The effect of activators Na2SO4 and CaCl2 on early microstructure deve
lopment of lime-fly ash pastes was examined. The control lime-fly ash
pastes did not show a significant strength up to 28 days. The addition
of 4% Na2SO4 increased both the early and later strengths of lime-fly
ash pastes significantly. CaCl2.2H(2)O (4%) was less effective at 3 a
nd 7 days, but more effective at 28 days than Na2SO4. Scanning electro
n microscope observation and X-ray diffraction analysis indicated that
very limited amount of C-S-H, ettringite (AFt) and monosulphoaluminat
e (AFm) formed in the control pastes even at 28 days. C-S-H and AFt fi
lled most spaces between fly ash particles in the Na2SO4 activated pas
tes at 3 days, and the structure became denser with time. CaCl2 activa
ted pastes also exhibited a dense structure at 3 days, in which most v
oids between fly ash particles were filled with C-S-H and the solid so
lution of Ca4Al2[(SO42-)(X)(Cl-)(Y)(OH-)(2-2X-Y)].nH(2)O (X<1 and Y<2)
. The amount of the solid solution increased drastically from 7 to 28
days, which resulted in a denser structure and a higher strength than
Na2SO4 activated pastes.