The origins of the change in compressive
strengths of OPC pastes blended with 5% and
25% RHA at different curing times of 1, 7,
and 28 days were investigated. The compressive
strength gain with curing time for each paste
was primarily due to further extent of hydration,
as confirmed by the Ca(OH)2 content and the
SEM micrographs. At 1 day, the compressive
strength of the pastes was found to decrease
systematically with the increased RHA
addition, as a result of the dilution effect. At 7
and 28 days, the OPC+5% RHA exhibited
the highest compressive strength, followed by
the plain OPC paste and the OPC+25% RHA
paste, respectively. The increase in compressive
strength of the OPC+5% RHA paste at 7 and
28 days could not be reasoned by the pozzolanic
effect because of the very low pozzolanic
activity of the RHA. From microstructural
examination, however, it was evident that the
essentially inert RHA may play some roles in
accelerating the growth of ettringite, hence
partly increasing the strength of the OPC+5%
RHA paste at 7 and 28 days. The reduced
compressive strengths of the OPC+25% RHA
paste with respect to the plain OPC paste at 7
and 28 days were most likely to be explained
by the dilution effect. In addition to these
effects, the vast difference in compressive
strength of the OPC+5% RHA and the OPC+
25% RHA pastes might also be reasoned by
the different homogeneity of the RHA
dispersion in these two pastes.