the voids in the concrete, thus increased the compressive strength of
concrete. Similar optimum level of POFA replacement level was also
reported by Sujivorakul et al. (2011), even though the increment of
compressive strength found in this study was lower. However, when
the POFA replacement levels were increased to 20% and 25%, a
decrease in the compressive strength of up to 14% was noticed for the
OPSC. The decrease in the compressive strength could be attributed
to the dilution effect, in which there is reduced amount of OPC, and
consequently lower amount of CH, which is a by-product of cement
hydration. This created a shortage of CH to react with the increased
amount of POFA, and thus reduced the overall pozzolanic reaction as
compared to the mixes with lower POFA content.