON. The Expansion of Portland Cement Mortar Bars during Disintegration in Sulphate Solutions. Eng. J. 10, 3 (1927). D. WoLOCHOW. Determination of the Sulphate Resistance of Portland Cement. Proc. Am. Soc. Testing Mats. 52, 250 (1952). T . THORVALDS0N, V. A. VIGFUSSON, and R. K. LARMOUR. The Action of Sulphates on the Components of Portland Cement. Trans. Royal Soc. Canada, 3rd Series, 21, Sec. III, 295 ( 1927) . W. C. HANSEN, L. T . BROWNMILLER, and R. H. BOGUE. Studies on the System Calcium Oxide-Ferric Oxide. J. Am. Chem. Soc. 50, 396 (1928). T. THORVALDSON, D. WoLOCHow, and V. A. VIGFUssoN. Studies on the Action of Sulphates on Portland Cement-part IV. Can. J. Res. 6,485 (1932). T. THORVALDSON. Chemical Aspects of the Durability of Cement Products. Proc. 3rd Intern. Symp. on the Chem. of Cement, Cement and Coner. Assoc., 436 (London, 1952) . W. C . HANSEN, L. T. BROWNMILLER, and R. H. BOGUE. Phase Studies on the System CaO-Fe 2O 3 -SiO 2 • J. Am. Chem. Soc. 48, 1261 (1926). A. G. FLEMING. The Development of Special Portland Cements in Canada. Eng. J. 16, 215-23, 260-67 (1933). T. THORVALDSON, V. A. VIGFUSSON, and D. WOLOCHOW. Studies on the Action of Sulphates on Portland Cement-part III. Can. J. Res. I, 385 (1929). T. THORVALDSON, V. A. VIGFUSSON, and D. WoLOCHOW. Studies on the Action of Sulphates on Portland Cement-part II. Can. J. Res. I, 359 (1929). T . THORVALDSON and V . A. VIGFUSS0N. The Effect of Steam Treatment of Portland Cement Mortars on Their Resistance to Sulphate Action. Eng. J. 11, 174 (1928) . T. THORVALDSON and G. R. SHELTON. Steam Curing of Portland Cement Mortars : A New Crystalline Substance. Can. J. Res. 1, no. 2, 148 ( 1929). T. THORVALDSON and N. S. GRACE. The Hydration of the Aluminates of Calcium. Can. J. Res. 1, no. 1, 36 ( 1929). D. T . MATHER and T. THORVALDSON. The Action of Saturated Steam on Dicalcium Ferrite and on Tetracalcium Aluminoferrite. Can. J. Res. B15, 331 (1937). T. TuORVALDSON and D. WoLocHow. The Action of Sulphate Solutions on Steam-Cured Composite Cement Mortars. Proc. J. Am. Concrete Inst. 34,241 (1938) . T. THORVALDSON. Effect of Chemical Nature of Aggregate on Strength of Steam-Cured Portland Cement Mortars. J. Am. Concrete Inst. 27, no. 7, 771 (1956). T . TuoRVALDSON and V. A. VIGFUSSON. The Action of Water on Tricalcium Silicate and Beta Dicalcium Silicate. Trans. Royal Soc. Canada, 3rd Series, 22, Sec. III (1928) . V. A. VIGFUSSON, G . N. BATES, and T. THORVALDSON. Hydrothermal Synthesis of Calcium Hydrosilicates. Can. J. Res. 2,520 (1934) . N. B. KEEVIL and T. TuoRVALDSON. The Hydration of Dicalcium Silicate and Tricalcium Silicate. Can. J. Res. Bl 4, 20 (1936). 0. K. JoHANNSON and T . THORVALDSON. Studies on the Thermochemistry of the Compounds Occurring in the System CaO-Al 2 O 3-SiO 2-part V. J. Am. Chem. Soc. 56, 2327 (1934) . T. THORVALDSON, N. S. GRACE, and V. A. VIGFUSS0N. The Hydration of the Aluminates of Calcium-part II. Can. J. Res. 1, 201 (1929). T. TH0RVALDS0N, W. G. BROWN, and C. R. PEAKER. Studies on the Thermochemistry of the Compounds Occurring in the System Ca0-AI 2 O 3-SiO 2 - part IV. J. Am. Chem. Soc. 52, 3927 (1930).

CONTRIBUTIONS OF THORBERGUR THORVALDSON TO RESEARCH

17

32. G. M. HARRIS, W. G. SCHNEIDER, and T. THORVALDSON. The Hydration of the Aluminates of Calcium-part IV. Can. J. Res. B21, 65 (1943 ) . 33. H. JoHNSON and T. THORVALDSON. The Hydration of the Aluminates of Calcium-part V. Can. J. Res. B21, 236 (1943 ). 34. W. G. SCHNEIDER and T. TuoRVALDSON. The Hydration of the Aluminates of Calciumpart III. Can. J. Res. B21, 34 ( 1943). 35. W. G. SCHNEIDER and T . THORVALDSON. The Dehydration of Tricalcium and Aluminate Hexahydrate. Can. J. Res. B19, 123 ( 1941). 36. T. THORVALDSON and W. G . SCHNEIDER. The Composition of the "5 : 3" Calcium Aluminate. Can. J. Res. B19, 109 (1941 ). 37. T. THORVALDSON, W. G. BROWN, and C. R. PEAKER. Studies on the Thermochemistry of the Compounds Occurring in the System CaO-Al 2O 3-SiO 2-part I. J. Am. Chem. Soc. 51, 2678 (1929) . 38. T. THORVALDSON and W. G . BROWN. Studies on the Thermochemistry of the Compounds Occurring in the System CaO-AI 2O~-SiO 2-part II. J. Am. Chem. Soc. 52, 80 (1930) . 39. T. THORVALDSON, W. G. BROWN, and C . R. PEAKER. Studies on the Thermochemistry of Compounds Occurring in the System CaO-Al 2O 3-SiO 2-part III. J. Am. Chem. Soc. 52, 910 (1930). 40. G. C. PRICE. Investigation of Concrete Materials for the South Saskatchewan Dam. Proc. Am. Soc. Testing Mats. 61, 1155 ( 1961 ) . 41. J. w. T. SPINKS, H. W. BALDWIN, and T. THORVALDSON. Tracer Studies of Diffusion in Set Portland Cement. Can. J. Technol. 30, 20 (1952). 42. W. A. G. GRAHAM, J. W. T . SPINKS, and T . THORVALDSON. The Mechanism of the Hydration of Tricalcium Silicate and B-dicalcium Silicate. Can. J. Chem. 32, 129 (1954) . 43. F. W. Brnss and T . THORVALDSON. The Hydration of Plaster of Paris. Can. J. Chem. 33, 870 (1955). 44. F. W. BIRSS and T . THORVALDSON. The Mechanism of the Hydration of Calcium Oxide. Can. J. Chem. 33,881 (1955) . 45. T. THORVALDSON, F . W. BIRSS, and K. G . MCCURDY. Calcium Exchange in Systems of CaO.SiO 2 .yH 2 O-Ca(OH) 2-H 2O. Proc. 4th Intern. Symp. on the Chem. of Cement, 1 (Washington, 1960) . 46. E.G. SWENSON and T. THORVALDSON. The Alcohol-Glycerol Method for the Determination of Free Lime. Can. J. Chem. 29,140 (1951) . 47. E. G. SWENSON and T. THORVALDSON. The Quantitative Determination of the Free Monoxides of Calcium, Strontium, and Barium, and of Calcium Ethylate by the AlcoholGlycerol Method. Can. J. Chem. 30, 257 ( 1952) . 48. E. G . SWENSON and T . TH0RVALDSON. Detection of Lignosulphate Retarders in Cement Suspensions and Pastes. Am. Soc. Testing Mats. Spec. Tech. Pub. no. 266 (1959). 49. T. TH0RVALDS0N. Portland Cement and Hydrothermal Reactions. Symp. on the Chem. of Cement (Stockholm, 1938). 50. T. THORVALDSON. Silicates of Calcium. Can. Chem. and Proc. Ind. 25, 197 (1942). 51. T . THORVALDSON. The Education of a Chemist. Can. Chem. and Proc. Ind. 26, 368 (1942). 52. T. THORVALDSON. Reactions in the Solid State. Trans. Royal Soc. Canada, 3rd Series, 38, Sec. III (1944). 53 . T. TH0RVALDS0N. Some Aspects of the Chemistry of Portland Cement. Can. Chem. and Proc. Ind. (1949) . 54. T. THORVALDSON. The Training of Chemists for Industry. Chemistry in Canada 3 (1951). 55. T. THORVALDSON, D. WOLOCHOW, and V. A. VIGFUSSON. Studies on the Action of Sulphates on Portland Cement-part I. Can. J. Res. 1, 273 (1929). 56. C. J. MACKENZIE and T. THORVALDSON. Differentiation of the Action of Acids, Alkali Waters, and Frost on Normal Portland Cement Concrete. Eng. J. 9, 79 (1926). 57. C. J. MACKENZIE. Concrete Mixtures in Alkali Soils. Eng. J. 3, 176 (1920) . 58. C. J. MACKENZIE. Rep. of Committee on Deterioration of Concrete in Alkali Soils. Eng. J. 6, no. 2, 57 (1923). 59. C. J. MACKENZIE. Concrete Deterioration in Alkali Soils. Eng. J. 8, no. 11,462 (1925). 60. MEL THISTLE. The Inner Ring. The Early History of the National Research Council of Canada (Univ. Toronto Press, 1966), p. 294.

21

The Chemistry of Sulphate-resisting Portland Cements

W. C. HANSEN* PROGRESS IN SCIENCE depends upon the accumulation of data and upon the formulation of theories describing the mechanisms by which reactions under investigation, either chemical or physical, produce observed effects. Usually no one investigator is able to present a completely correct theory for a given effect and, accordingly, proposed theories are generally revised, possibly by both the originator and others. The finally accepted theory is usually the result of the efforts of several people, and that was the case with portland cement. Impure limestones when calcined were discovered to yield products that behaved differently, in their reactions with water, from the products obtained when pure limestones were calcined. Studies of these impure limestones led to the development of portland cement. The history of this development has been reviewed by Bogue ( 1 ) . Having discovered that cements could be prepared by calcining blends of calcareous, argillaceous, and siliceous materials, investigators were faced with the problem of explaining what happened during calcination and during the reaction of the calcined product with water. While they were busy with these problems, engineers discovered that products fabricated from portland cements were not always performing satisfactorily when exposed to saline waters. This opened up a new field of research. The literature on the development of sulphate-resisting cements has been reviewed (2). Accordingly, in this paper, only the research and theories that contribute primarily to the subject of why concrete is attacked by saline waters and why certain cements produce concretes that have relatively high resistance to attack by these waters are discussed. Included with this is an outline of the several mechanisms by which concrete may be attacked by salts and by which the substitution of calcium aluminoferrites for tricalcium aluminate increases the sulphate resistance of cements. Readers interested in the nature of the cement minerals and their reactions with water are referred to papers presented at the last two symposia on the chemistry of cement (3, 4).

*W. C. Hansen, Consulting Chemist, Valparaiso, Indiana. An internationally known cement chemist, after some years in research work with the Portland Cement Association, Dr. Hansen joined the Universal Atlas Cement Company from which he retired as Director.

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

19

CONSTITUTION OF PORTLAND CEMENT A number of early investigators used the microscope in an effort to determine the chemical composition of the compounds present in hydraulic limes and portland cement. According to Lea and Desch (5), Le Chatelier as early as 1883 observed four crystalline phases in thin sections of portland cement clinkers and in 1887 Tornebohn gave these phases the names alite, belite, celite, and felite. Today it is known that these four crystalline phases are primarily CaS, 1 C2S, c~A, and a calcium aluminoferrite; the composition of the last named, however, differs somewhat from one clinker to another and can be expressed in terms of C 2F plus C6A2F. It is also known that all of these phases probably contain minor amounts of some or perhaps all of the other components of the cement. For example, the C 3S phase is probably never pure C 3S but always contains A and M. Hansen, Brownmiller, and Bogue ( 6) concluded from a study of a portion of the system C-A-F that a compound of the composition C 4AF existed and formed a complete series of solid solutions with C 2F. The calcium aluminoferrite phase in portland cement was therefore believed to be C4AF when the A/F ratio was 1 or greater than 1. Later work (7) showed that the alumina-bearing end member of this series of solid solutions has the composition CsA 2F rather than C4AF. For a number of years, however, investigators considered the iron-bearing phase in portland cement to be C 4AF and many prepared and studied products of this composition. Recent work ( 8) indicates that in many cements the iron-bearing phase does have a composition close to C4AF.

EARLY STUDIES ON SULPHATE RESISTANCE When he started the Canadian studies on sulphate resistance, Thorvaldson prepared and studied the behaviours of the pure cement minerals in water and in solutions of calcium and magnesium and sodium sulphates, CaSO4 , MgSO 4 and Na2SO4 • Thorvaldson, Vigfusson, and Larmour (9) studied mortar bars of the following compositions: I. II. III. IV. V.

1 part C 3S + 5 parts sand; 1 part C2S 5 parts sand; 1 part C3S 0.25 parts CaA 5 parts sand; 1 part C2S 0.25 parts C 3A 5 parts sand; 0.53 parts C3 S 0.26 parts C2S 0.21 parts C 3 A

+ + +

+

+ +

+

+ 7.5 parts sand.

In 2 and 8 per cent solutions of Na2SO4, bars of I and II expanded no more than did similar bars stored in water. However, in solutions of MgSO 4 of about the 1 The customary procedure is followed in this paper of expressing CaO, A1 2O3 , Fe2 O3 , SiO2 , MgO and H 2 O respectively as C, A, F, S, M, and H.

20

PERFORMANCE OF CONCRETE

same concentrations, bars of I and II expanded gradually. The incorporation of C 3 A with the silicates, bars III, IV, and V, destroyed the high resistance to sulphate observed for bars of I and II. The increased rate of expansion was more marked for bars of III and V made with C 3S than for bars of IV made with C 2S. Bars containing C 4 AF and C 2F were later introduced into the series. The results with those bars showed that the substitution of either C 4 AF or C 2F for C3 A markedly increased the resistance of the bars to attack by solutions of MgSO 4 • This is shown by the results for 1: 10 mortars (10) given in Table I. No calcium sulphate was added to the pure compounds, as is the case with portland cement. It may be unsafe, therefore, to conclude that the behaviours of the compounds in these specimens might be identical with their behaviours in portland cements containing calcium sulphate. TABLE I EXPANSIONS AND TENSILE STRENGTHS OF] :JO MORTAR BARS IN 0.)5 MOLAR MAGNESIUM SULPHATE SOLUTIONS* Composition of cement per cent by weight C3S

100 50 80 80 80 80 80 80 40 40 40 40 40 40

BC2S C3A CsA3

100 50

40

40

20

20

40 40 40 40 80 80 80 80 80 80

20

20

20

20

CA

20

20

20

C3As C2F C 4 AF

20

20

20

20

20

20

Expansion (per cent)

Exposure (days)

Strength (psi)

1.12 1.26 1.05 4.86 4.69 6 .27

200 200 325 26 26

65 38 42

1.48 2.04

200 160

3.08 3.58 3.00 3.32

II IO

5 .50

20

20

20

I.II

+1.22

IO IO

45 16 450 200

3.05 +2 .10 +0.85

35 210 300 200

0.85

1100

low low very low very low

48

bar firm low very low fairly firm low

86 60

fairly firm

60 81 87

no test no test

*Data from Thorvaldson, Wolochow, and Vigfusson (IO).

In 1926 the Portland Cement Association Fellowship at the National Bureau of Standards began an extensive investigation of the volume stability of 1 :2 mortar bars made with pure compounds, cements prepared in a small experimental kiln, and commercial cements stored in water and in solutions of sulphates. The results of these studies were published in several papers and were summarized by Bogue ( 11). Data for bars made with pure compounds and with some of the experimental

21

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

cements are given in Table II. These studies differed from those of Thorvaldson and associates in that 1 :2 mortars were used instead of the very lean mortars used by the latter. Accordingly the expansions and deteriorations proceeded at slower rates. TABLE II EXPANSIONS AT 1 AND 5 YEARS OF 1 :2 MORTAR BARS IN WATER AND 2 PER CENT SOLUTIONS OF SODIUM AND MAGNESIUM SULPHATES* Partial compositions of cements(% by weight) C3S

100

C 2S

Expansion for storage indicated ( %) Water

Sodium sulphate

C 3 A C4AF

I-year

100

15

0.012 0.013 ** ** 0. 137 0 .060

5 15 26 6 10 15 19 24 5 15 24 6 10 15 20 24

Laboratory cements 0 . 108 0 . 149 0.020 0 .067 0.087 0.023 0.044 0 .073 0.312 0.079 0 . 123 0.010 0.022 0 .066 0.104 0 .016 0 .059 0 .094 0.072 0 .014 0 .040 0.047 0.077 0 .043 ** 0.101 0.135 ** 0.058 0 .089 ** 0 .053 0.073 0.053 0 . 111 0.062 0.056 0.092 0 .280 0.038 0.064 0.315 ** 0.034 0 .061 0.056 ** 0.081

100

42.5 42.5

42 .5 42.5

15

36 40 40 52 50 50 52 54 34 36 37 50 43 48 51 51

48 37 25 33 31 25 18 12 45 33 23 28 33 22 14 8

7 5 5 5 5 6 5 5 11 11 11 12

II

12 12 12

100

5-year Pure compounds

1-year

5-year

0.012 0 .012 ** 0.500 0 . 112 0 . 160

Magnesium sulphate I-year

5-year

0.006 ** ** 0.032 0.057 0 .080 0 . 100 3-yr** 0 .079 0 .078 0 .082 ** 3-yr**

** ** ** ** **

0.075 0.125 0.350 0.063 0.057 0.115 0 .150 0.280 ** ** ** 0.195 0 .245 0 . 130 0.760 I .000

0.773 1.04 1.095 0 . 511 0.517 1.13 1.49 1.68

** ** ** ** **

*Data from R.H. Bogue (11). **Disintegrated.

