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A reply to a discussion by S. Chatterji of the paper “Mechanism of alkali-silica reaction and the significance of calcium hydroxide”
CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 193-194, 1992. Printed in the USA. 0008-8846/92 $5.00+.00. Copyright © 1991 Pergamon Press plc.
A REPLY TO A DISCUSSION by S. CHATTERJI OF THE PAPER "MECHANISM OF ALKALI-SILICA REACTION AND THE SIGNIFICANCE OF CALCIUM HYDROXIDE '''~
H.Wang and J.E.Gillott Dept. of Civil Engineering The University of Calgary Calgary, Alberta, Canada We would like to t h a n k Dr. Chatterji for his interesting and detailed comments on our paper. We agree with Dr. Chatterji t h a t our thinking owes a great deal to the pioneering work of Powers and Steinour though our new data has led us to propose what we feel are significant changes to the original hypothesis. Dr. Chatterji discusses the breakdown of the Si-O-Si bridge illustrated in Fig. 2 of our paper and the possibility of ion exchange reactions. We feel t h a t the details of this reaction are likely to be complex and our Figure is intended only to illustrate the process schematically. It nevertheless seems to us that alkali ions will penetrate to reaction sites ahead of Ca ++ ions because the concentration of alkali is higher and because the hydraulic radius of the alkali ions is smaller. Therefore the initial products of reaction are likely to be an alkali-silicacomplex of the type responsible for expansion. W e think that once the alkali-silicacomplex has expanded it becomes easier for Ca ++ ions to penetrate the alkali-silicacomplex where exchange between alkali and Ca ++ ions then occurs. In the proposed mechanism the properties of alkali-silicacomplex and alkalilime-silica complex are separated. At equivalence, we believe that the alkali-silica complex is expansive whereas the alkali-lime-silicacomplex is non-expansive. It was shown in another paper of ours that the alkali-silicacomplex had a significant affinity for moisture and great expansivity, while the alkali-lime-silicacomplex had less affinityfor moisture and littleexpansivity (1).The proportion of alkali and lime in the complex was chosen to be similar to that reported in the literature for alkalisilicaproducts in concrete affected by A S R (2). Regarding the function of OH" in alkali-silicareaction,Dr. Chatterji considered the system of Ca(OH) 2 and NaCl as an alkali hydroxide free system and used the system to argue that alkali silicareaction does not need a high pH; he concluded that *CCR 21(4) 647-654 (1991)
193
194
DISCUSSIONS
Volo 22, No. 1
ASR m a y occur when the O H concentration is substantially lower t h a n that of the equivalence of Ca(OH) 2. In his series of publications, it was always stressed that Ca(OH) 2 provides the necessary driving force for OH" to break Si-O-Si bonds and alkali cations from NaC1 then penetrate to the reaction sites. The rate of formation of alkali-silica complex depends on alkali ions (3). This mechanism implied that from the viewpoint of Si-O-Si bond breakdown, O H plays ar.. important role. In a comprehensive review of alkali-aggregate reaction, Diamond stated that "the alkalisilica reaction is not fundamentally between alkali ions and siliceous aggregate, but rather between hydroxide ions and aggregate" (4). Using buffer solutions with different pH values prepared by mixing amino acetic acid and NaC1 solution together with 0.1 M NaOH solution, Tang and Han showed that the higher the pH in the solution the more the SiO 2 dissolved (5). Dr. Chatterji's test involving immersion of mortar bars in a 3 N NaOH solution produced lower expansions t h a n were found when bars were immersed in combined Ca(OH) 2 and NaC1 solution. In a rapid test developed by Oberholster and his colleagues opaline samples showed lower expansions when p u t into 1 N NaOH solution t h a n when tested according to ASTM C-227 (6). They attributed this to the absence of a pessimum effect in their test procedure. Possibly a similar factor influences results in Dr.Chatterji's test. Although a high concentration of O H ions in solution increases attack on silica, the reaction does not necessarily produce an expansive alkali-silica complex. Also it is known that increase of Na/Si ratio, decreases the viscosity of the alkali-silica complex which becomes flowable. We are asked to comment on the observations of Kilgour (7). To our knowledge, in the pure alkali solution, it is possible to produce a very soluble alkali-silica gel, therefore, more silica is expected in the solution . In the system containing both NaOH and Ca(OH)2, it is possible to produce a less soluble alkali-lime-silica complex, containing less silica in solution. REFERENCES 1. H. Wang and J.E. Gillott, "Effect of Ca(OH) 2 on alkali-silica reaction", To be published in Magazine of Concrete Research. 2. H. Krogh, Pro. of a Symposium on Alkali-Aggr. Reaction, Reykjavik, Iceland, p.131, (1975). 3. S. Chatterji, N. Thaulow and A. D. Jensen, Cement and Concr. Res. 19, 177, (1989). 4. S. Diamond, Cement and Concr. Res. 5, 329, (1975). 5. M.S. Tang and S.F. Han, Journal of Silicate, 9(2), 160, (1981). 6. R.E. Oberholster and G. Davies, Cement and Concr. Res. 16, 181, (1986). 7. S. Diamond, Proc. of 8th Intern. Conf. Alkali-Aggregate Reactions. Kyota, Japan, p.83, (1989).
