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A discussion of the paper “Mechanism of expansion associated with ettringite formation” by P. K. Mehta
Vol. 3, No. 5
651 DISCUSSIONS
A DISCUSSIONOF THE PAPER"MECHANISM OF EXPANSION
ASSOCIATED WITH ETTRINGITE FORR~TION" by P. K. MEHTA* Bryant Mather Concrete Laboratory U.S. Army Engineers Waterways Experiment Station Vicksburg, Mississippi 3g180 Professor Mehta's rather ingeneous hypothesis concerning the mechanism of expansion associated with ettringite formation and the observations that he has made that have led him to develop this hypothesis are most interesting.
I am
pleased that he does not show any enthusiasm for the direct crystal thrust hypothesis.
I am not convinced, however, from his presentation, that his mechan-
ism is necessary to explain the facts and inferences that are available relating to the phenomena of expansion associated with ettringite formation.
I agree
that i t can be assumed that the physical nature of the ettringite substance that is associated with an expansive mechanism is, as Professor Mehta suggests, a dense packed, finely divided material quite different in habit from the long slender fragile rosettes and tufts of needles that are so often observed and reported on by microscopists in examinations of old concrete. Such tufts of needles I believe must represent deposition of ettringite from solution and growth of crystals in spaces that are ample to accommodate the crystals growing therein. Such growth is not, in my view, associated with any expansion of the system in which the growth occurs. My failure to be convinced of the probable applicability of Professor Mehta's hypothesis arises from two considerations. First, i t seems to me to be a more elaborate and more generalized mechanism than is necessary and, second, i t suggests that any colloidal material of similar geometrical and electrical properties should behave in the same way in the presence of moisture. The data on expansive-cement systems hydrating in sealed containers indicate that external moisture is not needed to produce expansion associated with ettringite formation. I suggest, as I have previously, that the mechanism of expansion associated with ettringite formation is that the alumina-bearlng phase which is * CCR, ~, 1 (]973).
652
Vol. 3, No. 5 DISCUSSIONS
altered to e t t r i n g i t e remains as a solid in the cement paste until that paste has developed a degree of r i g i d i t y , and that thereafter solutions carrying ions appropriate to the nature of the alumina-bearing phase, so as to allow ettringite to form, migrate through pore space in the paste, react with the aluminabearing solid, i n - s i t u , so that a new phase, e t t r i n g i t e , is formed. The ettringite occupies a larger volume than the solid which participates in its formation. The result is a locally generated expansion as the e t t r i n g i t e forms in which the e t t r i n g i t e pushes the surroundings outward.
This mechanism, as is noted, assumes
in-situ conversion of one alumina-bearing solid to another and i t further assumes that the space occupied by the solution, which provides the other ingredients that combine with the alumina-bearing solid to form ettFingite, remains empty after these materials have combined to form the new solid.
This hypothesis,
therefore, deals with the problem of overall expansion of a relatively rigid solid but porous system by a reaction involving a net reduction in volume.
It
assumes that there w i l l be a net increase in a i r - f i l l e d space in concrete as a result of this reaction when the reaction occurs in a completely sealed system. The case of the sealed system i s , however, not that which normally exists with regard to concrete in service.
In concrete in service there is a charact-
e r i s t i c loss of water from the concrete after placement and curing and in those instances where sulfate attack, freezing and thawing, or other deleterious processes occur, water is available from the environment.
I f the concrete does
not have a s u f f i c i e n t l y large i n i t i a l free water content to supply the water needed to make all of the e t t r i n g i t e that can potentially be produced in the concrete or i f some of the water is lost by evaporation before all the potential e t t r i n g i t e is formed, then the ettringite-formation mechanism w i l l be interrupted before i t goes to completion and may be resumed when additional water becomes available later from the environment.
In the case of sulfate attack, not
only water but also sulfate ion is characteristically made available from the surroundings.
These circumstances, i t seems to me, explain why sulfate attack
is associated with exposure to moisture in the environment of service. I hope that others may find themselves in a position to conduct additional experimental work to explore not only the hypothesis here advanced, but also that advanced by Professor Mehta.
