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A discussion of the paper “Mechanism of expansion associated with ettringite formation” by P. K. Mehta
Vol. 3, No. 5
655
DISCUSSIONS
A DISCUSSIONOF THE PAPER"MECHANISMOF EXPANSION ASSOCIATED WITH ETTRINGITE FOI~MATION" by P. K. MEHTA* W. C. Hansen, Consulting Chemist 15S Maple Street Valparaiso, Indiana 46383 The author makes no mention of certain reports in the literature that i t seems must be considered in evaluating his proposed hypothesis of a mechanism of the expansion associated with the formation of ettringite. Reniche Kondo and Shunro Ueda(1) reviewed the literature on topochemical reactions and gave a model for the reaction of C3S with water, which was developed from the optical and electron microscopic examination of individual grains of C3S as they reacted with water. The f i r s t reaction is a reaction at the surface of the grain to form an intermediate product (tr) through which water passes to form an inner product ( t i ) ; (tr) dissolves slowly and portlandite and a C-S-H product precipitate in the liquid phase as outer products (to). According to this model one might be inclined to conclude that the 'early hydration products in the cement pastes are formed by through-solution reactions rather than solid state reactions.' However i t seems from theoretical considerations (2) and from experimental observations that (tr) must f i r s t form and then dissolve to form the (to) products, portlandite and C-S-H. Masaichi Okushika and coworkers(3) studied the reactions of individual grains of C4A3S, by optical and electron microscopic methods, with water in the absence and in the presence of gypsum and calcium oxide. These authors represent ettringite as TSH. They state, "In the case of C4A3S alone, a complex layer of C3A.CSH12and AI(OH)3 forms as the inner product of C4A3S, and this reaction is completed in 12-24 hours. As shown in Fig. 12, i t is remarkable that the outer shape and dimensions of each grain are almost unchanged throughout the hydration, and TSH, as an outer product out of solution." "C4A3S hydrates qulckly in the presence of CaO, and the outer shape of its grain is almost unchanged. WhenC4A3S is hydrated with CaS04, the particles of * CCR, 3, 1 (1973).
656
Vol. 3, No. 5 DISCUSSIONS
the former are rapidly covered with needle-shaped crystals of TSH." "On the other hand, when a mixture of C4A3S, CaSO4 and CaO adjusted to a molar ratio of CaO:AI203:CaSO4 = 4:1:3 is hydrated, C4A3S particles are covered with a thick coating composed of fine crystals of TSH. This coating begins to expand after about 6-8 hours and becomes at about 24 hours, 2-3 times larger than at the beginning.
I t seems that this expansion due to hydration of C4A3S
in the presence of much CaSO4 and CaO is closely related to formation of TSH by the reaction between S04--, permeating from outside, and C3A.CSHI2 or its solid solution formed on the interface of each particle of C4A3S." The explanation of the expansion in a system, such as a portland cement paste, in which the sum of the volumes of the reaction products is less than the sum of the volumes of the reactants, has been attempted by means of a model(4) in which the reacting grains, in the original paste, touch, through thin films of water, one another at certain points on their surfaces. As the grains expand, by topochemical reactions, they exert pressure on one another at these points and cause an increase in the volume as the reaction proceeds. I t seems that the results depicted by the author in his Fig. 2 can be explained on the basis that (1) in the dry cured specimen some of the C4A3~ and possibly intermediate products such as C3A.CSHI2 had not been converted to ettringite; unreacted C4A3S was observed in this sample.
(2) In the wet cured
specimen, the water imbibed from the atmosphere provided an environment in which the reactions could proceed rapidly toward completion. References I.
Masaichi Okushima, Renichi Kondo, Hiroshi Muguruma and Yoshizo Ono, Development of Expansive Cement with Calcium Sulphoaluminous Cement Clinker, Proco
2.
Fifth Int. Sym. on Chem. of Cement IV, pp. 426-427 (1970). W. C. Hansen, Basic Chemistry of Reactions of Aggregates in Portland Cement
3.
Concrete, J. of Metals, 2, 412-415 (1967). Renichi Kondo and Shunro Ueda, Kinetics and Mechanism of the Hydration of
4.
Cements, Proc. Fifth Int. S~nn. on Chem. of Cement I l l , pp. 213-215 (1970). W. C. Hansen, Performance of Concrete edited by E. G. Swenson, Univ. of Toronto Press, p. 28 (1968).
