A discussion of the paper “mechanism of sulfate attack on portland cement concrete - another look” by P.K. Mehta

CEMENT and CONCRETE RESEARCH. Vol. 14, pp. 151-152, 1984. Printed in the USA. 0008-884~/84 $3.00-0. Copy.'ight (c) 1984 Pergamon Press, Ltd.

A D I S C U S S I O N OF THE P A P E R "MECH~.:~ISM OF S U L F A T E A T T A C K ON P O R T L A N D , C E M E N T C O N C R E T E - A N O T H E R LOOK" BY P.K. M E H T A

S. C h a t t e r j i Technological Institute 2630 Taastrup, Denmark

In his paper Prof. M e h t a has c o m m e n t e d that the e f f e c t of g y p s u m f o r m a t i o n on the s u l f a t e a t t a c k has not b e e n p r o p e r l y e x p l a i n e d in the l i t e r a t u r e . As far as I know, Lea was the first to p r o p o s e that g y p s u m f o r m a t i o n may cause d a m a g e to P o r t l a n d c e m e n t concrete and e x p l a i n e d this on the a s s u m p t i o n of a solid state conv e r s i o n of Ca(OH) 2, p r e s e n t in the specimen, to g y p s u m (i). Since then it has been d i s c u s s e d m a n y times. In 1968 an a l t e r n a t i v e exp l a n a t i o n of the above damage was p r o p o s e d in terms of the crystal g r o w t h p r e s s u r e (2). In this paper M e h t a has also m a d e two very i n t e r e s t i n g o b s e r v a tions. T h e s e are that (i) the f o r m a t i o n of even Type 2 e t t r i n g i t e by itself is not c a p a b l e of c a u s i n g an e x p a n s i o n (p. 404, par. i); and (ii) e t t r i n g i t e can cause e x p a n s i o n only w h e n the s t i f f n e s s of the c e m e n t paste has a l r e a d y been l o w e r e d by s o m e t h i n g else (p. 406). However, these o b s e r v a t i o n s were a l r e a d y a n t i c i p a t e d in 1963 (3). R o s s e t t i et al have even shown that the e x p a n s i v i t y of all types of e t t r i n g i t e is d r a s t i c a l l y r e d u c e d if a small res t r a i n i n g p r e s s u r e is a p p l i e d to them (4). M e h t a p r o p o s e d that a "drop in the h y d r o x y l ions c o n c e n t r a t i o n and an i n c r e a s e in the s u l f a t e ions c o n c e n t r a t i o n of the e n v i r o n m e n t lowers the s t i f f n e s s of the c e m e n t p a s t e on one hand, and i n c r e a s e s the w a t e r - a d s o r p t i o n c a p a c i t y of the e t t r i n g i t e on the other hand" (p. 406) h e n c e the e x p a n s i o n and damages. In this c o n t e x t the p r o p e r t i e s of K type e x p a n s i v e c e m e n t are of i n t e r e s t (5). It has been shown that a Type K e x p a n s i v e c e m e n t can e x p a n d over i n i t i a l 14 days of h y d r a t i o n even t h o u g h its s t r e n g t h inc r e a s e s at the same time period, i.e. e x p a n s i o n and an i n c r e a s e in the s t i f f n e s s occur at the same time. The e x p a n s i o n has b e e n a t t r i b u t e d to e t t r i n g i t e f o r m a t i o n . As Type K e x p a n s i v e c e m e n t s p e c i m e n s w e r e in c o n t a c t only w i t h w a t e r n e i t h e r SO~ ion conc e n t r a t i o n in the e n v i r o n m e n t c o u l d i n c r e a s e nor the h y d r o x y l ion

CCR I_.33, 401 (1983). 151

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concentration could decrease. Apparently n e i t h e r of the two necessary conditions as p r o p o s e d by M e h t a w e r e f u l f i l l e d by T y p e K c e m e n t s . Prof. M e h t a ' s c o m m e n t on t h e s e r e s u l t s w i l l be of g e n e ral i n t e r e s t . REFERENCES i)

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Chatterji - Proc. 1968, III, 336.

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Rosetti, Chiocchio 23, 1983.

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