Although fly ash has been widely used in concrete as a cement replacement,
little work has been done on determining the degree of hydration of high-vo
lume fly ash/cement (FC) systems. In the present study, the degree of hydra
tion of the cement in Portland cement (PC) paste was obtained by determinin
g the non-evaporable water (Wn) content. The degree of reaction of the fly
ash in FC pastes was determined using a selective dissolution method. Based
on the relation between the degree of cement hydration and effective water
-to-cement (w/c) ratio, the degree of hydration of the cement in FC pastes
was also estimated. It was found that high-volume fly ash pastes underwent
a lower degree of fly ash reaction, and in the pastes with 45% to 55% fly a
sh, more than 80% of the fly ash still remained unreacted after 90 days of
curing while the hydration of the cement in high-volume fly ash pastes was
enhanced because of the higher effective w/c ratio for the paste. This effe
ct was more significant for the pastes with lower water-to-binder (w/b) rat
ios. Thus, preparing high-volume fly ash concrete at lower w/b ratios can r
esult in less strength losses. This paper also introduces a model to descri
be the relationship between the w/c ratio and the degree of cement hydratio
n and gel/space ratio. The gel/space ratios of the FC pastes, evaluated bas
ed on the proposed model, were found to be consistent with the gel/space ra
tio of PC pastes in terms of the relationship with compressive strength. Th
e gel/space ratio data correlated (inversely) linearly with mercury intrude
d porosity, but the former correlated more with compressive strength than t
he latter. (C) 2000 Elsevier Science Ltd. All rights reserved.