Experiments were performed to determine the effect of various chemical activators on strength of lime fly-ash pastes manufactured with two types of fly ash — a low calcium subbituminous ash (LFA) and a high calcium subbituminous ash (HFA). The strengths of blends of 80% fly ash and 20% hydrated lime were used to evaluate the pozzolanic reactivity of the ashes. Pastes were continuously moist-cured at 50 °C. The results indicate that the addition of small amounts of Na2SO4 and CaCl2 can increase the pozzolanic reactivity of both types of ash; this results in a significant improvement in strength. Na2SO4 has its predominant influence at early ages; its effect at later ages varies with the nature of fly ash used. In general, early-age strength increases with the amount of Na2SO4 dosage. Extent of strength improvement at later ages depends upon the dosage and type of fly ash used. The addition of CaCl2 has a variable influence on strength at early and intermediate ages. However, strengths at 90 days and 180 days are significantly improved by addition of 3–5 % CaCl2 activator. The use of up to 5% NaCl rather than CaCl2 did not result in substantial improvements to strength. X-ray analysis suggests that the addition of Na2SO4results in the formation of substantial amounts of ettringite (AFt). Addition of CaCl2 results in the formation of solid solution C4AH13-C3A.CaCl2.10H2O.