A discussion of the paper “the pore structure of paste in concrete.” by Winslow and Liu

CEMENTand CONCRETERESEARCH. Vol. 21, pp. 196-197,1991. Printedin the USA. 0008-8846/91. $3.00+00. Copyright(c) 1991 PergamonPressplc.

A DISCUSSION OF THE PAPER " TIlE PORE STRUCTURE OF PASTE IN CONCRETE." BY WlNSLOW AND LIU *

S.Chatterji

D e p a r t m e n t of C i v i l E n g i n e e r i n g Kanazawa U n i v e r s i t y J a p a n 920

In their p a p e r D r s . W i n s l o w and Liu have c h a l l e n g e d a g e n e r a l l y believed a s s u m p t i o n t h a t t h e s t r u c t u r e of t h e p a s t e f r a c t i o n i n m o r t a r or c o n c r e t e is the same as t h a t of a n e a t cement p a s t e . They have shown t h a t a t l e a s t the pore s t r u c t u r e of p a s t e d e p e n d s on t h e p r e s e n c e or a b s e n c e of a g g r e g a t e particles. I a g r e e w i t h them f u l l y ; a c t u a l l y I have b e e n s p r e a d i n g t h e same message s i n c e 1982 ( 1 ) . We have even been a b l e t o i d e n t i f y one of t h e main r e a s o n s f o r t h e d i f f e r e n c e s o b s e r v e d by Winslow and L i u . Portland cement p a s t e , m o r t a r and c o n c r e t e , made w i t h w o r k a b l e water/cement ratios, b l e e d which means t h a t t h e s o l i d p a r t i c l e s settle towards bottom and w a t e r comes a t t h e t o p . However t h e s i t u a t i o n i s n o t i d e n t i c a l in all three c a s e s . I n t h e c a s e of t h e n e a t cement p a s t e , n o r m a l l y , a l l t h e cement g r a i n s s e t t l e t o g e t h e r f o r c i n g a l l the b l e e d water to the top. In the case of m o r t a r or c o n c r e t e , a g g r e g a t e p a r t i c l e s s e t t l e a t different rates than t h e cement g r a i n s . As a r e s u l t of t h i s d i f f e r e n t i a l movement a p a r t o f b l e e d

FIG.1 h m i c r o g r a p h of a m o r t a r s e c t i o n showing c r y s t a l l i z a t i o n of c a l c i u m h y d r o x i d e (marked C) a r o u n d a q u a r t z g r a i n , Q . M a g n i f i c a t i o n 8OK. *CCR 20(2), 227-335 (1990) 196

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water always c o l l e c t s round the aggregate p a r t i c l e s ; t h e l a r g e r or f l a t t e r the aggregate particles t h e more b l e e d w a t e r c o l l e c t s r o u n d them. The situation does n o t improve e i t h e r by r o t a t i n g or v i b r a t i n g a m o r t a r or concrete sample. This internal bleeding is quite common i n m o r t a r or concrete and n e a r l y a l w a y s a b s e n t i n cement p a s t e . These w a t e r p o c k e t s subsequently get filled up by larger crystals of calcium hydroxide,etteringite etc. Fig.1 illustrates the point clearly. In Fig. 1 it can be s e e n t h a t l a r g e c a l c i u m h y d r o x i d e c r y s t a l s have formed r o u n d many of the s a n d g r a i n s . These c a l c i u m h y d r o x i d e c r y s t a l s a r e larger than those formed i n s i d e t h e p a s t e and can o n l y form i n w a t e r - f i l l e d spaces. The p o r e s t r u c t u r e s of t h e f i l l e d or p a r t l y f i l l e d w a t e r p o c k e t s w i l l be q u i t e d i f f e r e n t from t h a t of t h e m a t r i x . It is also expected that the pores i n t h e f i l l e d or p a r t l y f i l l e d w a t e r p o c k e t s w i l l be c o a r s e r t h a n t h o s e in the surrounding matrix. These e x p e c t a t i o n s are substantiated by the o b s e r v a t i o n s of D r s . W i n s l o w and L i u .

Reference Chatterji. Chaussees.

I n t r n . C o n f . C o n c r e t e of E a r l y Ages. E c o l e N a t i o n a l e des P o n t s P a r i s . 1982. 3 4 - 3 8 .

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