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Discussion of the paper “expansion properties of the mixture C4AS H12-CS and a hypothesis on the expansion mechanism” by V.A. Rossetti, G. Chicocchio, and A.E. Paolini
CEMENT and CONCRETE RESEARCH. Vol. 13, pp. 591-592, 1983. Printed in the USA. 0008-8846/83 $3.00 + 00. Copyright (c) 1983 Pergamon Press, Ltd.
DISCUSSION OF THE PAPER "EXPANSIO~I PROPERTIES OF THE MIXTURE C.A~H..-C~AND A HYPOTHESIS ON THE EXPANSION MECHANISM" 4 BYI~.A. ROSSETTI, G. CHICOCCHIO, AND A.E. PAOLINI*
P. K. Mehta Department of C i v i l Engineering University of C a l i f o r n i a Berkeley, CA 94720
The w r i t e r would l i k e to commend the authors for a very i n t e r esting paper which contains experimental data on mechanism of expansion associated with e t t r i n g i t e formation. I t is shown that stoichiometric mixtures for e t t r i n g i t e formation produced large l i n e a r expansions (400-500%) only when the contact solution had high calcium hydroxide concentration (1600 mg/l). In f a c t , the data in Fig. 4 show that the expansion increased at an exponential rate when calcium hydroxide concent r a t i o n was raised from 500 to 1600 mg/l. However, the authors were not able to find a s i m i l a r correlation between thecalcium hydroxide concentration and the surface area of the e t t r i n g i t e formed. The w r i t e r would l i k e to suggest that the lack of correlation may be due to the method used for the measurement of the surface area. Many researchers have shown that the BET method using nitrogen as adsorbate is not suitable for surface area measurements of poorly c r y s t a l l i n e hydrates. For instance, Winslow and Diamond ( I ) found approximately 700 m2/g surface area for hydrated cement pastes by Small Angle X-ray Scattering ~AXS) method, whereas BET surface areas by nitrogen adsorption are usually lower by an order of magnitude. Using the SAXS method, Chen and Mehta (2) obtained 153 m2/g surface area from an e t t r i n g i t e specimen which was not permitted to dry. Again, this value is about an order of magnitude greater than the nitrogen adsorption surface areas reported by the authors. Since the volume of water adsorbed by the e t t r i n g i t e formed under high-lime concentration was 3-4 times greater than the e t t r i n g i t e formed under low-lime concent r a t i o n , i t may be deduced that the surface area of the material in the former case was much higher. From experimental observations on freshly prepared specimens the authors concluded that water adsorption was a factor in the e t t r i n g i t e expansion phenomenon. However, since the dried specimens of e t t r i n g i t e gave r e l a t i v e l y small expansions on rewetting, the authors proposed that crystal growth also played a role in the e t t r i n g i t e - r e l a t e d expansions. *~CR., I~2, 577-585 (1982) 591
592
Vol. 13, No. 4 P.K. Mehta
This conclusion, which is not supported by d i r e c t experimental evidence, is questionable. I t is well known that large amounts of big crystals of e t t r i n g i t e are found in the hydration products of many cement types which show high strength but l i t t l e or no expansion. Furthermore, the transformation of smaller crystals into bigger ones should lead tQ densification or shrinkage rather than expansion. The w r i t e r would l i k e to suggest that the differences of expansion in the two cases arose, not because of the existence of two e t t r i n g i t e expansion mechanisms, but probably due to d i s s i m i l a r i t i e s in the s t i f f n e s s . In moist s o l i d s , removal of the adsorbed water layers is known to cause an increase in the London van der Waals' forces of a t t r a c t i o n , thus increasing strength and s t i f f n e s s ( e l a s t i c modulus). Unless the s t i f f ness is lowered, the s t r a i n caused by a given stress can be limited. This is why in expansive cements during the f i r s t 2-3 days hydration, when the cement paste s t i f f n e s s is low, the expansions associated with e t t r i n g i t e are large. S i m i l a r l y , in the case of sulfate attack on highCRA cement pastes immersed in NapSOm solutions, the w r i t e r has observed t~at lowering of the e l a s t i c mod~lu~ associated with gypsum formation usually preceded the onset of a large increase i n ~ e rate of expansion. Also, during the high-expansion period, often there was no additional e t t r i n g i t e formed and no increase in the size of e t t r i n g i t e crystals.