One object of the study was to determine the effect of C 3 S content when the C 3 A and C 4AF contents were relatively constant. An increase from 40 to 50 per cent in the C 3S content appeared to have no significant effect upon the expansions. Another object was to determine the effect of increasing the C 4 AF content at the expense of the C 2S content. The data show that this increase of C4AF and decrease of C2S increased the expansions. The results of these and other studies clearly indicated that cement with relatively high resistance to sulphate could be prepared by increasing the F / A ratio of the clinker, either by decreasing the alumina content of the kiln feed or by adding additional iron-bearing material to it, both of which reduce the potential C 3 A content of the clinker. European investigators (2) also had reached the

22

PERFORMANCE OF CONCRETE

conclusion that C 3 A was the least resistant of the cement minerals to attack by sulphates. Miller and Manson ( 12, 13) determined the sulphate resistance of 119 commercial cements when exposed as mortar bars in sulphate solutions in the laboratory and in the water of Medicine Lake in South Dakota. As a result of the findings of Thorvaldson and others, Miller and Manson had their cements analyzed completely and used the data to compare the resistance on the basis of the calculated compound compositions of the cements. This showed a definite relationship between calculated C 3 A content and sulphate resistance. They then arranged for 19 of the cement mills to modify the compositions of their kiln feeds so as to decrease the potential C 3 A contents and to increase the potential C 4 AF contents. These cements showed a resistance to sulphate much improved over that of the unmodified cements. SPECIFICATIONS FOR SULPHATE-RESISTING CEMENTS According to Moreen (14 ), the Technical Committee on Cement, Lime, and Plaster of the U.S. Federal Executive Committee, under the leadership of P. H. Bates, undertook in the fall of 1935 the preparation and revisions of the then current specifications for normal portland cement. The Committee decided on five separate specifications as follows: (a) normal portland cement, (b) high earlystrength cement, ( c) moderate heat of hardening cement, ( d) low-heat cement, and (e) sulphate-resisting cement. Later the Committee found no need for both moderate heat of hardening and low-heat cements and on September 30, 1936, they adopted specifications for four cements. The chemical requirements, under the title Federal Specification SS-C-211, for sulphate-resisting cement were as follows: Loss on ignition (max.), 3.0 per cent: AhO 3 (max.), 4.0 per cent MgO (max.), 4.0 per cent SO 3 (max.), 2.0 per cent Fe2Oa (max.), 4.0 per cent

SiO2 (min.), 24.0percent C 3 A (max.), 5.0percent A/F, 0.7 to 2.0 per cent.

In 1940 the American Society for Testing and Materials adopted a tentative specification for a Type V cement, sulphate-resisting, which was identical with the requirements of SS-C-211 (15). It may be seen that this specification limited the C 3S content by setting a minimum limit on the SiO2 content and limited the aluminoferrite content by setting a maximum limit on the Fe2O 3 content. The latest ASTM specification (16) has no limit on the SiO2 content and accordingly no limit on the C 3 S content; it has maximum limits of 5.0 per cent on both MgO and C3A; it limits calcium aluminoferrite by specifying that the aluminoferrite plus 2(C3A) contents shall not exceed 20.0 per cent.

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

23

LATER STUDIES ON SULPHATE RESISTANCE

In 1940 ( 17), the Portland Cement Association undertook an extensive investigation of the performance in the field of concrete in various types of structures and exposures. One phase of this programme, which included 27 cements, was the exposure of 6- by 6- by 30-in. concrete beams in sulphate soils in Sacramento, California. Three mixes designed to have 1½-in. slump with cement contents of 4-, 5Jf-, and 7-sack/cu yd of concrete were used in making beams; these were designated mixes 1, 2, and 3 respectively. Two basins, 1 and 2, were built to hold the specimens and the soil. The soil in basin 1, which was a natural alkali soil from Willows, California, contained about 10 per cent of soluble material, mostly Na2 SO4 • Magnesium sulphate and relatively alkali-free soil were added to the Willows soil to give a soil for basin 2 with about the same salt content as that of basin 1 but in which the ratio of Na 2SO4 to MgSO 4 was about 2: 1. The specimens, at ages of a few weeks, were placed in the basins and the soil was added to a depth of about 3 in., leaving about half of the beam exposed above the soil. Tap water was added to a level just above the surfaces of the beams; this was the standard level to which the basins were filled whenever water was added to them. Winter rains caused the basins to overflow; therefore, the water was analyzed each spring and salts were added as required to give approximately the original salt contents. During the summer and fall, water was added to the basins whenever the soil began to appear dry. At approximately one-year intervals, the beams were examined visually and rated for extent of deterioration. A rating of 1 indicated a perfect beam, 2 indicated slight rounding of the edges and corners, 3, 4, and 5 indicated greater and greater deterioration, and 6 indicated complete failure. It was expected that the rate of deterioration would be greater in the mixed sulphate soil, basin 2, than in the primarily Na2SO4 soil of basin 1. However, the reverse was found to be the case. Close examination ( 17) revealed that, when the surfaces of the beams became dry, the salts deposited on their surfaces in basin 1 turned into a powdery solid, whereas in basin 2 they formed a glass-like coating. It seemed evident that this glass-like deposit was sealing the surfaces against evaporation of water from them. A study of the properties of the salts showed that as water evaporated and the solution became saturated, Na2SO 4 .10H2O was the salt deposited on the beams in basin 1. This salt dissociated into powdery Na2SO4 and water vapour under the temperature conditions and humidity of the atmosphere in Sacramento during the summer and fall. On the other hand, the salt deposited on the beams in basin 2 was Na2SOF MgSO4.2H2O, which does not dehydrate under those atmospheric conditions. Evaporation of the water deposited this salt as a continuous glass-like layer over the surface of each beam. This protected the beam from the migration of water

24

PERFORMANCE OF CONCRETE

through it, whereas, the powdery salt formed on the beams in basin 1 was no protection against the migration of water through the beam to the surface as water evaporated from it after the surface became dry. Hence all conclusions from this phase of the study, with respect to the durability of the concrete in sulphate soils, have been limited to the data from the beams in basin 1. The specimens of mixes 1 and 2 ( 4- and 5)~-sack concrete) deteriorated rather rapidly and the data do not provide a very sound basis for judging the long-time performance of the cements in higher quality concrete. However, the data substantiate one conclusion of all investigators in this field, that is, that concrete must be highly impermeable if it is to perform satisfactorily in alkali soils and water. The specimens of mix 3 (7-sack concrete) are deteriorating at a relatively slow rate and the data from them are useful in showing the long-time performance of concrete that is exposed at intervals to drying, a condition in which the migration of salts into and through the specimens should be at a much greater rate than that into specimens that are merely immersed in water or water-saturated soil. Table III compares the values calculated for the C 3A contents of the cements with the ratings of the beams in basin 1 after exposure for 18 years ( 18). This table also contains data for the expansions of mortar bars of these cements tested in accordance with ASTM method C-452 ( 16). The C 3 A contents of these cements were calculated on the basis that the calcium aluminoferrite phase had the composition C 4 AF and that none of the A or F was contained in any of the other cement minerals. Errors in these calculations of C 3A contents because of these assumptions might be greater for some cements than for others. Such errors might be responsible, to some extent, for the less than perfect relationships between calculated C 3 A contents and the ratings of the beams and the expansions of the mortar specimens. CALCIUM SULPHOALUMINATES According to Lerch, Ashton, and Bogue ( 19), Candlot appears to have been the first ( 1890) to establish the formation of a definite compound by the interaction of aqueous solutions of calcium aluminates and calcium sulphate. A number of investigators restudied this system and confirmed the existence of such a compound. Lerch, Ashton, and Bogue made a thorough study of the system and established the existence of two compounds: a high sulphate compound with the composition C 3 A.3CaSO 4 .31H2 O and a low sulphate compound with the composition C 3 A.CaSO4.12H2O. The high sulphate form occurs naturally and has been given the name ettringite. There is not complete agreement as to the water content of this compound, but it seems likely that the crystals in equilibrium with the mother liquor contain 32 moles instead of the 31 as reported by many investigators. Because of its formation in large quantities in concrete attacked by sulphates, this compound has been called "cement bacillus." Midgley and Rosaman ( 4) believe that the ettringite phase in hardened cement pastes may be a solid solution phase in which some sulphate ions are replaced by hydroxyl ions.

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

25

TABLE III FIELD RATINGS VS EXPANSIONS OF MORTAR BARS BY ASTM METHOD C-452 Expansions at days indicated : C-452**

Cement number

Cale. C3A*

Rating* 18-yr

7

14

21

28

84

365

23 51 42 41 25 43A 24 43 21 34 22 16 14 13 17 33 31 15 11 12 18

2.1 2 .2 2 .7 3 .4 3.8 3.9 4.4 4.8 5.1 5.2 5.4 6.4 7.l 8.9 9 .1 9 .2 9 .6 9.9 10 .9 11 .2 12.2

1.2 1.3 1.4 1.4 1.2 1.2 1.3 2.7 1.4 2.7 1.6 3.1 2.2 2.0 3.2 3 .1 3.3 3 .4 2 .1 3.5 4 .1

0 .020 0.018 0.022 0.025 0 .030 0 .026 0 .017 0 .038 0.027 0.025 0 .036 0.030 0 .028 0.027 0 .039 0 .039 0.039 0.043 0 .045 0.058 0 .085

0.027 0.025 0 .030 0.033 0 .040 0 .037 0.023 0 .051 0.038 0 .033 0 .048 0 .044 0.039 0.042 0 .048 0 .060 0 .060 0 .066 0 .067 0.088 0 .135

0 .033 0.030 0 .035 0 .038 0 .047 0 .044 0 .027 0 .061 0.046 0 .043 0 .058 0 .054 0 .048 0.052 0 .062 0 .076 0 .076 0.090 0 .085 0.114 0 .180

0 .038 0 .033 0 .040 0.042 0.052 0 .049 0.030 0.068 0.052 0.066 0.066 0.062 0.054 0 .060 0.072 0.093 0.092 0 .117 0 .102 0 .135 0 .224

0 .059 0 .049 0 .057 0 .063 0.075 0 .071 0.043 0.100 0 .083 0.067 0.107 0.103 0.091 0 .104 0.137 0.240 0.279 0 .177 0 .222 0 .329 0 .741

0.093 0.086 0 .089 0 .114 0 .121 0 .112 0.081 0 .124 0 .144 0 . 167 0.225 0 .216 0.166 0 .375 0 .325 0 .253 0 .353 0 .216 0 .645 0 .733

21T 16T 33T llT 12T 18T

5 .4

1.2 1.3 2 .0 1.6 3.4 3.9

0 .058 0 .057 0.096 0 . 103 0.139 0 . 191

0 .065 0 .065 0 .116 0.123 0 .166 0.235

0.093 0.106 0 .258 0.338 0 .425 1.295

0.162 0 .192 0.255 0.359 1.15

6.8 9.3 11.0 11.1 12 .2

Air-entraining cements

0 .035 0.033 0.051 0.055 0.070 0 .093

0 .049 0 .047 0 .075 0.081 0.108 0 .146

*William Lerch. Significance of Tests for Sulfate Resistance. Proc. ASTM 61, 1043 (1961). **William Lerch . A Performance Test for the Potential Sulfate Resistance of Portland Cement. ASTM Bull. no. 212, 37 (1956).

A number of investigators have attributed the deterioration of concrete in sulphate-bearing water to the formation of ettringite. It appears that most investigators have assumed that, since this highly hydrated salt occupies much more volume than the C 3 A from which it formed, its formation by a through-solution process in pores of the concrete would cause expansion and destruction of cement paste. According to Blondiau (20), however, Le Chatelier concluded that expansion was caused primarily by the reaction of the dissolved calcium sulphate with solid CaA to produce solid ettringite in situ. That is, in this reaction the C 3 A does not dissolve in the water but reacts directly with ions of calcium sulphate and water to yield a solid product. Such reactions have been referred to as solid-liquid and topochemical reactions and have been discussed in some detail (21). Apparently Le Chatelier also believed that hydrated aluminates, such as C 4 A.aq. and C 3 A.aq., could also undergo this solid-liquid reaction because he decided that the most effective test procedure for determining the sulphate resistance of a cement would be to measure expansions of specimens prepared from fully reacted pastes.

26

PERFORMANCE OF CONCRETE

Those who support the solid-liquid reaction, and there appears to be an ever increasing number, probably accept the principle that a crystal cannot enter the solution phase as ions or molecules, without first reacting with the solvent in a manner that overcomes the energy by which the atoms or ions in the crystal are bonded to one another. Van Arkel (22) has discussed this process in some detail for the dissolution of NaBr, which forms the hydrate NaBr.H 20. According to van Arkel, NaBr dissolves completely in an excess of water but forms NaBr.H 20 in a limited amount of water. When a crystal of NaBr is exposed to water, the first water molecules are taken up in a regular manner in the lattice of the crystal and surround the positive ions. When additional molecules of water are taken up, the ions are loosened from the structure and each ion goes into solution with its portion of the water molecules attached to it. This take-up of water furnishes the energy required to break the ionic bonds in the crystal and thus allow the dissolution of the crystal. It seems evident from this mechanism that relatively insoluble compounds should form as solids on the surfaces of the reacting crystals at higher rates than those at which they can dissolve. If the reacting crystals are in contact with one another, except possibly for a thin layer of adsorbed water, this formation of the reaction product as a solid on the surface could cause the crystals to exert pressure against each other and, in a rigid paste, cause it to expand. Chatterji and Jeffery (23) suggest that conversion of C4 AH 13 to the low sulphate sulphoaluminate might be a solid-state reaction, which could cause expansion in concrete. They point out that the crystal structure of C4 AH 13 is such that sulphate ions can replace two hydroxyl ions, by an ion exchange, as illustrated in the following equation: C4AH13 + SOl- +water= C3A.CaS04.H12 + 2(0H-) 277 .37 313.68 The numbers 277.37 and 313.68 are the volumes of the two solids which show that the volume of the solid phase increases about 14 per cent if this reaction occurs. There appears to be no reason why this reaction should not occur as a solid-liquid reaction if the compound C4 AH 13 is present as a component of the hardened cement paste and sulphate ions diffuse into the paste. This will be discussed in more detail in a later section. SOLID-LIQUID REACTIONS It seems that chemists have generally assumed that when solids undergo reactions in an aqueous medium, the solids first dissolve in the water and, if the reaction products are relatively insoluble, the reaction products precipitate or crystallize as solids. Kohlschtitter (24) appears to have been one of the first, if not the first, to realize that in some reactions the reaction product formed on the surface of the solid without the solid's going into solution. These were called topochemical reactions. It also appears that Kohlschtitter and co-workers did not realize as van

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

27

Arkel did that the topochemical reaction precedes the process of dissolution of a solid in a liquid. This appears also to be unrecognized by some investigators today, because one finds statements in the literature on cement to the effect that in the early stages, when there is a relatively large amount of water present, the cement minerals react with water by a through-solution process. This appears to be the process only because the reaction products dissolve to some extent from their sites of formation on the surfaces of the crystals and crystallize in the water-filled space. According to Strelkov (25), however, Bailkov, from investigations carried out between 1923 and 1932, concluded that the theories proposed by Le Chatelier and by Michaelis did not fully explain the setting and hardening of pastes of plaster of Paris, hemihydrate-CaS04 .½H2 0. According to Bailkov a chemical reaction takes place during the hardening of hemihydrate on the surfaces of the hemihydrate crystals without preliminary dissolution, which is that of the formation of gypsum, CaS04.2H20; this is followed by dissolution of the newly formed gypsum until a saturated solution is produced with respect to this layer of gypsum on the surfaces of the original crystal of hemihydrate. A further chemical reaction between water and hemihydrate takes place in contact with the liquid; the gypsum formed is insoluble in the saturated solution phase. This process is a unidirectional diffusion of water into the lattice of the hemihydrate crystals resulting in the fixation of water by the crystal. Since the volume of the dihydrate is approximately equal to the sum of the volumes of the hemihydrate and the combined water, there occurs at the reacting surface a local expansion of the volume of the solid which sets up stresses in the boundary zone. When these stresses exceed a certain value, the reaction layer breaks into separate fragments or crystallites; that is, they cleave from the reacting surface. Glasson (26) describes a similar mechanism for the reaction of calcium oxide with water. According to Bailkov, this cleavage of the reaction layer disperses this layer, in the form of colloidal particles or gel, into the water-filled space. Because this gel is a polydisperse substance whose particles have different solubilities a recrystallization is produced by way of solution of the finer and growth of the larger particles or crystallites. Strelkov concluded from his later studies that this mechanism, described by Bailkov, applied also to the reaction of portland cement with water. He points out that the reaction products migrate into the water-filled space by two processes. In the one process they go into solution and in the other they are forced away from the reacting surface by the cleavage mechanism. Some investigators have argued that most of the reaction of the cement minerals with water, or with other components of the cement paste, must proceed by a through-solution process because the topochemical or solid-liquid process would destroy the hardening cement paste by expansion. This argument appears not to recognize,that the products formed on the surface of the solid minerals may dissolve and diffuse into water-filled space and that these products may also cleave, as colloidal particles, from the reacting surfaces when the layer of reaction product reaches a certain thickness.

