Figure 8, it is interesting to note that the
ettringite needles in the OPC+5%RHA paste
are larger than in the plain OPC paste, both
at 7 and 28 days, (the difference is more
pronounced at 7 days). The growth of
ettringite seems to be accelerated by the
presence of 5 wt% RHA, possibly due to the
greater amount of porosity or the improved
hydration conditions (e.g., a larger amount of
water) accompanying the RHA addition.
Although the origin of this accelerated ettringite
growth is still unclear, its existence may partly
help explain the superior compressive strength
of the OPC+5%RHA paste compared with
the plain OPC paste at 7 and 28 days. With the
possibility of the pozzolanic effect being ruled
out, the slight reduction in the Ca(OH)2
content at 7 and 28 days of the OPC+5%
RHA paste from that of the plain OPC paste
should, therefore, be primarily due to the
dilution effect