Effects of clays on fracture properties of cement-based materials

Effects of alumino-silicate clays on fracture properties of cement-based materials are reported in this paper. Two clays, illite and kaolinite, were used to replace 20% of Type I portland cement in different mixtures. Pores size distributions of hardened specimens were measured using mercury intrusion porosimetry, and compressive and fracture tests were conducted. It was found that when alumino-silicate clays are incorporated as substitutes for a fraction of the cement, the total porosity of hardened mixtures increases compared to that of hardened cement paste with no clay addition. The increase occurred primarily in the fraction of finer pores. However, the addition of clays decreased porosity for mortars. The addition of illite clay in cement-based mortars slightly decreased the compressive strength and the critical stress intensity 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 values of K_IC, CTOD_C and the compressive strength. These results indicate that appropriate use of silica fume and alumino-silicate clays may make cement-based materials stronger and more ductile.