651 DISCUSSIONS
A DISCUSSIONOF THE PAPER"MECHANISM OF EXPANSION
ASSOCIATED WITH ETTRINGITE FORR~TION" by P. K. MEHTA* Bryant Mather Concrete Laboratory U.S. Army Engineers Waterways Experiment Station Vicksburg, Mississippi 3g180 Professor Mehta's rather ingeneous hypothesis concerning the mechanism of expansion associated with ettringite formation and the observations that he has made that have led him to develop this hypothesis are most interesting.
I am
pleased that he does not show any enthusiasm for the direct crystal thrust hypothesis.
I am not convinced, however, from his presentation, that his mechan-
ism is necessary to explain the facts and inferences that are available relating to the phenomena of expansion associated with ettringite formation.
I agree
that i t can be assumed that the physical nature of the ettringite substance that is associated with an expansive mechanism is, as Professor Mehta suggests, a dense packed, finely divided material quite different in habit from the long slender fragile rosettes and tufts of needles that are so often observed and reported on by microscopists in examinations of old concrete. Such tufts of needles I believe must represent deposition of ettringite from solution and growth of crystals in spaces that are ample to accommodate the crystals growing therein. Such growth is not, in my view, associated with any expansion of the system in which the growth occurs. My failure to be convinced of the probable applicability of Professor Mehta's hypothesis arises from two considerations. First, i t seems to me to be a more elaborate and more generalized mechanism than is necessary and, second, i t suggests that any colloidal material of similar geometrical and electrical properties should behave in the same way in the presence of moisture. The data on expansive-cement systems hydrating in sealed containers indicate that external moisture is not needed to produce expansion associated with ettringite formation. I suggest, as I have previously, that the mechanism of expansion associated with ettringite formation is that the alumina-bearlng phase which is * CCR, ~, 1 (]973).
652
Vol. 3, No. 5 DISCUSSIONS
altered to e t t r i n g i t e remains as a solid in the cement paste until that paste has developed a degree of r i g i d i t y , and that thereafter solutions carrying ions appropriate to the nature of the alumina-bearing phase, so as to allow ettringite to form, migrate through pore space in the paste, react with the aluminabearing solid, i n - s i t u , so that a new phase, e t t r i n g i t e , is formed. The ettringite occupies a larger volume than the solid which participates in its formation. The result is a locally generated expansion as the e t t r i n g i t e forms in which the e t t r i n g i t e pushes the surroundings outward.
This mechanism, as is noted, assumes
in-situ conversion of one alumina-bearing solid to another and i t further assumes that the space occupied by the solution, which provides the other ingredients that combine with the alumina-bearing solid to form ettFingite, remains empty after these materials have combined to form the new solid.
This hypothesis,
therefore, deals with the problem of overall expansion of a relatively rigid solid but porous system by a reaction involving a net reduction in volume.
It
assumes that there w i l l be a net increase in a i r - f i l l e d space in concrete as a result of this reaction when the reaction occurs in a completely sealed system. The case of the sealed system i s , however, not that which normally exists with regard to concrete in service.
In concrete in service there is a charact-
e r i s t i c loss of water from the concrete after placement and curing and in those instances where sulfate attack, freezing and thawing, or other deleterious processes occur, water is available from the environment.
I f the concrete does
not have a s u f f i c i e n t l y large i n i t i a l free water content to supply the water needed to make all of the e t t r i n g i t e that can potentially be produced in the concrete or i f some of the water is lost by evaporation before all the potential e t t r i n g i t e is formed, then the ettringite-formation mechanism w i l l be interrupted before i t goes to completion and may be resumed when additional water becomes available later from the environment.
In the case of sulfate attack, not
only water but also sulfate ion is characteristically made available from the surroundings.
These circumstances, i t seems to me, explain why sulfate attack
is associated with exposure to moisture in the environment of service. I hope that others may find themselves in a position to conduct additional experimental work to explore not only the hypothesis here advanced, but also that advanced by Professor Mehta.