A REPLY TO A DISCUSSION by S. CHATTERJI OF THE PAPER "MECHANISM OF ALKALI-SILICA REACTION AND THE SIGNIFICANCE OF CALCIUM HYDROXIDE '''~
H.Wang and J.E.Gillott Dept. of Civil Engineering The University of Calgary Calgary, Alberta, Canada We would like to t h a n k Dr. Chatterji for his interesting and detailed comments on our paper. We agree with Dr. Chatterji t h a t our thinking owes a great deal to the pioneering work of Powers and Steinour though our new data has led us to propose what we feel are significant changes to the original hypothesis. Dr. Chatterji discusses the breakdown of the Si-O-Si bridge illustrated in Fig. 2 of our paper and the possibility of ion exchange reactions. We feel t h a t the details of this reaction are likely to be complex and our Figure is intended only to illustrate the process schematically. It nevertheless seems to us that alkali ions will penetrate to reaction sites ahead of Ca ++ ions because the concentration of alkali is higher and because the hydraulic radius of the alkali ions is smaller. Therefore the initial products of reaction are likely to be an alkali-silicacomplex of the type responsible for expansion. W e think that once the alkali-silicacomplex has expanded it becomes easier for Ca ++ ions to penetrate the alkali-silicacomplex where exchange between alkali and Ca ++ ions then occurs. In the proposed mechanism the properties of alkali-silicacomplex and alkalilime-silica complex are separated. At equivalence, we believe that the alkali-silica complex is expansive whereas the alkali-lime-silicacomplex is non-expansive. It was shown in another paper of ours that the alkali-silicacomplex had a significant affinity for moisture and great expansivity, while the alkali-lime-silicacomplex had less affinityfor moisture and littleexpansivity (1).The proportion of alkali and lime in the complex was chosen to be similar to that reported in the literature for alkalisilicaproducts in concrete affected by A S R (2). Regarding the function of OH" in alkali-silicareaction,Dr. Chatterji considered the system of Ca(OH) 2 and NaCl as an alkali hydroxide free system and used the system to argue that alkali silicareaction does not need a high pH; he concluded that *CCR 21(4) 647-654 (1991)
193
194
DISCUSSIONS
Volo 22, No. 1
ASR m a y occur when the O H concentration is substantially lower t h a n that of the equivalence of Ca(OH) 2. In his series of publications, it was always stressed that Ca(OH) 2 provides the necessary driving force for OH" to break Si-O-Si bonds and alkali cations from NaC1 then penetrate to the reaction sites. The rate of formation of alkali-silica complex depends on alkali ions (3). This mechanism implied that from the viewpoint of Si-O-Si bond breakdown, O H plays ar.. important role. In a comprehensive review of alkali-aggregate reaction, Diamond stated that "the alkalisilica reaction is not fundamentally between alkali ions and siliceous aggregate, but rather between hydroxide ions and aggregate" (4). Using buffer solutions with different pH values prepared by mixing amino acetic acid and NaC1 solution together with 0.1 M NaOH solution, Tang and Han showed that the higher the pH in the solution the more the SiO 2 dissolved (5). Dr. Chatterji's test involving immersion of mortar bars in a 3 N NaOH solution produced lower expansions t h a n were found when bars were immersed in combined Ca(OH) 2 and NaC1 solution. In a rapid test developed by Oberholster and his colleagues opaline samples showed lower expansions when p u t into 1 N NaOH solution t h a n when tested according to ASTM C-227 (6). They attributed this to the absence of a pessimum effect in their test procedure. Possibly a similar factor influences results in Dr.Chatterji's test. Although a high concentration of O H ions in solution increases attack on silica, the reaction does not necessarily produce an expansive alkali-silica complex. Also it is known that increase of Na/Si ratio, decreases the viscosity of the alkali-silica complex which becomes flowable. We are asked to comment on the observations of Kilgour (7). To our knowledge, in the pure alkali solution, it is possible to produce a very soluble alkali-silica gel, therefore, more silica is expected in the solution . In the system containing both NaOH and Ca(OH)2, it is possible to produce a less soluble alkali-lime-silica complex, containing less silica in solution. REFERENCES 1. H. Wang and J.E. Gillott, "Effect of Ca(OH) 2 on alkali-silica reaction", To be published in Magazine of Concrete Research. 2. H. Krogh, Pro. of a Symposium on Alkali-Aggr. Reaction, Reykjavik, Iceland, p.131, (1975). 3. S. Chatterji, N. Thaulow and A. D. Jensen, Cement and Concr. Res. 19, 177, (1989). 4. S. Diamond, Cement and Concr. Res. 5, 329, (1975). 5. M.S. Tang and S.F. Han, Journal of Silicate, 9(2), 160, (1981). 6. R.E. Oberholster and G. Davies, Cement and Concr. Res. 16, 181, (1986). 7. S. Diamond, Proc. of 8th Intern. Conf. Alkali-Aggregate Reactions. Kyota, Japan, p.83, (1989).