28

PERFORMANCE OF CONCRETE

It was pointed out in 1952 (3) that these surface reactions explained why portland cement products continued to expand over long periods of time when stored in water. Later (27) an attempt was made to show by means of an idealized model that these expansions should never be very large and, accordingly, should not destroy the cement paste. Figure 1 is this idealized model and is supposed to represent an arrangement of the solid (S) and liquid (L) units as cubes in a cement paste consisting of 40 parts cement and 60 parts water by volume. The ratio 40:60 is 1.0:1.5 and the cube roots of 1.0 and 1.5 are 1.0 and 1.15 respectively. Figure 1 is a schematic representation of a plane through such a paste. The lines ab, cd, and so forth are 1 unit long and the lines be, ef, and so forth are 0.15 units long. This figure shows the grains to be separated from one another by layers

Lo

So b

S2 e

S1

d

C

a

L1

L2

S3

S4

L5

13

f

L4 g

S7

FIGURE

L7

Sg

s6

Lg

I . Suggested arrangement of particles in a cement paste.

of liquid. In this particular paste, the average thickness of the layers of liquid at the points at which the particles are closest to one another is 0.15 units. If this figure is realistic, and there appears to be no reason to believe that it is not, then the grains of cement, in a paste of this water-cement ratio, can expand to more than double their volume without interference from a neighbouring grain except at

29

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

certain points of closest approach. For example, when the edge ab of cube S2 expands, it will tend to expand into space represented by line be, and the edge cd of cube S0 will also tend to expand into this space. Table IV gives the molecular weights and densities of the compounds referred to in Table V, which gives data for reactions that are generally accepted as possible reactions in cement pastes. The numbers immediately below the formula for the compound is the volume in cubic centimetres of the grams of the compound taking part or being formed in this reaction ( except the small number in parentheses in the right-hand side of each reaction) . This is the number of cc by which the volume of the reactants exceeds the volume of the reaction products. These calculations show, what is well known for chemical reactions, that the reaction products occupy less volume than the volume of the reactant. This means, if the reactants are dissolved in the liquid phase of the cement and then precipitated as the reaction products shown in the equations of Table V, that the volume of the paste should contract and not expand. Hence, the fact that portland cement products expand when kept moist is not compatible with the through-solution theory. This will be discussed later. TABLE IV MOLECULAR WEIGHTS AND DENSITIES OF COMPOUNDS REFERRED TO IN TABLE

Formula of compound CaSO 4 .2H2O 3CaO.Al2O3 3CaO.SiO 2 2CaO.SiO 2 Ca(OH),

H 20

4CaO. Al 2O 3 .13H 2O 3CaO .2SiO 2 .3H 2O 3CaO . Al,O 3 . 3CaSO4 . 32H,O 3CaO.Al2O3 .CaSO4.12H,O 3CaO .Al 2O 3 .6H,O

V*

Molecular weight

Density

Volume per molecular weight (cc)

172.1 270 .2 228 .0 172.0 74.0 18.0 560.2 342.0 1254.5 622.3 378 .2

2 .32 3.04 3 . 15 3 .27 3.24 1.00 2.08 2.73 1.73 1.95 2.52

74.1 88.8 72.3 52 .6 33.2 18 .0 269.2 125 .3 725 . 1 319 . 1 150 .0

*Data from reference 27.

The second row of numbers under the equations of Table V show the ratio of the volume of the principal reactant to the volume of the solids produced by the reaction. For example, one volume of C 3 A produces 8.2 volumes of ettringite by the reaction of Equation 1 in Table V. On the other hand, Equation 5 in Table V shows that one volume of C 3S produces only 1.55 volumes of C 3S2H 3 +CH. For this case of C 3 S in which 1 volume of C 3 S expands to 1.55 volumes, the cube root of 1.55 is 1.16. If the cubes S0 and S2 of Figure 1 are crystals of C 3S, when they react with water the edges cd and ab would tend to expand equally in both directions. They would each tend to expand 0.08 units into space represented by line be.

30

PERFORMANCE OF CONCRETE

TABLE V DATA FOR POSSIBLE REACTIONS IN PORTLAND CEMENT PASTES*

Tricalcium Aluminate

Equation 1 Volumes(cc) Ratio, original volume to new volume Equation 2 Volumes (cc) Ratio, original volume to new volume Equation 3 Volumes (cc) Ratio, original volume to new volume Equation4 Volumes (cc) Ratio, original volume to new volume

8 .2

3CaO.Al,O 3 88 .8

+ CaSO4.2H 2O + IOH,O = 3CaO .Al,O 3 .CaSO4.12H 2O 74 . 1 180 .0 319 . 1 + (23 .8)

3CaO.Al , O 3 88 .8

+ Ca(OH), + 12H,O = 4CaO .Al,O 3 . 13H,O 33 .2 216 .0 269 .2 + (68.8)

3CaO.Al,O 3 88.8

+ 6H2O = 3CaO .A1 2 O 3 .6H,O 108 .0 150 .0 + (46.8)

3 .6

3.0

1.69 Trica/cium silicate

Equation 5 Volumes (cc) Ratio, original volume to new volume

2(3CaO .SiO,) 144 .6

Equation 6 Volumes (cc) Ratio, original volume to new volume

2(2CaO .SiO,) 105.2

+ 6H2O = 3CaO .2SiO,.3H,O + 3Ca(OH)2 108 .0 125.3 99 .6 + (27 .7) 1.55 Dicalcium silicate

+ 4H, O = 3CaO .2SiO,.3H,O + Ca(OH), 72.0 125 .3 33 .2 + (18.7) 1.51

*Data from reference 27.

Hence, the two cubes tend to expand a total of 0.16 units into a space of 0.15 units. Each cube had an original length of 1 unit and each now has expanded to a length of 1.16 units. The length available for the expansion of one-half of each S2 and S0 cube is the sum of their lengths 0.5 0.5 plus the 0.15 units of water-filled space between them or 1.15 units. The expansion then may be 1.16 - 1.15 X 100 / 1.15 or 0.87 per cent over the available space. From this it appears that C 3 S could produce less than one per cent expansion in this paste if it were transformed directly into solid C3S2H3 3CH by the reaction of Equation 5 (Table V) . Less expansion would be produced by C 2S. This degree of interference could occur only at certain points at which the grains approached each other closely, when the crystals had completely reacted with water and when the reaction products were

+

+

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

31

fully hydrated. At lower water-cement ratios, the space available at these points would be less. The amounts of unreacted material in the hardened paste, however, increase as the water-cement ratio decreases. Gonnerman, Lerch, and Whiteside (28) gave expansions for neat cement specimens of 418 commercial cements stored in water for 15 years. All of the cements passed the ASTM pat test for soundness. The range of expansions for the 15-year period was 0.095 to 0.379 per cent. In general, the expansions in water increased with increasing expansion in the autoclave test and seemed to be related to the free Cao and MgO contents. These data indicate that the principal cement minerals do not produce much expansion in concrete. When C 3 A combines with water and CaSO4 to form ettringite, 1 volume of C 3A yields 8.2 volumes of reaction product. Since the crystals of C 3A should be more or less uniformly distributed among the other crystals, there should be water-filled space available to accommodate most of the expansion except at points at which the crystals of C 3 A are in close contact with other crystals of either C 3 A or one of the other cement minerals. Lerch (29) has shown that the 7-day expansions of mortar bars in water generally decrease as the SO3 content of the cement increases to some optimum amount and then increase. The optimum amount of SO:i appears to be the amount that is converted to ettringite within the first 24 hours. This seems to indicate that the ettringite formed while the paste is plastic does not produce significant expansion and one beneficial effect of SO3 is that it accelerates the rate at which C 3 A reacts so that much of its reaction occurs during the first 24 hours. Also, it is often possible to see rather large crystals of ettringite and calcium hydroxide in the hardened cement paste, an indication that there is considerable dissolution of these compounds and recrystallization during the period in which the liquid phase can readily contact these compounds as they form . This process will be discussed in more detail later. THROUGH-SOLUTION REACTIONS Burke and Pinckney (30) were among the early investigators of the causes of the deterioration of concrete in sulphate waters. They treated powdered hardened cement pastes with water and solutions of sulphates and from the results of chemical analyses of the filtrates and residues concluded that the reactions causing deterioration were double decomposition reactions as illustrated by the following equations: Ca(OHh Ca(OH)2

+ MgSO4 + 2H2O = CaSO4.2H2O + Mg(OH)z + Na2SO4 + 2H2O = CaSO4.2H2O + 2NaOH

They calculated that one volume of Ca(OH)z yielded 3.13 volumes of CaSOF 2H2O Mg(OHh. On the assumption that the reaction products tended to be deposited in the space occupied by the Ca(OHh, they concluded that this increase

+

32

PERFORMANCE OF CONCRETE

in space required by the reaction products was responsible for the disruption of the cement paste, causing the specimens to bulge, crack, and crumble. This might be true if the solid Ca(OH)z reacted as a solid with dissolved sulphate and water to yield the solid reaction products. However, this cannot be true if the Ca(OHh dissolves and reacts with dissolved sulphate and water. One theory appears to be that sodium sulphate, for example, diffuses into a pore in the hardened paste and there reacts with Ca(OHh as the latter dissolves to form gypsum which occupies a greater volume than the Ca(OH) 2 because one mole of Ca(OHh occupies a volume of 31.6 cc compared with 74.2 cc for one mole of CaSO 4 .2H2O. The following calculations ( 2) were offered to demonstrate that this reaction could not produce expansion in a pore that permitted the entrance and escape of water and dissolved salts. For ease of calculation, it was assumed that a pore, which could be visualized as lined with solid Ca(OHh, had a capacity of 42.6 cc. If 31.6 cc, one mole, of Ca(OH) 2 dissolved from the walls of the pore and reacted with one mole of Na2SO4 from the ambient solution, the reaction would produce 74.2 cc of solid CaSO4.2H2O. The original space of 42.6 cc in the pore plus 31 .6 cc of space formed when the Ca(OHh dissolved equals a total of 74.2 cc which is the volume of the CaSO4.2H2O formed by the reaction. Since this volume fills the pore, the pore cannot contain water to dissolve additional Ca(OH)z for reaction with additional Na2SO 4 • Accordingly it appears that the reaction will stop for lack of water before any expansion occurs. This result will be the same for a pore of any size. It seems obvious from such calculations that concrete cannot be caused to expand and crack by the simple mechanism of filling pores with solids by a through-solution process. The double decomposition reactions of equations 1 and 2 can destroy concrete by dissolving the cementing phases and precipitating noncementing phases such as Mg(OHh Later a mechanism will be described to explain the expansion of concrete caused by the formation of Mg(OH)z. Results by Thorvaldson, Wolochow, and Vigfusson (10) demonstrated the inability of the crystallization of gypsum in mortar bars to cause expansions and disintegration of mortar bars made with calcium silicates as the cements. The data for 1: 10 calcium silicate mortar bars stored for 3 years in 0.50M and 0.15M solutions of Na2 SO4 and in saturated solutions of CaSO 4 are given in Table VI. Table VII gives similar data for mortar bars made from calcium silicates as the cements and stored in 0.15M and 0.50M solutions of MgSO4. It may be seen from Table VI that some of the bars made with calcium silicates show slight contractions at 3 years and others expanded very slightly, 0.02 to 0.06 per cent. The authors point out the possibility of the bars expanding after longer periods of storage and the possibility of the strengths having been affected because microscopical examination of the bars showed the presence of large quantities of gypsum. It seems, however, that these results indicate that Na 2SO4 reacted with Ca(OHh and converted the latter into gypsum without causing expansions of the specimens. On the other hand, the data of Table VII indicate that the reaction of MgSO4 with Ca(OH)z caused large expansions of the bars made with calcium silicates in relatively short

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

33

TABLE VI EXPANSIONS OF CALCIUM SILICATE MORTAR BARS IN SULPHATE SOLUTIONS* Number 3A IA 21A 3B 1B 21B JC 21C JS 21S

Silicate C3S C3S C3S BC 2S BC2S BC 2S rC2S rC2S 1 :1 C 3S:C2S 1 :I C 3S:C 2S

Solution

Expansion-3-yr (per cent)

0 . SOM Na 2SO 4 0.15M Na 2SO 4 Satd.CaSO 4 0 .05M Na 2SO4 0. I SM Na2SO 4 Satd.CaSO., 0 . l 5M Na 2SO4 Satd.CaSO., 0 . l 5M Na 2SO 4 Satd.CaSO.,

0.02 -0.02 0.04 0 .03 -0 .02 -0 .02 0.06 -0 .01 0.02 0 .02

*Data by Thorvaldson, Wolochow, and Vigfusson (10). TABLE VII EXPANSIONS OF] :10 CALCIUM SILICATE MORTAR BARS IN MAGNESIUM SULPHATE SOLUTIONS* Number 4A 2A 2B 2S 2C

Silicate

Solution (moles)

Exposure (days)

Expansion (per cent)

C3S C 3S BC 2S 1:1C3S:C 2S rC 2S

0 .50 0.15 0 . 15 0 . 15 0 . 15

210 200 200 325 1000

2 .2 1.1 1.3 1.05 1.00

*Data by Thorvaldson, Wolochow, and Vigfusson (10).

periods of time. The reason for the lack of expansion in Na 2 SO4 is discussed above and that for the expansion in MgSO 4 will be discussed later. With respect to strengths, the authors prepared 1: 5 C3 S and C 2S mortar bars and tested them for tensile strengths after exposure for 4 years in 0.6M solutions of Na 2SO4 and in distilled water. The strengths were 232 and 198 psi respectively for the C 3S specimens and 256 and 187 psi respectively for the C 2S specimens, which show that the strengths were not harmed by the crystallization of gypsum in the specimens. Miller and Manson ( 13) pointed out that strengths were not satisfactory as measures of the deterioration because in many cases compressive strengths of cubes stored in sulphate solutions were higher than those of cubes stored in water. In other words, precipitation of salts in the hardened cement paste seemed to augment the strength. Table I gives expansions and tensile strengths, after storage in MgSO 4 solutions, for 1: 10 mortar bars made with either CsS or C2S or combinations of the two with various calcium aluminates, C 2F and C 4 AF. These data show that replacement of 20 per cent of the silicates with calcium aluminates markedly increased the

34

PERFORMANCE OF CONCRETE

expansions and decreased the tensile strengths. In general, replacements with either C2F or C4AF did not markedly affect either the strengths or expansions. The bars made with cements of C 3S plus the calcium aluminates expanded appreciably more than did bars made with C 2S and calcium aluminates. One might expect that the hardened pastes made with C 3 S and containing greater amounts of Ca(OHh would react more rapidly and to a greater extent with sulphate than would bars made with C2S which would contain less Ca(OHh, It appeared from such data that the C 3S content of a sulphate-resisting cement should be as low as is consistent with satisfactory strength, and the first U.S. specification for such a cement (14) placed a limit on the SiO 2 content which was equivalent to placing a maximum limit on the C3S content. POROSITY AND STRUCTURE OF HARDENED PORTLAND CEMENT PASTE Early investigators' observations that cement products lost and gained weight during drying and wetting were taken as evidence that the products were porous. It is recognized (31, 32) that this loss and gain in weight is not indicative of a simple system containing a number of holes and channels. For example, Powers and Brownyard (33) reported that ettringite loses 22 moles of water when dried over magnesium perchlorate. Generally this loss and gain of water by hydrated crystals has not been considered in studies of the porosity of portland cement products, with the result that some erroneous concepts as to the physical properties of hardened cement paste probably occur; for example, when an attempt is made to calculate the diameters of the pores of a cement paste by means of the Kelvin equation. Powers and Brownyard (33) presented the hardened cement paste as a product consisting of colloidal- and coarser-size particles of the reaction products, unreacted cement grains, gel pores, and capillary pores. One definition of a pore, as applied to stone, is a "small interstice admitting absorption or passage of liquid." Interstice is defined as "a space between one thing and another." Various types of evidence show that hardened cement paste is made up, in a large part, of colloidal-sized particles. Since these are discrete particles, they must be separated from one another by some medium. Also since these particles are formed in the presence of an aqueous medium, either liquid water or water vapour, the medium separating them must be water. The mechanism described by Strelkov and by Glasson for the reactions of cement and calcium oxide with water can explain the origin of the gel pores in portland cement paste. According to this mechanism, a grain of cement adsorbs and reacts with water to form a layer of reaction product on its surface. When the stresses in this layer become excessive, the layer of reaction products cleaves from this reacting surface in the form of crystallites. The surfaces of these crystallites

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

35

which are in contact with water are coated with adsorbed water and the new surfaces, formed by the cleavage, will become coated immediately with adsorbed water. The reacting surface uncovered by this cleavage will also tend to adsorb water immediately after it is uncovered. The act of cleavage separates, to some degree, the newly formed crystallites from the reacting surface and the adsorption of water by the newly formed surfaces on the crystallites and on the reacting surface generates forces that will tend to separate the particles. It has been pointed out by zur Strassen (32) that adsorbed water on the hydrated silicates of a hardened cement paste is bound to the silicate lattice by the same forces as in the inter-layer water and, accordingly, is likely to have a structure oriented with respect to the oxygen layers of the silicate lattice. In other words, the water molecules in the adsorbed water are less randomly oriented than are those in bulk water. The randomness of orientation, however, increases with each successive layer of water molecules adsorbed by the surface when it is immersed in water, until the arrangement of water molecules in a layer of water is almost identical with the arrangement of the water molecules in bulk water. The properties of this last layer of adsorbed molecules of water are those of bulk water. For example, this layer will evaporate at the same vapour pressure as that at which bulk water evaporates. Successive layers of molecules of water evaporate from this surface as the vapour pressure of the ambient atmosphere is progressively reduced. One sees from this that if two particles of a solid that are almost in contact with each other are immersed in water, the two particles will adsorb water and repel each other until they are separated by a mass of water which is in equilibrium with bulk water. Once this separation has been effected the thickness of the adsorbed mass will not increase because the surface forces of the solid particles have been completely satisfied. If the water is evaporated from this system by lowering the vapour pressure of the system, the particles will remain separated to this extent until all of the bulk water has evaporated. Then, when water begins to evaporate from the adsorbed mass between the particles, the particles draw closer to each other. Hence, it is seen that adsorbed water can serve two purposes in a portland cement paste, that is, it can force the solid particles apart and it can draw them together. In other words, the property of solid surfaces to adsorb water and change the properties of water from those of bulk water is one factor operating in the swelling and shrinkage of hardened cement pastes during wetting and drying. Another factor is the gain and loss of combined water by the crystals of the reaction products. When a second layer of reaction product cleaves from the reacting surface, each new crystallite is separated from a previously formed crystallite by a shell of adsorbed water. As the newly formed crystallite adsorbs water it will tend to force this crystallite and its shell of adsorbed water farther into the water-filled space. As this process continues, the reacting particle becomes surrounded by a threedimensional network of reaction product crystallites and their shells of adsorbed water; that is, the reaction process is building a three-dimensional system composed of a reacting grain and a water-filled space surrounded by and separated from each