References (I)
D. N. Winslow and S. Diamond, J. Amer. Ceram. Soc., 5~7, 193 (1974).
(2)
S. S. Chen and P. K. Mehta, Cem. Concr. Res., I~2, 257 (1982).
DISCUSSION OF THE PAPER "EXPANSIO~I PROPERTIES OF THE MIXTURE C.A~H..-C~AND A HYPOTHESIS ON THE EXPANSION MECHANISM" 4 BYI~.A. ROSSETTI, G. CHICOCCHIO, AND A.E. PAOLINI*
P. K. Mehta Department of C i v i l Engineering University of C a l i f o r n i a Berkeley, CA 94720
The w r i t e r would l i k e to commend the authors for a very i n t e r esting paper which contains experimental data on mechanism of expansion associated with e t t r i n g i t e formation. I t is shown that stoichiometric mixtures for e t t r i n g i t e formation produced large l i n e a r expansions (400-500%) only when the contact solution had high calcium hydroxide concentration (1600 mg/l). In f a c t , the data in Fig. 4 show that the expansion increased at an exponential rate when calcium hydroxide concent r a t i o n was raised from 500 to 1600 mg/l. However, the authors were not able to find a s i m i l a r correlation between thecalcium hydroxide concentration and the surface area of the e t t r i n g i t e formed. The w r i t e r would l i k e to suggest that the lack of correlation may be due to the method used for the measurement of the surface area. Many researchers have shown that the BET method using nitrogen as adsorbate is not suitable for surface area measurements of poorly c r y s t a l l i n e hydrates. For instance, Winslow and Diamond ( I ) found approximately 700 m2/g surface area for hydrated cement pastes by Small Angle X-ray Scattering ~AXS) method, whereas BET surface areas by nitrogen adsorption are usually lower by an order of magnitude. Using the SAXS method, Chen and Mehta (2) obtained 153 m2/g surface area from an e t t r i n g i t e specimen which was not permitted to dry. Again, this value is about an order of magnitude greater than the nitrogen adsorption surface areas reported by the authors. Since the volume of water adsorbed by the e t t r i n g i t e formed under high-lime concentration was 3-4 times greater than the e t t r i n g i t e formed under low-lime concent r a t i o n , i t may be deduced that the surface area of the material in the former case was much higher. From experimental observations on freshly prepared specimens the authors concluded that water adsorption was a factor in the e t t r i n g i t e expansion phenomenon. However, since the dried specimens of e t t r i n g i t e gave r e l a t i v e l y small expansions on rewetting, the authors proposed that crystal growth also played a role in the e t t r i n g i t e - r e l a t e d expansions. *~CR., I~2, 577-585 (1982) 591
592
Vol. 13, No. 4 P.K. Mehta
This conclusion, which is not supported by d i r e c t experimental evidence, is questionable. I t is well known that large amounts of big crystals of e t t r i n g i t e are found in the hydration products of many cement types which show high strength but l i t t l e or no expansion. Furthermore, the transformation of smaller crystals into bigger ones should lead tQ densification or shrinkage rather than expansion. The w r i t e r would l i k e to suggest that the differences of expansion in the two cases arose, not because of the existence of two e t t r i n g i t e expansion mechanisms, but probably due to d i s s i m i l a r i t i e s in the s t i f f n e s s . In moist s o l i d s , removal of the adsorbed water layers is known to cause an increase in the London van der Waals' forces of a t t r a c t i o n , thus increasing strength and s t i f f n e s s ( e l a s t i c modulus). Unless the s t i f f ness is lowered, the s t r a i n caused by a given stress can be limited. This is why in expansive cements during the f i r s t 2-3 days hydration, when the cement paste s t i f f n e s s is low, the expansions associated with e t t r i n g i t e are large. S i m i l a r l y , in the case of sulfate attack on highCRA cement pastes immersed in NapSOm solutions, the w r i t e r has observed t~at lowering of the e l a s t i c mod~lu~ associated with gypsum formation usually preceded the onset of a large increase i n ~ e rate of expansion. Also, during the high-expansion period, often there was no additional e t t r i n g i t e formed and no increase in the size of e t t r i n g i t e crystals.
References (I)
D. N. Winslow and S. Diamond, J. Amer. Ceram. Soc., 5~7, 193 (1974).
(2)
S. S. Chen and P. K. Mehta, Cem. Concr. Res., I~2, 257 (1982).