ACCELERATION OF THE REACTIVITY OF FLY-ASH BY CHEMICAL ACTIVATION

Experiments were performed to determine the effect of various chemical activators on strength of lime fly-ash pastes manufactured with two t ypes of fly ash - a low calcium subbituminous ash (LFA) and a high cal cium subbituminous ash (HFA). The strengths of blends of 80% fly ash a nd 20% hydrated lime were used to evaluate the pozzolanic reactivity o f the ashes. Pastes were continuously moist-cured at 50 degrees C. The results indicate that the addition of small amounts of Na2SO4 and CaC l2 can increase the pozzolanic reactivity of both types of ash; this r esults in a significant improvement in strength. Na2SO4 has its predom inant influence at early ages; its effect at later ages varies with th e nature of fly ash used. In general, early-age strength increases wit h the amount of Na2SO4 dosage. Extent of strength improvement at later ages depends upon the dosage and type of fly ash used. The addition o f CaCl2 has a variable influence on strength at early and intermediate ages. However, strengths at 90 days and 180 days are significantly im proved by addition of 3-5% CaCl2 activator. The use of up to 5% NaCl r ather than CaCl2 did not result in substantial improvements to strengt h. X-ray analysis suggests that the addition of Na2SO4 results in the formation of substantial amounts of ettringite (AFt). Addition of CaCl 2 results in the formation of solid solution C(4)AH(13)-C(3)A.CaCl2.10 H(2)O.