36

PERFORMANCE OF CONCRETE

other by a mass of reaction product crystallites, which are surrounded and separated from each other by shells of adsorbed water. The reacting grain now is not in direct contact with liquid water and must draw its water from the shells of adsorbed water on the reaction product crystaIIites. The reacting grain can draw water from these shells so long as its surface forces are not in equilibrium with the shell of adsorbed water. As it draws water from adjacent shells of adsorbed water, water will tend to diffuse from the water-filled space through other shells of adsorbed water to replace the water lost by the adjacent she11s, which is being adsorbed and is reacting with the active surface. If the rate at which the water can diffuse from the water-filled space becomes less than that at which the reacting surface can adsorb and react with water, however, the thickness of the she1ls of adsorbed water close to the reacting surface will decrease. Adsorption by each newly formed reaction product crystallite will have to move an increasing mass of reaction product crystallites and shells of adsorbed water in order to form an adsorbed shell that is in equilibrium with bulk water. Even if the rate at which water can reach the reacting surface is equal to the rate at which it reacts with the reacting surface, the ability of the surface forces of the newly formed reaction product crystaIIites may not be such as to adsorb sufficient water to form she1ls of the same thickness as the previously formed shells because these surface forces may be incapable of moving the mass of the reaction product the required distance. If one pictures this process of forming shells of adsorbed water as one in which the vapour pressure of the water in the shell decreases as the thickness of the shell decreases, it is possible to visualize a condition under which the vapour pressure of the shells in the proximity of the reacting surface becomes so low that the rate of reaction of this surface becomes infinitesimally slow. In other words, from a practical standpoint one might say that the reaction of the grain of cement has stopped because it cannot react with water vapour at that pressure. It is seen from this that there are two types of forces (if one neglects the process of dissolution of the reaction products) acting to cause the reaction between grains of cement and water and to cause dispersion of the reaction products into the original water-filled space of the plastic paste. The first are the chemical forces that cause chemical reactions and the second are the surface forces that cause adsorption of water by solids. It is also seen that the static force exerted by the mass of the reaction products plus their shells of adsorbed water must be overcome for the chemical reactions and dispersion to continue until the reacting grain is consumed. In pastes with relatively low water-cement ratios, this static force counteracts these forces sufficiently to halt, from a practical standpoint, the reaction of the cement with water before all of the cement has reacted and probably while there are still pockets filled with bulk water. Turning now to Figure 1, it seems likely that when the reaction of the cement with water stops or becomes very slow, a small water-filled space remains at the approximate centre of the L units and that a small fragment of unreacted cement remains at the approximate centre of the S units. The space between the water-

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

37

filled space and the fragment of unreacted cement is filled with a three-dimensional network of what is commonly called cement gel. This is composed largely of colloidal particles separated from one another by shells of adsorbed water. These shells are the gel pores of Powers and Brownyard. The thickness of these shells may vary from a maximum in the environment of the water-filled space and may gradually decrease to a minimum in the environment of the fragment of unreacted cement. According to Verbeck ( 34), the mean diameter of the gel pores, assuming that they are cylindrical, probably varies between about 0.000001 and 0.000008 mm and the water-filled pockets or capillaries may vary between about 0.000008 and 0.013 mm. Chemists may have some reservations about applying the name gel to this three-dimensional network of colloidal reaction products and shells of adsorbed water formed in the manner described here. This attitude does not seem to conform to the following statement by Glasstone and Lewis (35): "Under certain conditions it is possible to coagulate a sol in such a manner as to yield a semi-rigid, jelly-like mass which includes the whole of the liquid present in the sol; this product is known as a gel." Prior to and for some time after the second symposium on the chemistry of cement ( 36), it was rather generally accepted that the cement minerals dissolved congruently and then precipitated as gelatinous hydrated products. The product and manner of formation seemed to meet the requirements of a gel and, accordingly, the structure and properties of the hardened paste were considered in terms of a gel. In many gels the solid phase appears to be a continuous phase and the liquid phase to exist in holes in the solid structure. The classic example is that of silica gel, very nearly inelastic in the sense that it does not change volume significantly upon wetting and drying. Cement gel is different in that it very definitely changes volume upon wetting and drying, indicating a very fundamental difference in the mechanisms by which silica gel and cement gel imbibe and lose water. The mechanism of water vapour condensing and filling pores with rigid walls serves very well for silica gel but not for cement gel. The mechanism of combination of water with partially or completely dehydrated crystals of reaction products and of adsorption of water on surfaces of colloidal particles in a predetermined three-dimensional network seems to serve very well for cement gel; that is, the degree to which cement gel can shrink and expand was determined at the time of formation by the thickness of the shells of adsorbed water and the degree of hydration of the reaction products. According to Taylor and Glasstone (37), Graham applied the term gel to all types of coagulated or precipitated colloids. From this standpoint and in keeping with the literature, it seems proper to refer to hardened portland cement paste as a gel and to the space occupied by the shells of adsorbed water as gel pores. In doing this, one is faced with the proper name for the pressure exerted when the dried paste adsorbs water and expands. Thorvaldson ( 3), in closing a discussion of theories of sulphate resistance, made the following statement: "Many observations such as these suggest that volume changes in mortars are controlled by osmotic

38

PERFORMANCE OF CONCRETE

forces concerned with the swelling of gels, that the chemical reactions condition the gel system and destroy cementing substances while the formation of crystalline material is incidental to these chemical reactions, and that the increased resistance to volume change with increased richness of mix may not be primarily due to decreased permeability but rather to the more prolonged maintenance of conditions within the mortar unfavourable to the swelling of gels." Ogston (38) says the term "osmose," quoted as first used in 1854, is a noun derived from a Greek word meaning push, and is defined as a tendency of fluids separated by porous septa to pass through these and mix with each other, the action of passage and admixture, and diffusion through a porous septum or membrane. Ogston goes on to point out that in any system that contains several components there is latitude in the permeability of the membrane. It may be permeable only to water or it may be permeable to salt and water. He also points out that terms such as "osmotic presure," "total osmotic pressure" and "colloidal osmotic pressure" have been applied to different situations. In the case of a mass of cement gel surrounding a water-filled capillary pocket, some of the shells of adsorbed water, gel pores, are in actual contact with the bulk water in the pocket. Accordingly, there is no membrane separating these shells of adsorbed water from the bulk water. Water is taken up by the shell because its entropy differs from that of bulk water. This take-up of water produces pressure against the mass surrounding the gel pore and will continue to increase so long as the take-up of water can move the surrounding mass or until the entropy of the adsorbed water in the centre of the shell is the same as that of the bulk water in the pocket. This take-up of water can be visualized as a dilution of the adsorbed water. Solute in a solution decreases the randomness of the arrangement of the water molecules in a solution; and when water molecules diffuse through a membrane from a less to a more concentrated solution, the resulting dilution of the more concentrated solution is an increase in the randomness of the arrangement of the molecules of water. It seems from this that the process of migration of molecules of water from either a shell of greater thickness or from bulk water into a shell of adsorbed water is a type of osmosis and that it is proper to consider that the pressure created by this migration is osmotic pressure. Under some conditions, the pressure produced by this take-up of water by a partially dried paste may be the result both of adsorption of water and of crystals chemically combining with water. In some climates the volume changes produced in concrete structures by the loss and gain of chemically combined water may contribute to the deterioration of the concrete caused by salts. CRYSTAL GROWTH AS A SOURCE OF EXPANSION One of the early investigations on sulphate problems carried out by the National Bureau of Standards (39) involved experiments in which salt solutions were

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

39

evaporated through the walls of cement-sand-mortar pipes. One conclusion from the study was that portland cement concrete or mortar, if porous enough, could be disintegrated by the mechanical force exerted by the crystallization of almost any salt in its pores. This type of disintegration is seen in such places as the area immediately above the waterline on concrete structures which are partially immersed in salt-bearing water or soils and on the surfaces of dams, retaining walls, and so forth from which water is evaporating as it migrates through the concrete. The scaling and disintegration of the concrete beams stored in the sulphate soils at Sacramento, California ( 17) were to some extent caused by the crystallization of salts in the pores of the concrete. Griffin and Henry ( 40) discuss this type of disintegration with respect to sodium chloride as a component of the mixing water. This type of disintegration has been discussed ( 41) in the light of work by Taber ( 42) and others. Becker and Day ( 43) appear to have been the first to demonstrate conclusively that a crystal can grow and lift an object when loaded with a weight. Taber, in an extensive investigation of the behaviour of crystals under different environments, confirmed the results of Becker and Day. He found that a weighted crystal in a container grew on all faces in a supersaturated solution but that the edges of the bottom face tended to grow faster than the other faces. These crystals tended to form a pyramid with stepped sides and a concave bottom face. A section through such a crystal, in an exaggerated form, is depicted in Figure 2.

FIGURE

2. Diagram depicting section through a crystal grown under a weight.

Taber explained the growth of such a crystal as follows: a crystal resting on the bottom of a dish is always separated from the floor of the dish by a film of solution, and the bottom face of the crystal is under the greater pressure because it is supporting the weight of the crystal plus the applied load. The solution in contact with a crystal whose solubility increases with pressure has a maximum concentration under this face of the crystal. As the whole solution becomes more concentrated, either by evaporation or cooling, the denser portions settle to the bottom of the dish under the force of gravity. This raises the concentration of the solution at the bottom of the dish above that of the solution in equilibrium with the bottom face of the crystal. Solute molecules then tend to diffuse through the film separating the crystal face from the dish. As this occurs and the solution under the edges of the crystal becomes supersaturated with respect to those edges, however, the forces of crystallization draw the solute molecules onto the edges. This causes the edges

40

PERFORMANCE OF CONCRETE

to grow both horizontally and vertically. Very few solute molecules get beyond the edges. The remainder of the bottom surface grows slowly in relation to the edges and this surface gradually becomes concave. As the denser solution settles toward the bottom, the concentration of the solution increases from the top surface. The rates of growth of the uncovered faces increase with depth of immersion. This results in the formation of crystals with larger bases than tops, as depicted in Figure 2. Taber concluded from this work that a crystal will grow in the direction in which external forces oppose growth if the surfaces on which the forces are acting are in contact with a solution that is supersaturated with respect to it, and if the growing crystal is composed of a substance the solubility of which increases with pressure.Then for any increase in the forces opposing growth a corresponding increase in the concentration of the solution is necessary. Taber also studied the behaviour of water and solutions in porous materials and found that solutions in subcapillary pores-pores in which the surface forces of the walls extend across the liquid between the walls-are not readily nucleated. The solution in such pores will be supersaturated with respect to solution in contact with crystals in larger pores. Figure 3 depicts two ways in which large pores might be connected to smaller pores in a concrete structure. Large pore A, which is open at the surface of the structure, is connected to small pore B which leads into the interior of the structure. Large pore D is connected to small pore C, which extends to the surface of the structure, and to small pore E, which leads into the interior of the structure. Referring to the section on porosity and structure, pores A and D may be visualized as remnants of water-filled spaces, capillary pockets, and instead of being connected to single small pores, their walls consist of the three-dimensional network of reaction products with their shells of adsorbed water. However, the simple diagram of Figure 3 simplifies the discussion that is to follow. Suppose that the concrete depicted in Figure 3 is partially immersed in and is saturated with a solution of a salt at a given temperature. If water evaporates from the surface of the structure, crystals will form in the larger pores A and D but will not form in the smaller pores B and E. This crystallization of salt in pores A and D reduces the concentration of salt below that in pores B and E. Solute molecules or ions will diffuse from the solutions in the smaller pores into the larger pores and the crystals in the larger pores will grow. As pores A and D become filled with crystals, the crystals, because of their weight, will exert pressure on one another, will dissolve at the interfaces, and grow at other faces. This will cause them to grow into a single crystal, the bottom face of which is in contact with the solution in the smaller pore. If the larger pore is open, as depicted for A in Figure 3, solute molecules adding onto the bottom of the crystal will cause the crystal to grow above the surface as was observed by Griffin and Henry ( 40). If the larger pore is connected to the surface by a smaller pore C, as depicted for pore D, the growing crystal, being unable to grow into pore C, will exert pressure on the bottom shoulders of pore C. If the pressure is sufficient to overcome the tensile strength of the concrete, the concrete immediately surrounding pore C

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

41

will spall from the surface of the structure. If the concrete does not spall, the pressure exerted against the shoulders of pore C can cause the concrete to expand and crack. What has been said for salt crystals applies equally to ice crystals; that is, ice can grow in a capillary pocket by drawing water from gel pores and can cause either spalling or expansion and cracking. :.

..

. ....... . .. . C ·.. :

....

•••()

V

~: •

.. . o ·. A -~ ·.• 0 • II·." . .. . ... V •• D ... :" • .

.

•

0

V 0

. ,f

..

0 0

~

B

"

V

V

E v

•

' . "·

p

3. Diagram depicting a section through a concrete slab containing large pores A and D and smaller pores B, C, and E.

FIGURE

This type of attack is probably responsible for considerable damage to concrete in service. For example, Tuthill ( 44) points out that dry concrete in dry sulphatebearing soils will not be attacked; whereas, when saturation is continuous in strong sulphate-bearing water, the attack will be rapid and severe. He also points out that it is even more severe where saturation and drying are frequently alternated. Tuthill ( 44, p. 282, Figure 5) shows a picture of badly spalled surfaces on a concrete structure. The caption of the figure reads: "Concrete under ground was intact, but rising sulfate solution caused exposed surface to spaU away as salt crystals developed in concrete pores as moisture evaporated." Griffin and Henry reported on whiskers of sodium chloride growing on walls of rooms in an airconditioned building. It was observed that the whiskers emerged through a layer of paint and caused the surface of the walls to flake off to a depth of 1/32 in. The salt crystals came from sea-water used as mixing water in the concrete. As pointed out earlier, the concrete beams in basin 2 at Sacramento became coated with a glass-like coating of salt that must have interfered with the migration of salt through the beams. Under such a condition, there would be little tendency, if any, for the formation of salts in the pores near the surface as discussed above. Actually, the rates of deterioration of the surfaces of these beams were very much lower than those for the beams stored in basin 1.

42

PERFORMANCE OF CONCRETE

REACTIONS OF CALCIUM ALUMINATES AND ALUMINOFERRITES IN AQUEOUS SOLUTIONS The chemist interested in portland cement and its reaction products is primarily interested in reactions that occur in solutions that are saturated with respect to Ca(OHh because, except in rather unusual circumstances, the liquid phase of either the plastic or the water-saturated hardened cement paste is saturated with respect to Ca(OH)z. In the section on sulphoaluminates, it was pointed out that Lerch, Ashton and Bogue showed that C 3 A formed two compounds with calcium sulphate that were stable in aqueous solution saturated with Ca(OH)z, i.e. ettringite, a high-sulphate form, and a monosulphate or low-sulphate form, C 3 A.CaSO 4 .13H2O. According to Malquori and Cirilli (3), McIntire and Shaw prepared the high-sulphate iron compound, C 3 F.3CaSO 4 .aq. and Malquori and Caruso prepared the low-sulphate iron compound, C3 F.CaSO4 .aq. Malquori and Cirilli demonstrated the existence of a series of solid solutions between the high-sulphate aluminate and the high-sulphate ferrite compounds as well as a series of solid solutions between the low-sulphate aluminate and the low-sulphate ferrite compound. Malquori and Cirilli studied the sulphate resistance of some of their products by determining the rates at which the products combined with sulphate in a solution saturated with respect to both Ca(OHh and CaSO 4 .2H2O. With C 3 A.aq. and C 4 A.aq. the ratio of SO 3 :R2O3 in the solid reached 3 in 15 days, whereas with a solid solution in which the A:F ratio was 3 the SO3 :R2O 3 ratio was 1.3 at 15 days and 2.4 at 60 days compared with 0.5 and 1.3 for a solid solution with an A: F ratio of one and with ratios of 0.4 and 0.9 for C 4 AF.aq. Flint, McMurdie, and Wells (45) prepared the compound C 3AS3 and members of a series of solid solutions of it with C 3 AH 6 , and the compound C 3 FS 3 and members of a series of solid solutions of it with CaFH6 • They also found that C 3 AH 6 and C 3 FH 6 as well as C3AS3 and C3FS3 formed complete series of solid solutions. These compounds and members of the solid solutions are known as garnets and hydrogarnets. It is now known that C3AH 6 is stable in a solution saturated with Ca(OH)z, whereas pure C 3FH6 probably is not. These investigators prepared their products in glass vessels and found that their C 3 AH 6 and C 3 FH6 preparations were contaminated with silica. Hence, their data do not show the existence of pure CaFHo. Flint and Wells (46) studied the sulphate resistance of members of the hydrogarnet solid solutions series by exposing 0.5-gram samples in 100 ml of a solution made by dissolving 100 grams of Na 2SO 4 in one litre of a saturated solution of Ca(OH)z. With the preparation C 3AHo containing a small amount of silica, ettringite began to form within a few hours; with a silica content of 7.7 per cent the product was not perceptibly altered at 4 months after which ettringite appeared;

