3.3. Compressive strength
According to the results in Table 6, it could be observed that
generally the OPSC specimens, regardless of the curing condition,
the compressive strengths attained at the age of 3-d were about
90% of the corresponding 28-d compressive strengths. The high
early strength gain of OPSC could be largely attributed to the low
water-to-binder ratio adopted in the study.
At the age of 1-d, it was found that the control mix M0 had
higher compressive strength compared to the other POFA-blended
OPSC mixes. The lower early age compressive strength of the POFAblended
concrete was due to the lower cement content of the
mixes, resulting in a low C3S content (Tangchirapat and
Jaturapitakkul, 2010). Moreover, as mentioned by Safiuddin et al.
(2011), the decrease in early age strength of POFA-blended concrete
was due to the slow pozzolanic activity of POFA.
When the curing period was increased to 3 d, it was found that
the POFA-blended OPSC had higher strength gain compared to the
control mix. For the AC regime, the strength gain from 1-d to 3-d
could be as high as 49% for the POFA-blended OPSC compared to
that of only 11% for the control OPSC. In the case of WC regime, the
corresponding strength gain was as high as 51% for POFA-blended
OPSC while it was only 18% for the control concrete. In addition,
the strength gain was higher when the POFA replacement levels
were increased. Similar observation was noticed when the curing
period was increased to 7 d and 28 d. This phenomenon was likely
to be caused by the pozzolanic reaction of POFA, which imparted
strength to the concrete at later stages due to the formation of CSH
gels upon reaction of the POFA with the CH produced from cement
hydration.
In terms of the effect of curing condition, it was noticed that the
28-d compressive strengths of all the OPSC specimens were
generally higher in the case of WC compared to other curing
regimes, while similar range of compressive strength was found in
the case of AC and PWC (Fig. 4). In the WC regime, the specimens
were kept in water throughout which ensured the continuation of
the hydration process efficiently and subsequently increased the
compressive strength. In addition, the presence of water moderated
the heat of hydration in the concrete, limiting the possibility of the
occurrence of internal cracking. The compressive strength of the
water-cured control OPSC specimens was about 18% higher
compared to the corresponding specimens subjected to AC and
PWC regime whereas the range of 5e14% was found in the case of
the POFA-blended OPSC. This indicates a possibility of reduction of
the sensitivity of OPSC subjected to poor curing in the presence of
POFA. As high content of cement was used in the production of the
OPSC, the low water-to-binder ratio prevented complete hydration reaction with POFA, thus improving the compressive strength
under such curing conditions. Since the ground POFA had greater
fineness than OPC, the filler effect of the ground POFA particles
could also reduce the sensitivity of the OPSC towards poor curing.
According to Fig. 4, there is an optimum amount of POFA
replacement level which contributed to the highest compressive
strength of OPSC. Generally, about 10e15% POFA replacement levels
gave rise to the compressive strength of OPSC compared to the
control concrete under all types of curing condition and the highest
28-d compressive strength of 42.4 MPa was obtained in water-cured
OPSC containing 10% POFA. Increment of up to 9% compared to the
OPSC without POFA could be observed in this investigation. The
increase in the compressive strength could be likely due to the
pozzolanic activity of POFA (Safiuddin et al., 2011). Besides that, the
finer ground POFA particles could impart packing effect and filled