EFFECTS OF CLAYS ON FRACTURE PROPERTIES OF CEMENT-BASED MATERIALS

Effects of alumino-silicate clays on fracture properties of cement-bas ed materials are reported in this paper. Two clays, illite and kaolini te, were used to replace 20% of Type I portland cement in different mi xtures. Pores size distributions of hardened specimens were measured u sing mercury intrusion porosimetry, and compressive and fracture tests were conducted. It was found that when alumino-silicate clays are inc orporated as substitutes for a fraction of the cement, the total poros ity of hardened mixtures increases compared to that of hardened cement paste with no clay addition. The increase occurred primarily in the f raction of finer pores. However, the addition of clays decreased poros ity for mortars. The addition of illite clay in cement-based mortars s lightly decreased the compressive strength and the critical stress int ensity factor (K(IC)), but increased the compressive toughness and the critical crack tip opening displacement (CTOD(C)). The replacement of 20% cement with 12% silica fume and 8% illitic clay increased the val ues of K(IC), CTOD(C) and the compressive strength. These results indi cate that appropriate use of silica fume and alumino-silicate clays ma y make cement-based materials stronger and more ductile.