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

43

when the composition was C 3 ASH4, the product showed no alteration in 6 months. When the compositions were C 3 A 0 . 66F 0 . 33S0 . 5.aq. and C 3 A 0 . 5F 0 . 5S0 _5.aq. some amorphous material surrounded the grains at 6 months. When the compositions were either CaA 0.r.Fo.5S2.aq. or C 3A 0 . 5F 0 . 5S2.5.aq. the products were unaltered at 6 months. Their sample of C 3 FH6 , which contained some silica, also was unaltered at 6 months. These results show that the incorporation of either F or S into the lattice of CaA increases the resistance to attack by sulphate. Turrisiana ( 4 7) and co-workers studied the hydrated aluminates and found that C4A.aq. and C4F.aq. and their solid solutions have almost identical X-ray diffraction patterns and DT A curves. The same is true of C 3 AH 6 and its solid solutions with the theoretical C 3 FH6 . Therefore, these analytical methods are not useful in determining the degree to which F replaces A in these hydrates. Turrisiana also pointed out that C 3 AH13 , which might be considered to be C 3 A.CH.H 12 , does not form a series of solid solutions with C 3 A.CaSO 4H 12 as some investigators believe. He claims that these two compounds form an intercrystalline mixture in which the crystals might be viewed as being made up of alternating layers of one and the other compound. Microscopically these crystals appear to be homogeneous and the indices of refraction increase regularly with increasing CaSO 4 :CaO ratio. However, the X-ray patterns show them to be a mixture of C 4A.aq. and CaA.CaSO4.aq. This is also true of C 4F.aq. and C 3 F.CaSO 4 .aq. Seligmann and Greening ( 48) studied the rates of reactions with water and sulphate of pure preparations of C 3 A, C2F, C 6 AF2, C 4AF, and C 6 A2F. A sample of the paste was placed in one compartment of a two-compartment cell, in which the compartments were separated by a porous membrane, and water was placed in the other compartment. This arrangement permitted the reacting sample to obtain additional water as the reaction proceeded. The courses of the reactions were determined by XRD patterns of the surfaces. The basic composition of the pastes was one mole of the reactant to one mole each of CaSO 4 .2H 2 O and Ca(OHh For example, the mixture C 3 A:CaSO 4 .2H 2O :Ca(OH)2 was made into a paste with a water-cement ratio of 0.40 by weight. One conclusion from the study of this composition was that C 3 A reacted with water and calcium sulphate in three stages, as follows: Stage I , C 3 A combines with water and calcium sulphate to form ettringite and it terminates when the gypsum is depleted. Stage II, C 3 A reacts with water and ettringite to form calcium monosulphate, CaA.CaSO4.H14-15• Stage III, remaining C 3 A reacts with water, monosulphate, and Ca(OHh to form a solid solution of C4A.aq. Ca( OH) 2 Similar pastes in which the ferrites replaced C 3 A showed that the reaction of the aluminoferrites with water and sulphate was relatively slow. In one molar composition of 2/ 3C3 A : 1/3C4AF:CaSO4.2H2O:Ca(OHh, shaken with excess water in a flask, all of the gypsum had not reacted during a period of 17 days. From this and other experiments it was concluded that the C4AF not only reacts much more

+

44

PERFORMANCE OF CONCRETE

slowly than does CaA but that the aluminoferrite phase retards the normal reaction of Stage I for CaA. In all cases the reaction behaviour of the mixtures containing the aluminoferrites corresponded to the three stages found for the C 3 A pastes, but the reactions occurred at slower rates. Schwiete and Iwai ( 49) studied the reactions of C 3A and aluminoferrites with Ca(OH) 2 in saturated solutions of Ca(OHh by means of XRD patterns of the reaction products. The procedure consisted of grinding samples for some period of time in a ball mill with solid Ca(OH) 2 in a saturated solution of Ca(OHh, in the ratio of 5 parts water to 1 part solid, at 50°C. Portions of this slurry were transferred to plastic bottles. One portion was stored at 5°C and another at 25°C for one month. The solids were removed by filtration and dried at room temperature. The ratios of C3 A or aluminoferrite to Ca(OHh in the solid mixtures were such as to yield C 4 A.aq. or C 4 (A,F) .aq. At 5°C, C 3 A formed C 4 AH13 and at 25°C it formed mostly C4 AH 13 with some C3AH6 . At 5°C, C 4 AF and other aluminoferrites formed C4 (A,F)H 13 and C 3 (A,F)HG, whereas, at 25°C, C 3(A,F)HH was formed with only small amounts of C 4 (A,F)H 13 • They found that in a solid solution series in which F replaced A in C 3 AH 6 the lattice parameters increased linearly up to a ratio of F:(A+F) = 0.8. This indicates that a compound of the composition C 3FH6 is not stable at 25°C. From the standpoint of the chemistry of portland cement, these results indicate that at the temperatures at which concrete is usually placed and cured the reaction product of CaA in the absence of sulphate is largely C 4 AH.aq., whereas that of an aluminoferrite is largely C 3( A,F) HG. Schwiete and Iwai then studied the influence of C 3 S on the reactions of the aluminoferrites. The basic mixture was C 4 AF + 2Ca(OH) 2 to which was added C3S over the range of 0 to 5 moles C 3S. The methods of study were similar to those described above. As pointed out earlier, Flint, McMurdie, and Wells showed that S could replace H in either C 3 AH 6 or C 3FH6 to form hydrogarnets. Schwiete and Iwai found that a series of solid solutions was formed in which the lattice constant decreased linearly with increasing S content until the composition C 6 (A,F)S2Hs was reached. Excess C 3 S formed afwillite. They exposed various members of this series to Na2 SO 4 by grinding 300 mg with 30 ml of 0.lN solution of Na 2SO 4 for 24 hours and then allowing the slurry to stand for one month at 25°C. They found that the specimens in which the molar ratio of S:F was less than 1.5 expanded during this period and that expansion increased as this ratio decreased. They analyzed the residue by XRD for ettringite and hydrogarnet and found that the ettringite phase decreased as the S:F ratio increased from 0 to 1.5. The hydrogarnets with S:F ratios of 1.5 and 2.0 were almost free from attack by sulphate under this treatment with Na2SO4 • This certainly could be considered to be a severe treatment. According to Midgley and Rossman ( 4), Flint and Wells claimed to have prepared the compound C 3 A.3Ca(OHhaq. with an XRD pattern indistinguishable from that given by ettringite. This compound was found to form a complete

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

45

series of solid solutions with ettringite. Midgley and Rossman were unable to prepare this compound, however, but they did obtain preparations which DT A and XRD studies indicated to be solid solutions of ettringite with C:iA.3Ca(OHhaq. That is, their results indicated that there is at least a partial series of solid solutions between ettringite and the theoretical hydroxy compound C3A.3Ca(OHhaq. They observed that the ettringite phase in hardened pastes had a d value a little lower than that for the pure ettringite and also that the temperature of the DT A peak was higher than that for the pure compound. This led them to suspect that the ettringite phase in portland cement pastes might be a solid solution of ettringite and this hydroxy phase. Studies of these properties of hardened pastes up to ages of 6 months led them to conclude that the phase first formed is pure ettringite and that this phase contains increasing amounts of the hydroxy compound with age. It is seen from this review, that under non-equilibrium conditions a hardened cement paste might contain all of the following phases: C 3 A.3CaSO 4 .aq.; C 3 A.CaSO 4 .aq.; C3(A,F).3CaSO 4 .aq.; C 3 {A,F) .CaSO 4 .aq.; C4A.aq.; C4(A,F).aq.; C 4 F.aq.; C3 AH 6 ; CaASx,aq.; C 3 {A,F)S,.aq. At present it appears to be impossible to distinguish between some of the non-iron-bearing and iron-bearing phases by means of either DTA or XRD. Also, DTA and XRD will probably not be able to reveal the presence of these phases when they are present as extremely small and possibly almost amorphous particles. For example, Brunauer, Kantro, and Copeland (50) concluded that some of the Ca(OH) 2 in hardened pastes is amorphous. Also Heller and Ben-Yair (51) were unable by DTA and XRD methods to observe increases that were expected in the ettringite contents of hardened cement pastes. They suggested that the ettringite changed to a gel which either exists as a separate phase or forms part of a solid solution series with the hydrated calcium silicate phase. Seligmann and Greening ( 48) observed three separate stages in the reaction of C 3 A and aluminoferrites with water and calcium sulphate. Their work indicated that there was little or no formation of C 4 A.aq. or C 4 (A,F).aq. until all of the high-sulphate phase had been converted to the low-sulphate phases. Midgley and Rosaman, however, observed an ettringite phase in the hardened cement pastes at 6 months. Under the solid-liquid reaction mechanism, such as that described earlier, after most of the original sulphate has combined as the high-sulphate phases, relatively large amounts of the remaining C 3 A and aluminoferrite phases will be converted to hydrated phases that contain little or no sulphate. In other words, much of any formation of low-sulphate phases that occurs will be the result of reactions of these hydrated phases with sulphate that is slowly migrating into their environment from the environments of the high-sulphate phases. In concrete with a relatively high cement factor and low water-cement ratio, which is used for structures to be exposed to sulphate-bearing water, much of the reaction of the cement grains with water must take place with adsorbed water instead of with bulk water. Under such conditions, one would expect the reaction

46

PERFORMANCE OF CONCRETE

products to be extremely fine-grained and to have very poorly developed crystal structure. The only reaction products that might be clearly indentifiable by DT A and XRD methods in these hardened cement pastes are probably those that are formed during the first few days of the reaction period. Since the larger grains of cement are aggregates of most or all of the cement minerals, it can be visualized that, at 6 months for example, the reaction products in the regions most distant from the reacting grain would contain high-sulphate phases; a region somewhat closer to the reacting grain would contain low-sulphate phases, and the region closest to the unreacted core would contain phases relatively free of sulphates. In most experiments to determine the sulphate resistance of cement products in which specimens were used, the specimens were exposed to solutions of sulphates from ages of a few days to a few months. Therefore, in the hardened cement pastes upon which data are available, this picture probably represents fairly accurately the nature of the paste at the time at which the specimens were exposed to the solutions of sulphates. This picture probably equally represents fairly accurately the nature of the cement paste in concrete structures when they are first exposed to sulphate soils and waters. There appears to be no question about the magnesium and sulphate ions being the principal components of sulphate-bearing soils and waters that are responsible for the deterioration of concrete structures in such environments. This discussion will, therefore, be limited to the effects of magnesium and sulphate ions recognizing that future research may find that some of the other ions in these environments may play some part in the deterioration of concrete structures. When a specimen is immersed in such a solution and is saturated with water, water molecules and magnesium and sulphate ions will diffuse toward the interior of the specimen through the gel pores; which, as previously described, are layers of adsorbed water. In the course of this diffusion the ions will enter the solution contained in the capillary pockets. The course of each of these ions through the mass of gel produced by one grain of cement will be followed separately with respect to the behaviour of that ion toward the alumina-bearing phases. EXPANSION PRODUCED BY SULPHATE IONS As outlined above, each cement grain produces a gel phase proceeding from the site of the cement grain toward the site of space originally filled with water. The gel adjacent to the water-filled space contains, in addition to hydrated calcium silicate products, high-sulphate alumina-bearing phases. This region will be referred to as region I. Adjacent to region I is region II, in which the aluminabearing phases are low-sulphate compounds. Adjacent to region II is region III, in which the alumina-bearing phases contain little or no sulphate. Region III may or may not be in contact with an unreacted core of the cement grain.

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

47

The liquid phase in region I contains a concentration of sulphate ion which is in equilibrium with the high-sulphate phases. Therefore, sulphate ions diffusing into this region pass through without having any significant effect upon the reaction products. When the sulphate ions reach region II, they can combine with the lowsulphate products to produce the high-sulphate phases and will cause the cement paste to expand, if they react in accordance with the solid-liquid reaction mechanism outlined in an earlier section. When sulphate ions diffuse into region III, they may or may not be able to react with the alumina-bearing phases in this region. The phases in this region produced from C:A may, in accordance with the work of Schwiete and Iwai, be largely C 4 AH.aq., which readily reacts with sulphate to produce both the low- and high-sulphate aluminoferrites. The products produced from the aluminoferrite phase will be C 3 (A,F)H 6 , which in the pure form does not readily react with sulphate. Since this product is formed in the presence of the reaction products of C 3 S and C 2S, however, it probably contains silica that increases its resistance to reaction with sulphates. The reaction products of C 3 A with water in region III might be capable of rather rapid reaction with sulphate and of causing expansion, whereas those produced by the reaction of the aluminoferrite phase may be almost inert toward sulphate ion and will therefore not be the source of an expansive reaction in this region. This difference between the behaviour of C 3 A and aluminoferrite when they react with water, at temperatures at which the laboratory and field specimens have been exposed to sulphate-bearing water, seems to explain readily why the resistance of a given cement to attack by sulphate can be increased by increasing the ratio of F: A in the kiln feed. Kalousek and Adams (52) have presented evidence indicating that some of the sulphate in cement goes into the hydrated calcium silicate phase; specifically, that A and F as well as sulphate go into this phase. There does not appear to be any evidence by which one can speculate as to what effect this might have upon the sulphate resistance of cement products. In this connection it is unfortunate that in all the studies of sulphate resistance of the pure minerals no studies were made with mixtures of the minerals with calcium sulphate. A number of investigators (2) have presented data which show that curing concrete or mortar specimens at temperatures at or above 100°C under pressure greatly increases their resistance to attack by sulphates regardless of the C 3 A content of the cement. These are the conditions that would convert C 3 A and aluminoferrites to hydrogarnets, which as shown by Flint and Wells and by Schwiete and I wai, are highly resistant to attack by sulphates. REACTIONS OF MAGNESIUM ION WITH HARDENED CEMENT PASTE As pointed out earlier, Thorvaldson, Wolochow, and Vigfusson (10) found that mortars made from C 3 S and C 2S, as the cements, expanded in solutions of MgSO 4 • This indicated that the chemical composition of the solid particles of the hardened

48

PERFORMANCE OF CONCRETE

cement paste probably plays little part, if any, in the expansion of cement products by magnesium ion. The liquid phase in a gel pore is saturated with respect to Ca(OHh When magnesium ion diffuses into this solution it will, because of the very low solubility of Mg(OH)z, cause the precipitation of Mg(OHh in accordance with the following equation: Ca(OHh

+ MgSO4 = Mg(OH)z + CaSO

4•

Also because of its low solubility, Mg(OHh precipitates as extremely fine or colloidal particles. As already pointed out, the water in the gel pores, shells of adsorbed water, is in equilibrium with the surface forces of the colloidal reaction products. Precipitation of colloidal Mg(OHh in the gel pore introduces new surface forces that upset this equilibrium. In order to re-establish equilibrium between the surface forces and water, additional water will be drawn into the gel pore. This can be accomplished only by enlargement of the pore by "osmotic pressure." Thorvaldson, Wolochow, and Vigfusson found that specimens made with mixtures of CnS and C2S containing various amounts of calcium aluminates behaved differently in solutions of Na2SO4 than they did in solutions of MgSO 4 • Bars exposed to Na 2 SO4 shed their surfaces continuously with the hard cores expanding as much as 2 per cent before they fell to pieces. In MgSO 4 , the bars did not crumble but retained their shape and remained fairly firm until they reached a very high expansion. This indicates that sulphate and magnesium ions produce expansions by different mechanisms. However, in the case of MgSO 4 both mechanisms are operating, whereas in the case of Na2SO4 only the sulphate ion is producing expansion. It must be recognized also that bars made with the pure minerals did not contain sulphate as portland cement does. Heller and Ben-Yair ( 51) made an extensive study by DT A and XRD methods of the products in bars exposed to solutions of Na 2SO4 and MgSO 4 • They found large quantities of ettringite and only small quantities of the low-sulphate sulphoaluminate. They could not identify either Mg(OHh or magnesium silicates in samples of the specimens but they did identify these materials in samples of salts scraped from the surfaces of the bars. Their results indicated that the amount of ettringite did not increase significantly with increased time of exposure to the sulphate solutions. They suggested that the ettringite may have changed into a gel that formed a separate phase or formed a solid solution series with the hydrated calcium silicate phase, resembling "phase X" described by Kalousek. Their results suggest that some of the products in the hardened cement paste may be too small and too poorly crystallized to react in the manner of larger and better crystallized particles when studied by DT A and XRD methods. With respect to "phase X," Kalousek ( 53) analyzed hardened cement pastes using a DT A method for high- and low-sulphate sulphoaluminates and gypsum. He reported results for cements containing SO3 in the range of 0.57 and 4.08 per cent which

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

49

showed that from 0.57 to 1.96 per cent of SO3 by weight of the cement was unaccounted for by the DT A results. He suggested that the SO3 had become a part of "phase X."