655
DISCUSSIONS
A DISCUSSIONOF THE PAPER"MECHANISMOF EXPANSION ASSOCIATED WITH ETTRINGITE FOI~MATION" by P. K. MEHTA* W. C. Hansen, Consulting Chemist 15S Maple Street Valparaiso, Indiana 46383 The author makes no mention of certain reports in the literature that i t seems must be considered in evaluating his proposed hypothesis of a mechanism of the expansion associated with the formation of ettringite. Reniche Kondo and Shunro Ueda(1) reviewed the literature on topochemical reactions and gave a model for the reaction of C3S with water, which was developed from the optical and electron microscopic examination of individual grains of C3S as they reacted with water. The f i r s t reaction is a reaction at the surface of the grain to form an intermediate product (tr) through which water passes to form an inner product ( t i ) ; (tr) dissolves slowly and portlandite and a C-S-H product precipitate in the liquid phase as outer products (to). According to this model one might be inclined to conclude that the 'early hydration products in the cement pastes are formed by through-solution reactions rather than solid state reactions.' However i t seems from theoretical considerations (2) and from experimental observations that (tr) must f i r s t form and then dissolve to form the (to) products, portlandite and C-S-H. Masaichi Okushika and coworkers(3) studied the reactions of individual grains of C4A3S, by optical and electron microscopic methods, with water in the absence and in the presence of gypsum and calcium oxide. These authors represent ettringite as TSH. They state, "In the case of C4A3S alone, a complex layer of C3A.CSH12and AI(OH)3 forms as the inner product of C4A3S, and this reaction is completed in 12-24 hours. As shown in Fig. 12, i t is remarkable that the outer shape and dimensions of each grain are almost unchanged throughout the hydration, and TSH, as an outer product out of solution." "C4A3S hydrates qulckly in the presence of CaO, and the outer shape of its grain is almost unchanged. WhenC4A3S is hydrated with CaS04, the particles of * CCR, 3, 1 (1973).
656
Vol. 3, No. 5 DISCUSSIONS
the former are rapidly covered with needle-shaped crystals of TSH." "On the other hand, when a mixture of C4A3S, CaSO4 and CaO adjusted to a molar ratio of CaO:AI203:CaSO4 = 4:1:3 is hydrated, C4A3S particles are covered with a thick coating composed of fine crystals of TSH. This coating begins to expand after about 6-8 hours and becomes at about 24 hours, 2-3 times larger than at the beginning.
I t seems that this expansion due to hydration of C4A3S
in the presence of much CaSO4 and CaO is closely related to formation of TSH by the reaction between S04--, permeating from outside, and C3A.CSHI2 or its solid solution formed on the interface of each particle of C4A3S." The explanation of the expansion in a system, such as a portland cement paste, in which the sum of the volumes of the reaction products is less than the sum of the volumes of the reactants, has been attempted by means of a model(4) in which the reacting grains, in the original paste, touch, through thin films of water, one another at certain points on their surfaces. As the grains expand, by topochemical reactions, they exert pressure on one another at these points and cause an increase in the volume as the reaction proceeds. I t seems that the results depicted by the author in his Fig. 2 can be explained on the basis that (1) in the dry cured specimen some of the C4A3~ and possibly intermediate products such as C3A.CSHI2 had not been converted to ettringite; unreacted C4A3S was observed in this sample.
(2) In the wet cured
specimen, the water imbibed from the atmosphere provided an environment in which the reactions could proceed rapidly toward completion. References I.
Masaichi Okushima, Renichi Kondo, Hiroshi Muguruma and Yoshizo Ono, Development of Expansive Cement with Calcium Sulphoaluminous Cement Clinker, Proco
2.
Fifth Int. Sym. on Chem. of Cement IV, pp. 426-427 (1970). W. C. Hansen, Basic Chemistry of Reactions of Aggregates in Portland Cement
3.
Concrete, J. of Metals, 2, 412-415 (1967). Renichi Kondo and Shunro Ueda, Kinetics and Mechanism of the Hydration of
4.
Cements, Proc. Fifth Int. S~nn. on Chem. of Cement I l l , pp. 213-215 (1970). W. C. Hansen, Performance of Concrete edited by E. G. Swenson, Univ. of Toronto Press, p. 28 (1968).