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.