POZZOLANS Before it was discovered that cements with high resistance to attack by sulphates could be produced by controlling the composition of the cement clinker, considerable effort was made to find a pozzolan that could be used with any portland cement to increase the resistance to attack by sulphate ( 54). The results of these studies usually showed that some pozzolans increased significantly the sulphate resistance in laboratory tests of cements that showed relatively poor resistance without them, but had little, if any, effect with cements that showed relatively high resistance. In some cases, pozzolans had an adverse effect. The theory upon which these investigators were working was that uncombined Ca(OH)z produced during the reaction of the cement with water would combine with silica and alumina of pozzolans to produce colloidal hydrated calcium silicates and aluminates similar to the reaction products of cement with water. It was believed that : (a) uncombined Ca(OH)z had no cementing properties and could, by reaction with a pozzolan, be converted to products with cementing properties; (b) uncombined Ca(OH)z was more readily dissolved and, therefore, more readily leached from the concrete than was the combined Ca(OH)z from the reaction products-a highly questionable assumption; and ( c) the colloidal products produced by the reaction of Ca(OH)z with pozzolans would decrease the porosity of the hardened cement paste. In a relatively recent study, Polivka and Brown (55) found, in tests in which 25 per cent of the cementing ingredient was a pozzolan, that no improvement in the sulphate resistance of the cement in which the calculated CaA and C 4AF contents were 2.8 and 9.5 per cent respectively was produced by any of several pozzolans. With a cement in which the calculated CaA and C4AF contents were 11.7 and 8.2 per cent, respectively, the sulphate resistance of the cement was improved by the pozzolans but did not approach that of the cement with the low C3A content. Since crystals of Ca(OH) 2 can be readily identified microscopically in hardened cement pastes, it has generally been assumed that all of the free Ca(OH)z is present as relatively large crystals. Such crystals, having low specific surfaces compared with those of the colloidal reaction products, would have inferior cementing properties. Brunauer, Kantro, and Copeland ( 50), however, from X-ray diffraction analysis of products of the reaction of C2S and C 3S with water, concluded that significant amounts of the Ca(OH)z released when these compounds react when water is present in an amorphous form . In the mechanism previously described for

50

PERFORMANCE OF CONCRETE

the reaction of cement with water, it seems logical to expect that much of the Ca(OH)z released during this reaction would remain in the hardened paste as colloidal and possibly amorphous crystallites. These might have relatively good cementing properties. The well-formed and relatively large crystals of Ca(OH)z found in the cement paste were certainly formed by the dissolution and recrystallization of such crystallites. Most, if not all, of the reaction products in a hardened cement paste decompose in water and liberate Ca(OH) 2 to the solution. The rate of liberation of Ca(OH)z by these products probably is about as rapid as the rate of dissolution of Ca(OH)z crystals. Hence, from the standpoint of the rate of leaching of Ca(OH)z from concrete products, the absence of crystalline Ca(OH)2 in the hardened paste probably would not significantly affect this rate. It is possible to make calculations ( 4) that qualitatively show whether one could expect the use of pozzolan with portland cement to decrease the porosity. For example, if the pozzolan is silica, the following calculations A and B can be made, assuming that C 3S forms C 3S2H3 and that S CH form the same compound.

+

Calculation A 2(C3S) + 6H = C3S2H3 + 3CH Wt 456 108 342 222 108 140.2 99.5 Vol 144.7 Void-space (144.7 + 108) - (140.2 + 99 . 5) = 13 .0 Calculation B 2(C 3S) + 2S + 6H = 2(C3S2H3) Wt 456 120 108 684 108 280.4 Vol 144 . 7 52 V-S (144. 7 + 52 + 108) - 280.4 = 24.3

On the assumption that a calcined clay or a fly ash has the composition AS2 and that C 3S reacts with it to form C 3S2H3 and either C4AH 19 or C 4 AH 13 , the following calculations C and D can be made. Calculation C 14(C 3S) + 3(AS2) + 87H = IO(C3S2H3) + 3(C4AH, 9 ) Wt 3192 666 1566 3420 2004 Vol 1013.3 240.5 1566 1401.6 1120 V-S (1013.3 + 240.5 + 1566) - (1401.6 + 1120) = 198.2 Calculation D 14(C 3S) + 3(AS2) + 69H = l0(C3S2H3) + 3(C4AH 13 ) Wt 3192 666 1242 3420 1680 Vol 1013. 3 240. 7 1242 1401. 6 807. 7 V-S (1013.3 + 240.5 + 1242) - (1401.6 + 807 . 7) = 286.5

On the basis of the paste containing 40 per cent water by weight of solids, C3 S

+ pozzolan, the data of Table VIII are obtained if the above reactions go to

completion. For the reactions of the equations in calculations A and B above, the pozzolan

CHEMISTRY OF SULPHATE- RESISTING PORTLAND CEMENTS

51

is approximately 25 per cent of the C 3S by weight, and for the equations of calculations C and D, approximately 21 per cent. These are comparable to the quantities used commercially. Malquori ( 4) points out that portland-pozzolan cements generally require more water than portland cements. Hence, data given in the last column of Table VIII based on 40 per cent water for all cements may indicate less difference in porosities than actually would be obtained in service. TABLE VIII DATA PERTAINING TO C3S-POZZOLAN PASTES

Reaction number

Weight solids

Volume water*

1 2 3 4

456 576 3858 3858

182.4 230.4 1543 .2 1543 .2

Volume Volume Volume combined free Volume voids reactants H20 H20 252 .7 304 .7 2819.8 2498 .5

108 108 1566 1242

74.5 122 .4 -23 .2 301 .2

13.0 24 .3 298.2 289 .2

Volume Volume** total porosity voids (per cent) 87 .4 146 .7 590.4

34 .5 48 .1 *** 23.7

*40 per cent by weight of solids. **Volume of voids plus volume of free water as per cent of volume of reactants (solid plus water). ***40 per cent of water is insufficient for complete reaction.

It appears from the data of Table VIII that for the same water-cement ratio, the porosity of the portland-pozzolan cement concrete made with a highly siliceous pozzolan would be somewhat greater than that of a concrete made with the portland cement. On the other hand, these calculations indicate that the use of pozzolans containing both alumina and silica might reduce the porosity of the concrete. However, the data from these calculations, although qualitative, suggest that significant changes in the permeability and porosity of concrete may not result from the use of portland-pozzolan cements. Malquori points out that it has been repeatedly stated that pozzolanic cements do not possess any intrinsic, specific, chemical resistance to attack by sulphates because the alumina of the pozzolanic materials is found in the hardened cement paste in the form of potentially vulnerable calcium aluminates. It is true that ettringite forms only in solutions saturated with respect to Ca(OHh and that pozzolans are supposed to combine with the free Ca(OHh of the cement paste. It seems likely, however, that the Ca(OHh concentration of the liquid phase of portland-pozzolan cement concrete will be that of a saturated solution and sufficient for the formation of ettringite. Eitel (56) discusses the formation of ettringite in lime-pozzolanic mortars containing gypsum. It appears from this brief review that the sulphate resistance of a cement that will meet ASTM specifications for a sulphate-resistant cement will not be improved by the use of a pozzolan with it and that it might be harmed because of either increased porosity or the presence of aluminate phases that can react with sulphate.

52

PERFORMANCE OF CONCRETE

NEW EXPERIMENTAL TECHNIQUES

It was pointed out earlier that XRD and TDA methods do not distinguish between the hydrated calcium aluminates and calcium aluminoferrites expected to be present in hardened cement paste. Recently Wittman and co-workers (57, 58) used a method based on the Mossbauer Effect in the study of iron-bearing phases in cements and hardened pastes. In 1956 and 1957, R. L. Mossbauer (59, 60), while studying the scattering of gamma rays, compared the scattering of the 129 keV gamma ray of lr 191 by iridium and platinum. He observed an increase in scattering in Ir at low temperatures, a result counter to classical predictions. This was found to be true for certain other elements and was named the Mossbauer Effect. This effect arises from the energy associated with the recoil imparted to a nucleus of an atom by the emission of a low energy gamma ray and the characteristic energy of the lattice vibrations. These energies are dependent upon the chemical binding and the point symmetry of the lattice surrounding the atom. By the use of suitable equipment, it is possible to obtain Mossbauer spectra for certain elements. Since these spectra are associated with a particular atom, for example, the iron atom in calcium aluminoferrite, it appears that it should be possible to obtain the spectra for the pure iron-bearing compounds and use them for identifying those compounds in the hardened cement paste. The work by Wittman and co-workers appears to be a start in the development of such a method. Chatterji and Jeffery ( 61 ) describe a method of determining the three-dimensional arrangement of the products in a hardened cement paste and present three micrographs of fractured surfaces of such pastes at ages of 24 hours and 3 and 14 days. The instrument used is a scanning electron microscope. In this microscope a very narrow electron beam scans the surface of the specimen under investigation. The electrons that are scattered by the specimen are collected and the amplified current controls the brightness of the spot on the corresponding point of the raster of a television tube to form the image of the specimen. For non~conducting materials, such as cement and ceramics, an evaporated layer of gold-palladium or gold is generally deposited on the specimen to form the conducting layer. According to the authors, the micrograph of the fractured surfaces of the 24-hour specimen consisted of hexagonal crystals, which are either calcium hydroxide or calcium monosulphate, and needle crystals which may be either ettringite or calcium silicate hydrate. It was impossible in this specimen to distinguish differences between the needles. At 3 and 14 days some of the needles were fairly thick and long and others were thin. The authors believe that the former are crystals of ettringite and that the latter are crystals of calcium silicate hydrate. They state that the micrographs show that the needles of both types form an entangled mass in three dimensions and that further work should make it possible to formulate a model of the three-dimensional arrangement of hydration products

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

53

in cement pastes. They concluded that the well-developed morphology of the larger needles definitely indicates that they are formed by a through-solution mechanism, which, if the crystals are ettringite, is strong evidence against a solidstate formation of ettringite in cement pastes. The authors point out that this is important because of its significance in sulphate expansion of cement pastes. The supporters of the solid-liquid mechanism probably would readily agree that the larger crystals were formed by the crystallization from the liquid phase. However, they probably would not accept this as evidence that the initial reaction of C 3 A with water and sulphate was by a through-solution process. This is another case, as was pointed out earlier, in which the authors neglect to consider the reactions that precede the dissolution process. It seems likely that some of the fine needle crystals are ettringite crystals formed by the solid-liquid reaction and not dissolved and recrystallized into larger crystals. REFERENCES 1. R. H. BOGUE. A Digest of the Literature on the Constitution of Portland Cement Clinker.

Concrete (July 1926-Feb. 1927) . 2. W. C. HANSEN. Attack on Portland Cement Concrete by Alkali Soils and Waters : A Critical Review. Highway Res. Bd. Highway Res. Record no. 113, 1 (Washington, 1966) . 3. Proc. 3rd Intern. Symp. on the Chem. of Cement. Cement and Coner. Assoc. (London, 1952). 4. Proc. 4th Intern. Symp. on the Chem. of Cement. U .S. Dept. of Commerce, Natl. Bur. Stds. Monograph 43, vols. 1 & 2 (Washington, 1962) . 5. F . M. LEA and C. H. DESCH. The Chemistry of Cement and Concrete (Edward & Co., London) . 6. W. J. HANSEN, L. T . BROWNMILLER, and R. H. BoGUE. Studies on the System Calcium Oxide-Alumina-Ferric Oxide. J. Am. Chem. Soc. 40,396 ( 1928). 7. A. J. MAJUMDAR. The Ferric Phase in Cements. Trans. Brit. Ceramic Soc. 64, no. 2, 105 (1965). 8. STEPHEN BRUNAUER et al. Quantitative Determination of the Four Phases in Portland Cement by X-ray Analysis. Proc. Am. Soc. Testing Mats., 59, 1091 (1959) . 9. T. THORVALDSON, V. A. VIGFUSSON, and K. R. LARMOUR. The Action of Sulphates on the Components of Portland Cement. Trans. Royal Soc. Canada, 3rd series, 21, Sec. III, 295 (1927). 10. T. THORVALDSoN, D. WoLocHow, and V. A. VIGFUssoN. The Action of Sulphate Solutions on Mortars Prepared from Some Binary and Ternary Compounds of Lime, Alumina, Silica and Iron. Can. J. Res. 6, 485 ( 1932). 11. R. H . BOGUE. Studies of the Volume Stability of Portland Cement Pastes. Portland Cement Assoc. Fellowship, Paper no. 55, 1 (1949). 12. DALTON G. MILLER and PHILLIP W. MANSON. Laboratory and Field Tests of Concrete Exposed to the Action of Sulphate Waters. U.S. Dept. Agric. Tech. Bull. no. 358, 1 (1939). 13. DALTON G. MILLER and PHILLIP W. MANSON. Long-Time Tests of Concretes and Mortars Exposed to Sulphate Waters. Univ. Minnesota, Agric. Exp. Sta. Tech. Bull. no. 184, 1 (1951). 14. BEN MoREELL. The New Federal Specifications for Portland Cement. Proc. Am. Concrete Inst. 8, 435 (1937) . 15. C. H . SCHOLER. How Many Specifications for Cement. Am. Soc. Testing Mats. Bull. no. 40, 39 (1940) .

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PERFORMANCE OF CONCRETE

16. Am. Soc. Testing Mats. Standards, Cement, Lime and Gypsum, part IX (June 1966). 17. F. R. McMILLAN, T. E. STANTON, I. L. TYLER, and W. C. HANSEN. Long-Time Study of Cement Performance in Concrete. Portland Cement Assoc. Bull. no. 30, 1 (1949). 18. WILLIAM LERCH. A Performance Test for Potential Sulfate Resistance of Portland Cement. Am. Soc. Testing Mats. Bull. no. 212, 37 (1956) . 19. W. LERCH, F. W. ASHTON, and R.H. BOGUE. The Sulfoaluminates of Calcium. J. Res. Natl. Bur. Stds. 2, 715 (1929). 20. L. BL0NDIAU. Considerations Regarding the Le Chatelier-Anstett Test of Resistance to Chemical Attack by Calcium Sulfate. Rev. des. Mater. Const. no. 56, 189 (1961) . 21. W. C. HANSEN. Solid-Liquid Reactions in Portland Cement Pastes. Mats. Res. & Stds. 2,490 (1962). 22. A. E. VAN ARKEL. Molecules and Crystals (lnterscience Pub. Inc., New York, 1956). 23. S. CHATTERJI and J. W. JEFFERY. A New Hypothesis of Sulfate Expansion. Magazine of Concrete Res. 15, 83 (1963) . .. 24. H . W. KoHLSCHUTTER and LENELORE SPRENGER. Uber die Entwicklung der Topo-Chemie. Angewandte Chemie, 52, 197 (1939). 25. M . L. STRELKOV. Changes in the State of the Liquid Phase during Hardening of Cements and the Mechanism of Their Hardening. Reps. of Symp. on the Chem. of Cement, ed. P. P. Budnikov et al., p. 183 (1956). Slavic Language Microfilm no. R-02318 and Chem. Absts., 52, 6748 (1958). 26. D. R. GLASSON. Reactivity of Lime and Related Oxides-part III, Sorption of Liquid Water on Calcium Hydroxide (Wet Hydration) . J. Applied Chem. no. 10, 38 (London, 1960). 27. W. C. HANSEN. Discussion of paper by T. B. Kennedy, Significance of Tests for Calcium Sulfate in Hydrated Portland Cement Mortar. Proc. Am. Soc. Testing Mats. 61, 1038 (1961). 28. WILLIAM LERCH. The Influence of Gypsum on the Hydration and Properties of Portland Cement Pastes. Proc. Am. Soc. Testing Mats. 46, 1251 ( 1946) . 30. EDMUND BURKE and RUEBEN M. PINCKNEY. Destruction of Hydraulic Cements by the Action of Alkali Salts. Bull. Montana Agric. Coll. Exp. Sta. no. 81 (1910). 31. W. C. HANSEN. Porosity of Hardened Portland Cement Paste. Proc. Am. Concrete Inst. 60, 141 (1963) . 32. H. ZUR STRASSEN. Discussion of reference 31, Proc. Am. Concrete Inst. 60, 1301 (1963) . 33. T. C. PowERS and T. L. BROWNYARD. Studies of the Physical Properties of Hardened Portland Cement Paste. Proc. Am. Concrete Inst. 43, 469 ( 1946) . 34. GEORGE VERBECK. Pore Structure. Concrete and Concrete-making Materials. Am. Soc. Testing Mats. Spec. Tech. Pub. 169A, 211 (1966) . 35. SAMUEL GLASSTONE and DAVID LEWIS. Elements of Physical Chemistry (D. van Nostrand Co. Inc., New York, 1960) p. 585. 36. Symp. on the Chem. of Cement. The Royal Swedish Inst. Eng. Res. (Stockholm, 1938). 37. HUGH S. TAYLOR and SAMUEL GLASSTONE. Treatise on Physical Chemistry (D. van Nostrand Co. Inc., New York, 1951) p. 671. 38. A.G. OGSTON. When is Pressure Osmotic. Federation Proc. 25, 1112 (1966) . 39. P. H. BATES, A. J. PHILLIPS, and RUDOLPH WIG. Action of Salts in Alkali and Sea Water on Cement. Natl. Bur. Stds. Tech. Paper no. 12 (1913). 40. D. W. GRIFFIN and R. L. HENRY. Integral Sodium Chloride Effect on Strength, Water Vapor Transmission and Effloresence on Concrete. Proc. Am. Concrete Inst. 48, 751 (1961) . 41. W. C. HANSEN. Crystal Growth as a Source of Expansion in Portland-Cement Concrete. Proc. Am. Soc. Testing Mats. 63, 932 ( 1963). 42. STEPHEN TABER. The Growth of Crystals Under External Pressure. Am. J. Sci. 41 , 532 (1916). 43 . G. F. BECKER and W. L. DAY. The Linear Force of Growing Crystals. Proc. Washington Academy of Sciences, 7,282 (1905). 44. L. H . TUTHILL. Resistance to Chemical Attack. Concrete and Concrete-making Materials. Am. Soc. Testing Mats. Spec. Tech. Pub!. no. 275 (1966) .

CHEMISTRY OF SULPHATE-RESISTING PORTLAND CEMENTS

55

45. E. P. FLINT, HOWARD MCMURDIE, and LANSING S. WELLS. Hydrothermal and X-Ray Studies of Garnet-Hydrogarnet Series and the Relationship of the Series to Hydration Products of Portland Cement. J. Res. Natl. Bur. Stds. 26, 13 ( 1941). 46. E. P. FLINT and L. S. WELLS. Relationship of Garnet-Hydrogarnet Series to Sulfate Resistance of Portland Cements. J. Res. Natl. Bur. Stds. 27, 171 (1941 ). 47. RENATO TuRRISJANJ. The Process of Hydration of Portland Cement. L'lndustria Italiana de! Cemento 29, 189, 219, 244, 276, 282 (1959). Slavic Language Translation no. 6014132. 48. PAUL SELIGMANN and NATIIAN R. GREENING. Studies of Early Hydration Reactions of Portland Cement by X-Ray Diffraction. Highway Res. Bd. Highway Res. Record no. 62, 80 (Washington, 1964). 49. H. E. SCHWIETE and T. lwAJ. The Behavior of the Ferrite Phase in Cement during Hydration. Zement-Kalk-Gips. no. 9,379 (1964). 50. STEPHEN BRUNAUER, D. L. KANTRO, and L. E. COPELAND. The Stoichiometry of the Hydration of beta-Dicalcium Silicate and Tricalcium Silicate at Room Temperature. Am. Chem. Soc. 80, 1 (1958). 51. L. HELLER and M. BEN-YAIR. Effect of Sulfate Solutions on Normal and Sulfate-resisting Cements. J. Appl. Chem. 14, 20 (1964). 52. G. L. KALousEK and H. ADAMS. Hydration Products Formed in Portland Cement Pastes at 25 and 175 C. Proc. Am. Concrete Inst. 48, 77 ( 19 51 ) . 53. G. L. KALOUSEK. Analyzing SO 3-bearing Phases in Hydrating Cements. Mat. Res. & Stds. 5, 292 (1965). 54. R. E. DAVIS. Pozzolanic Materials and Their Use in Concrete. Symp. on Use of PozzoJanie Materials in Mortars and Concrete. Am. Soc. Testing Mats. Spec. Tech. Pub. 99, 3 (1949). 55. MrLos POLIVKA and EDWARD BROWN. Influence of Various Factors on Sulfate Resistance of Concrete Containing Pozzolans. Proc. Am. Soc. Testing Mats. 58, 1077 (1959). 56. WILHELM EITEL. Recent Investigations of the System Lime-Alumina-Calcium SulfateWater and Its Influence in Building Research Problems. Proc. Am. Concrete Inst. 53, 679 (1957). 57. VON FOLKER WITTMAN, FRANK PODELL, and WERNER WIEDMANN. Examination of the Hydration of Iron-containing Clinker Components by the Help of the Mossbauer Effect. Zeitschrift fiir angewandte Physik 19, 281 (1965) . 58. F. PODELL and F. WITTMANN. Mossbauer Effect of 14 keV-line of Fe 57 in the Superparamagnetic Ferrites of Portland Cement. Zeitschrift fiir angewandte Physik 20, 488 (1966). 59. GUNTHER K. WERlHEIM. Mossbauer Effect, Principles and Applications (Academic Press, New York, 1964 ). 60. RussELL L. COLLINS. Mossbauer Effect Spectroscopy. Industrial Res. p. 112 (Oct. 1966). 61. S. CHATTERJI and J. W. JEFFERY. Three-Dimensional Arrangement of Hydration Products in Set Cement Paste. Nature 209, 1233 (1966).

31

Some Studies on the Performance of Concrete Structures in Sulphate-bearing Environments

F. M. LEA* THE BEHAVIOUR of concrete in sulphate-bearing soils attracted attention as far back as the latter years of the nineteenth century. The records of laboratory and field studies since then are numerous and amongst them are the many outstanding contributions of the late Professor Thorvaldson. I first met Professor Thorvaldson at Saskatoon in 1929 and was privileged to keep in touch with him throughout all the years of his life in Canada and elsewhere. I am happy to have the opportunity of presenting this paper as a tribute to his memory. The main purpose of the paper is to discuss some cases of concrete exposed to sulphate action, but as a background a brief reference may be made to the occurrence of sulphate salts in clay soils and ground waters in Great Britain. The sulphates of calcium, magnesium, and alkali metals occur in Great Britain in many, though not all, parts of the Keuper Marls, and Lias, Oxford, Kimmeridge, Weald, Gault, and London Clays ( 1). The content of sulphur trioxide ( acid soluble) is generally below 1 per cent, but may in places rise to 2 to 3 per cent, or, as in the Keuper Marl where gypsum is the dominant sulphate, to much higher values. The distribution is irregular; the top few feet of the clay are often relatively free from sulphates owing to leaching by water, whereas in the North American alkali soils evaporation leads to high concentrations at the surface. The concentration of sulphates in the ground waters is usually more uniform over a restricted area than that of the clay itself, but it is liable to wide seasonal variations with rainfall. The level of the water table may rise close to the surface at the end of the winter and fall many feet below it in summer. The character of the environment surrounding any buried concrete can therefore usually only be defined very roughly. Even under the worst conditions, concentration above 500 parts S03 per 100,000 in the ground water are rare in Great Britain. The London clay probably carries a greater volume of buried concrete than any other strata. The content of acid-soluble sulphur trioxide varies from a few

*Sir Frederick Lea, Director of the British Building Research Station from 1945 until his retirement in 1965. An international authority on cement and concrete, Sir Frederick is well known for his book The Chemistry of Cement and Concrete by Lea and Desch, a second edition of which was published in 1956.

CONCRETE IN SULPHATE-BEARING ENVIRONMENTS

57

hundredths per cent to several per cent. Gypsum is always present and is the predominant sulphate in the clays of lower sulphate content with magnesium and alkali sulphates becoming increasingly important as the sulphate content of the clay rises. A classification of sulphate soil conditions into three classes with ground water sulphur trioxide contents < 30, 30 to 100, and > 100 parts per 100,000 and recommendations as to the types of concrete to be used in each case were issued in 1951 (2). Few cases of serious deterioration of concrete placed in recent years have been reported in the London area and this may well reflect a better choice of the type of cements to be used and a general improvement in quality of the concrete. There is now a tendency to relax somewhat the 1951 recommendations in cases where the predominant sulphate is calcium sulphate alone and to relate the above three classes more to the content of magnesium and alkali sulphates present. THE RELATIVE RESISTANCE OF DIFFERENT TYPES OF CEMENT TO SULPHATE ATTACK It is well known that there is a general relationship between the tricalcium aluminate content of portland cements and the resistance to sulphate attack. The serious discrepancies in this relationship, however, are evident from the earlier trials of D. G. Miller ( 3), those still proceeding in the Portland Cement Association Long-Time Study of Cement Performance in Concrete ( 4), and various trials in other countries. Although the limitation of the C 3A content to 5 per cent in specifications for sulphate-resisting portland cement ensures a certain minimum performance, it does not ensure equality of performance and it eliminates some cements of higher C 3 A content showing good performance. These discrepancies may, in part, arise from errors in the calculation of the C3 A content; but they still persist, and on occasions are increased, when this content is determined by X-ray diffraction. Important as is the C 3 A content as a classification test, other factors relating to cement mineralogy or to the structure of the set paste must also be taken into account. The reduction in sulphate resistance of a concrete under bending stress, compared with the unstressed concrete found by Stolnikov ( 5), and the attribution of this to an increase in the permeability of concrete under stress by Maltsov, Sokolov, and Staritsky (6), is worthy of note in this connection. Although high alumina cement has been used for many years in Great Britain for concrete exposed to severe sulphate conditions, experience with sulphateresisting portland cement and supersulphated cement in structures buried in sulphate-bearing soils is limited to more recent years. It is hardly surprising, therefore, that no real comparison can as yet be made of their relative performance in actual structures. Much of the literature on sulphate attack relates to concrete buried in neutral sulphate-bearing soils. It is the aim of this paper to widen the discussion by dealing

58

PERFORMANCE OF CONCRETE

with some rather different examples. The common feature distinguishing these examples from the more normal case of concrete buried in the soil is that the conditions that led to attack were created in the course of the construction work itself or by subsequent building nearby. ACID SULPHATE WATERS A combination of sulphuric acid with sulphate salts in the soil is rarely found in Great Britain except on sites that have been subject to industrial contamination. Natural conditions of this type can arise in marsh soils containing marcasite or pyrite; these, under suitable conditions, can oxidize to produce free sulphuric acid which is not fully neutralized by the bases present in the soil. Such conditions exist, for instance, in the moors around Osnabruck in Germany (7) . Similar conditions have been found in the middle Cambrian-lower Ordovician shales, known locally as "alum shales," in the Oslo region of Norway (8). Deterioration of concrete in these shales has long been a problem. They contain an unstable iron sulphide, pyrrhotite, of composition FeSi. 14 , which rapidly oxidizes to produce ferrous and ferric sulphate and sulphuric acid when the shale is crushed and exposed to weathering. Exceptionally, pH values in the ground water as low as 2.5 were recorded but a value of 5 to 6 was more normal. An unusual case of acidic conditions arising from the civil engineering operations themselves was found recently in construction of a sewer in a tunnel in the outer London area at Bexley, Kent. The tunnel lining consisted of bolted precast concrete segments made with sulphate-resisting portland cement. The site comprised alluvial clays, sand, and gravels of the Woolwich beds overlaying Thanet sand. Tests on the ground water showed a minimum of 60 parts and a maximum of 163 parts SO3 per 100,000. The tunnel was driven by compressed air. When the air pressure was released and the ground water returned to the periphery of the lining, the usual small leaks were observed at bolts and other places. Within a week along two sections of the tunnel mild steel bolts were seen to be badly corroded and the water lying in the invert of the tunnel was acidic. Ground water entering the tunnel showed in the sample of highest acidity a pH value of 1.8 and a content of 2.69 per cent SOs, 1.33 per cent Fe, equivalent to 3.61 per cent FeSO4 and 0.95 per cent H2SO4. A sample of sand adjacent to the tunnel showed the presence of pyrite, FeS 2, a mineral which is sporadically present in the Woolwich beds. Oxidation of pyrite according to the equation, 2 FeS2

+ 702 + H2O = 2 FeSO4 + 2H2SO4

would account for the acidity. The reaction was set in train by the use of compressed air which provided the necessary oxygen; with the removal of the source of oxygen the reaction would be expected to slow down. Evidence of this was

CONCRETE IN SULPHATE-BEARING ENVIRONMENTS

59

indicated by the rise in the pH from 1.8 to 3.5 over a period of some weeks. Over a period of three years the pH at another sampling point rose from 3 to 4-4.5. A secondary lining of supersulphate cement concrete had been constructed and after three years this was unaffected. The condition of the outer part of the lining could not be determined. CONCRETE GROUND FLOOR SLABS Various cases have occurred in the United Kingdom of sulphate attack on concrete ground floor slabs of houses, caused not by sulphates in the ground but by the filling placed underneath the slabs. The concrete slab is generally laid on a bed of hardcore whose depth depends on the filling necessary. If the hardcore contains sulphates there is a risk on wet sites that these may be drawn upwards into the concrete subfloor. Though the use for such purposes of hardcore containing damaging concentrations of sulphates is now prohibited in the 1965 building regulations, many instances of their use have occurred previously. In the mining areas hardcore had often been obtained from colliery waste tips which, after burning spontaneously, consisted of shale in a condition varying from underburnt to well burnt. Shortly after the use of some of these tips, concrete slabs cast on top of the colliery shale were found to be lifting as much as 6 in., generally in the centre, and cracking. In some cases this was accompanied by an outward thrust displacing walls. In the first major example of this trouble, some 46 out of 50 houses were affected within two years of construction. On another estate 160 out of 1200 houses were affected within four years and there were indications that this number would rise with time. Some of these examples were investigated to determine the extent of sulphate attack and its distribution through the concrete slab which was usually 4 in. thick. Case A The hardcore was brick rubble topped with burnt colliery shale. Analysis of samples from the concrete slabs of two houses given in Table I shows that the invading sulphate had hardly reached the top of the slab. Case B The hardcore consisted of 9 in. of ashes covered with 3 in. of burnt colliery shale containing 1.52 per cent SO3 • Samples of unattacked concrete and of an attacked 5 mm undersurface layer are given in Table II. CaseC The progressive movement of sulphate from burnt colliery shale hardcore into 3-in. concrete floor slabs on another estate is illustrated by the data in Table III.

60

PERFORMANCE OF CONCRETE

TABLE I COMPOSITION OF CONCRETE SLAB

(per cent)

House 1 Slab

top Undissolved Loss on ignition S03 total S03 soluble in lime water S03 as calcium sulphoaluminate*

Slab

bottom

top

70 .3

1.69

76.4 8 .05 0.43

75 .7 8 .3 1.23

0 .04

0 .24

0.05

0.13

0 .22

1.45

0 .38

1.10

72 .6 8.6 0.26

S03 in colliery shale filling

House 2

10 .5

3 .2

bottom

1.9

*Determined by difference between total S0 3 and that soluble in lime water (9). TABLE II COMPOSITION OF CONCRETE SLAB

(per cent)

Unattacked

Undissolved S0 3 total S0 3 soluble in lime water S0 3 as calcium sulphoaluminate

77 .4 0 .66 0.10 0 .56

Attacked (5 mm surface layer) 75 .4 2 .02 0 . 38 1.64

Recalculated as percentage of cement 2.95 S0 3 total S0 3 as calcium sulphoaluminate 2 .50

8 .20 6 .64

TABLE III SULPHUR TRIOXIDE CONTENTS

(per cent)

House A Top layer concrete Middle layer concrete Lower layer concrete Top layer colliery shale Lower layers colliery shale

HouseB

HouseC

0 .5

0.92

1. 75 1. 78 1.06

1.65 2.18 2 .2-5 .0

0 .56 0.62 3 .07 1.21 1.3

X-ray examination of the samples showed the presence of calcium sulphoaluminate in all the lower layers of the concrete. Investigations on a number of housing estates on which colliery shale had been used as a filling under the ground floor concrete slab have demonstrated the

61

CONCRETE IN SULPHATE-BEARING ENVIRONMENTS

variability in the incidence and rate of progress of sulphate attack. This is to be expected since the burnt colliery shale varies much in sulphur trioxide content while, in addition, the sites themselves must vary in their relative degree of wetness. Data on a large number of houses built from 1954 onwards on five different estates are shown in Table IV. TABLE IV INCIDENCE OF DAMAGE FROM BURNT COLLIERY SHALE FILLS

Area

l

2 3 4 5

Total no. of houses

1522 2385 1728 1289 1682

Number damaged in each year

1957

1958

1959

1960

1961

Total

%

1 19

33

12 58

20 46

6

5 12

21 73 29 14

54 229 29 25

3.5 9 .6 1. 7 1.9 0.9

3

15

The cement content of the concrete slabs would be likely to have been about 400 lb per cu yd, with 3/ 4 in. the maximum size aggregate. The permissible content of sulphur trioxide in hardcore for use as filling under, and in contact with, a concrete slab, may be put at less than 0.5 per cent. Many well-burnt colliery shales have sulphur trioxide contents below 0.5 per cent, but the material is so variable that it can only be safely used if a protective membrane is placed between the filling and the concrete slab to prevent transfer of water.

SULPHATE ATTACK ON FOUNDATIONS Though the details are incomplete, the following case affords an example of the changes that can occur in site conditions after construction. A building erected in 1935, consisting, in part, of 2-storey construction of 11-in. cavity brickwork and, in part, of I -storey construction in solid brickwork, was founded on a clay subsoil on concrete strip foundations about 3 ft below ground level. In 1949 cracks developed in the external walls of one half of the cavity brickwork that was then underpinned. The other half of the cavity brickwork was underpinned for the same reason in 1955. No further cracking appeared until 1958 when, after a particularly wet season, vertical cracks appeared on the external walls accompanied by cracking of plaster ceilings. At the same time cracks appeared for the first time in the solid brickwork walls that had not been underpinned. Trial holes showed that the water level in the ground was above the strip foundation level and that the underpinning concrete was heavily encrusted with a white deposit with all the appearance of severe sulphate attack. Samples of water from the trial holes contained 120 parts SO3 per 100,000 and clay samples 0.36 per cent. Magnesium sulphate was present as well as calcium sulphate, which tends

62

PERFORMANCE OF CONCRETE

to be the dominant sulphate in the particular area. A sample of the underpinning concrete contained 12.6 per cent SO3 calculated on the cement content. The C 3 A content of the cement used for this concrete was probably between 10 and 13 per cent. It appears that in the early years of this building the ground water level was well below the concrete strip foundation but that subsequent construction on an adjacent site had altered the ground drainage raising the water levels and exposing the concrete to attack. The deepening of the foundation by underpinning had also carried this concrete deeper into the wet zone. HIGH ALUMINA CEMENT IN SEA-WATER

Although high alumina cement concrete has a high resistance to the action of sulphates and sea-water, various cases have been reported of the unsatisfactory behaviour of marine structures ( 10). These failures have been associated with the conversion of the hexagonal mono- and di-calcium aluminate hydrates in the set cement into the cubic tricalcium aluminate hydrate (3CaO.Al2O3.6H::iO). It is well known that although this conversion leads to a marked reduction in strength, this reduction becomes progressively less serious as the water-cement ratio of the concrete is reduced (10-12). The conversion proceeds very slowly over a long period of years at 18°C but with rapidly increasing speed as the temperature is raised. The variable behaviour of high alumina cement concrete structures in practice has seemed to suggest that the rate at which the conversion occurs is a significant factor in determining its effect on the strength and durability of the concrete. 1 The purpose of this note is to discuss some evidence obtained in large-scale seawater trials and from an examination of a 40-year-old marine structure. High alumina cement was included in the trials on reinforced concrete in seawater started in 1929 by the Sea Action Committee of the Institution of Civil Engineers (13). Reinforced concrete piles ( 5 in. square and 5 ft long) were exposed at Sheerness Dockyard in southeast England in an open tank connected to the sea with their feet always immersed to a depth of 18 in. By a suitable mechanism a rise and fall of the level of the sea-water of about 1½-2Jf ft, depending upon the tide, was arranged to occur automatically twice daily following the normal tidal movement on a reduced scale. After the conclusion of the trials2 some undamaged piles, then approaching 30 years of age, were examined in the laboratory. Cubes of 2-in. side were cut from the outside of the piles and tested for strength, and samples of concrete were examined by differential thermal !Laboratory tests by H. G. Midgley, in course of publication, have shown that the loss in strength decreases as the rate of conversion falls. 2The actual sea-water exposure was only 10 years, as after 1939 the piles had to be left standing in empty tanks either dry or containing a few inches of rain-water.

63

CONCRETE IN SULPHATE-BEARING ENVIRONMENTS

analysis and X-ray diffraction to estimate the degree of conversion. 3 These estimates are subject to a probable error of up to + IO per cent. The only comparable strength data available at earlier ages were on 6- by 3-in. cylinders stored in water and artificial sea-water of three times normal concentration. As may be seen from Table V the lowest strengths occurred at the tops of the piles that were always in the air and the highest at the feet of those that were always TABLE V STRENGTH AND DEGREE OF CONVERSION OF SAMPLES CUT FROM HIGH ALUMINA CEMENT CONCRETE PILES AT AN AGE OF NEARLY 30 YEARS Compressive strength (lb. per sq . in.) Cement 108 Source oftest specimen

R*

Conversion (per cent)

Cement 109

M*

R

M

R

90

Sheerness piles Top Upper middle** Lower middle** Foot

8150 8960 8970 10230

4480 6110 7780 8870

5910 7370 10080 8510

3830 6270 8470 9730

Cylinders 6 X 3 in. Stored in water 7days I year 5 years 10 years

6890 7020 9400 8300

6230 6960 7400 8150

5850 7320 8260 7250

6100 7720 7960 7720

7470 8300 7500

7900 8680 8200

7580 8350 7150

7520 8520 8500

Stored in three times normal sea-water I year 5 years lOyears

Cement 108

50

M 90 (IO)*** (IO)*** (10)***

Cement 109 R

M

90

90

75

90

*R = concrete mix 1.73 gravel(½-¼ in.) : 0.87 sand:I cement by weight. Water-cement ratio 0.3; 1000 lb cement per cu yd concrete. M = concrete mix 3.3 gravel (½-¼ in.) : 1.7 sand:! cement by weight. Water-cement ratio 0.48; 600 lb cement per cu yd concrete. **Respectively 3 ft and 2 ft above foot of pile. "High" tide rose 3 to 4 ft above the foot according to the tide. "Low" tide was at 18-in. level. ***Little 3CaO .Al 2 O 3 .6H 2 O could be detected in these specimens but these low estimates may be subject to more error than the other values.

immersed. The strength of a 6- by 3-in. cylinder may be taken as 0.75 to 0.85 of that of a 2-in. cube. On this basis the strengths of the specimens from the feet of the 30-year-old piles were roughly similar to those of the 10-year-old cylinders and above the original 7-day strength. The specimens from the top of the piles show strengths corresponding to about 80 to 90 per cent of the 7-day strength at a water-cement ratio of 0.3, and 50 to 60 per cent at water-cement ratio of 0.48. These losses in strength are rather lower than those found when complete conversion is made to occur rapidly by curing at a high temperature. 3Examination by H . G. Midgley.

64

PERFORMANCE OF CONCRETE

The reduction in the percentage loss in strength on conversion with decreasing water-cement ratio accords with other data, but some other explanation must be sought for the retention of strength in the permanently immersed portions of the individual piles compared with the loss in the portions permanently in air. There is also little relation between the loss in strength and the degree of conversion in the different parts of a single pile. The foot of the pile would have been maintained at a temperature close to that of the continuously renewed sea-water whereas in the upper parts the temperature of the concrete would be determined by the air temperature and solar radiation and be higher over part of the year. In hindsight it is to be regretted that these temperatures were not recorded when the trials started nearly 40 years ago, but it is plausible to speculate that the process of conversion was slower in the lower than the upper parts of the piles on account of the difference in the temperature regime. The marine structure that was examined was a pier-head built on the south coast of England in 1926. The precast concrete piles and the cast-in-situ concrete superstructure were made with high alumina cement concrete composed of 3Jf parts coarse aggregate Oct in.), 1¾ sand, 1 cement by volume with a cube strength of 6500 lb per sq in. at 28 days. Inspections in 1955 and 1959 showed that the piles both below and above the low tide level were sound and the concrete very hard. The cast-in-situ concrete members in the deck-beams and the tidal zone members showed some cracking and spalling of the concrete and corrosion of the reinforcement. This was invariably found to be associated with a concrete cover of about 1 in. in comparison with the 2-in. cover specified. Apart from these defects the cast-in-situ concrete was in a hard condition as judged by chipping with a hammer. One of the piles was extracted and samples of the concrete examined by X-ray and DT A methods. The predominant compound identified in the set cement was CaO.Al2O3. lOH2O accompanied by Stratling's compound 2CaO.Al 2O:i.SiO2 .H 2O. None of the samples showed much sign of conversion. The maximum temperature attained by the sea in summer in this region does not exceed about 18°C. The results indicate that at this temperature in sea-water the rate of conversion can be surprisingly low. It is noteworthy that high alumina cement showed a good performance not only in the British sea-water exposure trials, but also in the German trials (14) at Wilhelmshaven and the Belgium trials (15) at Ostend, all in temperate climates. In contrast, the performance in the Norwegian trials at Trondheim was poor and the cause of this is not known for in this climatic region the rate of conversion would be expected to be low.

REFERENCES 1. G. E. BESSEY and F. M. LEA. Proc. Inst. Civ. Eng. part I, 159 (1953). 2. Bldg. Res. Sta. Digest no. 31, HMSO (1951).

CONCRETE IN SULPHATE-BEARING ENVIRONMENTS

65

3. D. G . MILLER and P. W. MANSON. Univ. Minnesota, Agric. Exp. Sta. Tech. Bull. no. 194 (1951). 4. U.S. Portland Cement Assoc. Res. Dept. Bull. no. 175 (1965). 5. V. V. STOLNIKOV. Trans. 8th Intern. Congress on Large Dams 3, 403 ( 1964) . 6. K. A. MALTsov, I. B. SoLoKov, and P. G . STARITSKY. 8th Intern. Congress on Large Dams. Communication C ( 1964). 7. A. BURCHARTZ. Deut. Ausschuss fiir Eisenbeton, 64 ( 1931). 8. J. MOURN and I. Tu. RosENQVIST. Proc. Am. Concrete Inst. 56,257 (1959) . 9. F. M. LEA. Chemistry of Cement and Concrete (Edward Arnold Publishers Ltd., London, 1956),p.599. 10. Summarized by A. M. NEVILLE. Proc. Inst. Civil Eng. 25, 287 (1963). 11. K. NEWMAN. Reinforced Coner. Review 5,269 (1960-61). 12. Institution of Structural Eng. Rep. on the Use of High Alumina Cement in Structural Engineering (London, 1964) . 13. F. M. LEA and C. M. WATKINS. The Durability of Reinforced Concrete in Sea Water. Natl. Bldg. Studies Res. Paper no. 30, HMSO (London, 1960) . 14. A. ECKHARDT and K. KRoNSHEIN. Deut. Ausschuss fiir Stahlbeton, 102 (1950). A. HUM• MELL and K. WESCHE, ibid., 124 (1956). K. WESCHE, RILEM Bull. 32, 291 (1966). 15. F. CAMPUS. Annales des Travaux Publics de Belgique no. 5, 419 (1960-61) ; F. CAMPUS, R. DANTINE, and M. DZULYNSKI. RILEM Symp. on the Behaviour of Concretes Exposed to Sea-water (Palermo, 1965). 16. 0. E. GJoRv, I. GuKILD and H.P. SuNDH. RILEM Bull. 32,305 (1966).

41

Field and Laboratory Studies of the Sulphate Resistance of Concrete

BRYANT MATHER* AT PRESENT, the principal approach to the production of sulphate-resisting concrete is to limit the allowable tricalcium aluminate (C 3 A) content of the cement to progressively lower limits as the sulphate concentrations that are expected to come into contact with the concrete in service increase. The Corps of Engineers and the U.S. Bureau of Reclamation, for example, require ( 1, 7) that, where sulphate concentrations exceed 0.20 per cent as water-soluble S04 in soil or 1000 ppm as S04 in water, Type V (sulphate-resisting) cement will be used; where concentrations are in the range 0.10 to 0.20 per cent in soil or 150 to 1000 ppm in water, Type II portland cement or Type IS (MS) portland blast-furnace slag cement will be used. Where the concentrations are lower than 0.10 per cent in soil or 150 ppm in water, no special precautions are needed. So far as is known, no significant deterioration of concrete due to sulphate attack has been encountered when these precautions have been taken and the estimated sulphate contents have not been significantly exceeded. In 1966 the British Standards Institution issued a specification for sulphateresisting portland cement ( 13) in which it is stated that "a considerable degree of sulphate resistance is conferred on portland cement if the tricalcium aluminate is limited to 3)f per cent." This may be compared with the limits of 5 and 8 per cent respectively on Type V and Type II in specifications used in the United States (2, 14). The limitation on C 3 A content of cement involves a calculation of C 3 A content based on the results of chemical analysis using the formula:

Per cent C 3 A

= 2.650

X per cent A}z0 3

-

1.692 X per cent Fe 20 3 •

CHEMICAL REACTIONS

The chemical reactions involved in the attack on concrete by magnesium sulphate, as provided, for example, by exposure to sea-water, have been given (3) as follows : *Bryant Mather, Chief, Concrete Division, U.S. Army Engineer Waterways Experiment Station, Jackson, Mississippi. Present Chairman of the HRB Concrete Division, a past President of ACI, and a past Chairman of ASTM Committee C-9, Concrete.

67

STUDIES ON THE SULPHATE RESISTANCE OF CONCRETE

(a) The magnesium sulphate reacts with hydrated C3A : 2(3CaO.A12O 3 .12H2O) 3(MgSO4.7H2O) calcium aluminate magnesium sulphate hydrate 3CaO.AbO3.3CaSO4.3lH2O 2Al(OH)3 3Mg(OHh 8H2O calcium aluminum aluminum magnesium sulphate hydrate hydroxide hydroxide ( ettringite)

+

+

+

+

(b) The magnesium sulphate also reacts with calcium hydroxide : Ca(OHh MgSO4.7H2O - CaSO4.2H2O Mg(OHh 5H2O calcium calcium sulphate hydroxide (gypsum)

+

+

+

(c) The gypsum formed in (b) or (d) also reacts with hydrated C 3 A : 3CaO.A}zO 3 .12H2O. 3(CaSO4.2H2O) 13H2O 3CaO.Al2Oa.3CaSO4.3 lH2O

+

+

(d) The magnesium sulphate also reacts with calcium silicate hydrate: 3MgSO4.7H2O- 3CaSO4.2H2O 3Mg(OH):i 3CaO.2SiO2. nH2O calcium silicate 2SiO2.nH2O 12H2O hydrate silica gel

+

+

+

+

( e) The magnesium hydroxide formed in (a) or (b) reacts with the silica gel formed in (d): 4Mg(OHh SiO2.nH2O - 4MgO.SiO2. 8.5H2O n - 4,520 magnesium silicate hydrate

+

+

Hansen and Offutt (5) have shown the following reactions of CnA, calcium sulphate (gypsum), and water, depending on which form of calcium aluminum sulphate hydrate is formed:

+

3CaO.Al2Oa 3CaSO4.2H2O 88.8 222.3 1 3CaO.Al2Oa CaSO4.2H2O 88.8 74.1 1

+

+ 25H2O -

3CaO.Al2Oa. 3CaSO4.3lH2O 714.7 ( 46.4) 8 l0H2O- 3CaO.Al2Oa.CaSO4.3lH2O 180.0 319.6 (23.3) 3.6 450

+

The numbers in the first line below the equations are participating volumes. The number in parentheses at the right is the volume added to that on the right side of the equation to make its volume equal to the sum of those on the left; thus both of these reactions involve a net reduction in volume of the reactants. The second line of numbers compares the relative volume of anhydrous C 3 A and the volume of calcium aluminum sulphate formed from it by sulphate reaction. In one case, the increase in volume is eightfold; in the other nearly fourfold .

68

PERFORMANCE OF CONCRETE

MARINE EXPOSURE In sulphate attack on concrete in sea-water, the sulphate is predominantly magnesium sulphate in a solution that contains much more chloride ion than sulphate ion. Lea ( 4) has suggested that the presence of the chlorides retards or inhibits the expansion of the concrete by sulphate attack but does not reduce the degree of reaction. He cited work attributing this effect to increased solubility of calcium sulphate and calcium aluminate sulphate in chloride solution. The Concrete Division, U.S. Army Engineer Waterways Experiment Station, has made many studies directly or indirectly concerned with sulphate-resisting concrete. When the Concrete Division was established in 1946 it assumed jurisdiction over an investigation, initiated in 1939, of the effects of variations in cement composition on the durability of concrete involving exposure of concrete specimens to sea-water at St. Augustine, Florida. Three specimens were made using each of 51 cements; one specimen was broken before it could be installed. In 1950, after 11 years, only 11 of 152 concrete specimens that were insta11ed had failed (12), and eight of these were made using cements having calculated C 3 A contents over 12 per cent. In 1966, 121 specimens were still under test; all had relative dynamic moduli above 98 per cent and relative pulse velocity values (V 2 ) above 80 per cent. Of the 31 no longer under test, 20 are recorded as "broken in handling" and 11 as having "failed." Of the 11 "failed" specimens, 8 are accounted for as follows: 3 specimens (cement E 58, calculated C 3 A content 17 per cent) failed in 1946; 3 specimens (cement E 32, calculated CaA content 14 per cent) failed in 1948; one specimen ( cement E 3, calculated C 3 A content 13 per cent) failed in 1946 and one specimen of the same group and content failed in 1950. The remaining three "failed" specimens represent one each of groups of three made using cements having 4 to 8 per cent C 3 A and perhaps might more accurately have been included in the "broken in handling" category.

TRICALCIUM ALUMINATE CONTENT In many localities Type V portland cement containing 5 per cent or less C 3 A calculated from chemical analysis is not readily available. The question has been asked whether some cements which, upon chemical analysis, yield values for percentages of Al 20 3 and Fe20 3 that cause the calculated C 3 A to exceed 5 per cent, might not be as sulphate-resistant as others whose calculated C 3 A content is 5 per cent, or less, because of possible incorporation of some of the Al 20 3 in constituents of the cement other than C3 A, in ways not contemplated by the phase equilibria relations upon which the calculations are based. In 1956 a sample of cement was received for study from the U.S. Army Engineer District, Los Angeles. An attempt was made to compare its C 3 A content estimated from X-ray diffraction data with

STUDIES ON THE SULPHATE RESISTANCE OF CONCRETE

69

that of other cements that had been studied by the light microscope, chemical analysis, and X-ray diffraction. In a report ( 8) data were given on 19 cements (Table I). Those ·cements which by quantitative microscope analysis were found to have C 3A contents lower than 4.8 per cent did not give X-ray diffraction peaks at 270A that were sufficiently well developed 1 to permit quantitative measurement of intensity. Correlation coefficients and "Student's" t were computed for the relations: (a) micrometric C3A vs X-ray intensity (r = +0.905, t 6.65), (b) calculated

=

DATA

ON

TABLE I 19 CEMENTS (8), (9)

Micrometric

Calculated

X-ray intensity, peak heightbackground at 2.70 A counts/sec

0 .1 0 .3 0 .7 2.0 3 .7 3 .7 4.4 4 .8 4.8 5.4 6.3 6 .3 6 .6 6 .9 7 .6 8 .7 9.8 10 .0 17.4

4 .5 4 .6 8 .9 3 .9 or 7 .4 7.2 6.8 6 .2 10.0 8.0 12.6 14.0 13 .3 9 .8 10 .6 11.3 13.4 12 .6 11 .2 17.3

(a) (a) (a) 120* 160* 155* 165* 210 130 245 225 235 235 225 230 245 290 250 450

C 3 A (%) Sample No. E-6 E-9 E-41 E-15 E-60 E-37 RC-318 E-33 RC-304 RC-299 RC-306 E-3 E-63 RC305 RC315 RC300 RC303 RC316 E58

*Peak too low and too poorly resolved for intensity measurement . In work reported earlier (8) no intensity measurement made; intensities measured later (9).

C3A vs X-ray intensity (r = +0.845, t = 5.04), (c) micrometric C3A vs calculated C3A (r +0.827, t 5.98) . The cement submitted for test did not have a peak at 2.70A that was well enough developed to scale; it had 3.9 per cent Al:!Oa and 2.6 per cent Fe2O3 and hence 6 per cent calculated C 3A. It was concluded that, had it been examined as clinker by microscope methods, it would have been found to contain less than 5 per cent C 3 A. It was assumed that the 5 per cent limit on C3A applicable to Type V cements was intended to refer to C3A that could, under optimum conditions of crystallization of the cement clinker, be found by microscope examination. The potential sulphate-resisting qualities of this cement were therefore expected to meet the requirements of the specifications for Type V cement.

=

=

1 X-ray diffraction equipment that became available later permits quantitative measurement of C 3 A contents below 4.8 per cent as indicated by microscope.

70

PERFORMANCE OF CONCRETE

In 1957, ten cement samples were received from the U.S. Army Engineer District, Omaha. Data on these cements are given in Table II (9). Only one of these cements gave a C 3 A peak well enough developed to scale; the intensity was 175 counts/sec above background. Four of the cements gave interfering peaks at about 2.72A, the significance of which has been discussed elsewhere (10). Four TABLE II DATA ON 10 CEMENTS (9) Expansion at

Cement serial no.

395 396 397 398 399 400 401 412 402 407 n.f.

1 yr of mortar

C3A (%)

MgO (%)

C 3A estimated from X-ray diffraction

6

0 .58 3.64 0.92 1.32 1.32 3.99 2.30 2.94 1.54 0.82

4-5