56
H. Asrah et al.
the brick denser and may contribute for better strength development compared to
unground POFA brick.
It was also noticed that the compressive strength of both compressed unground
and ultrafine POFA brick samples decreased with the increasing amount of POFA.
The likely factor in the reduction of strength for the brick at high POFA content is
possibly due to the lower amount of calcium silicate hydrate (CSH) formed. Since
POFA is categorized as a pozzolanic material, the strength gain of the resulting brick
is dependent on the reaction of silica (from POFA) with cement hydration (CH) to
form CSH. However, at high amount of POFA, there may be an insufficient amount
of CH formed from the cement hydration to react with the silica (Mo et al. 2017).
Hence, it limits the amount of CSH produced and resulted in lower strength of brick.
Nevertheless, both compressed unground and ultrafine POFA brick samples, and
control brick produced in this research have exceeded and satisfied the minimum
limits of 5 MPa as defined by BS 3921:1985. On the other hand, MS 76:1972 specifies
that the minimum strength required to produce load bearing brick is 7 MPa. Therefore,
all compressed brick samples produced in this research can be classified as load
bearing brick ranging from Class 1–Class 5. These results also indicated that POFA
has significant potential to be used as materials in the ICB production, especially
when it is crushed to ultrafine size.
(ii) Density Test
As illustrated in Fig. 4, the densities of compressed ultrafine POFA brick samples
were higher than the compressed unground POFA bricks at all curing ages. However,
the density of compressed ultrafine POFA brick was slightly lower than the control
brick at 24 days curing when 30 and 40% of ultrafine POFA were used in the mix.
With highest value in the chart, this indicated that the compressed ultrafine POFA
brick has a good pozzolanic reaction within the sample. The pozzolanic reaction
produced a secondary gel to interlock the bonding between the particles and enabled
a production of denser brick which was high in compressive strength. With finer size,
the ultrafine POFA provides a better filler effect, which filled up pore voids between
the particles and decreases the internal pores. The increment of the density is also due
to the fact that the finer POFA boosted the pozzolanic reaction with the by-product of
hydration to produce secondary CSH gel (Kroehong et al. 2011). The large amount of
CSH gel provides a path for the densification of brick, thus assisting pore refinement
to produce higher density brick as well as the brick strength. However, with an
increased in the amount of POFA, both ultrafine and unground POFA compressed
brick had shown a reduction in the density. This is probably due to the dilution effect,
which resulted from the excess amount of the POFA at fixed cement content. The
degree and the rate of hydration is reduced due to the increased amount of POFA
in total weight of paste, resulting in gradual reduction of the CH content and finally
leads to the arising issue of the reduction of CSH gel (Altwair et al. 2013).
(iii) Water Absorption
The results shown in Fig. 5 reveal that all compressed brick made with ultrafine
POFA (5.8–15.4%) has lower water absorption than those bricks made from
H. Asrah et al.
the brick denser and may contribute for better strength development compared to
unground POFA brick.
It was also noticed that the compressive strength of both compressed unground
and ultrafine POFA brick samples decreased with the increasing amount of POFA.
The likely factor in the reduction of strength for the brick at high POFA content is
possibly due to the lower amount of calcium silicate hydrate (CSH) formed. Since
POFA is categorized as a pozzolanic material, the strength gain of the resulting brick
is dependent on the reaction of silica (from POFA) with cement hydration (CH) to
form CSH. However, at high amount of POFA, there may be an insufficient amount
of CH formed from the cement hydration to react with the silica (Mo et al. 2017).
Hence, it limits the amount of CSH produced and resulted in lower strength of brick.
Nevertheless, both compressed unground and ultrafine POFA brick samples, and
control brick produced in this research have exceeded and satisfied the minimum
limits of 5 MPa as defined by BS 3921:1985. On the other hand, MS 76:1972 specifies
that the minimum strength required to produce load bearing brick is 7 MPa. Therefore,
all compressed brick samples produced in this research can be classified as load
bearing brick ranging from Class 1–Class 5. These results also indicated that POFA
has significant potential to be used as materials in the ICB production, especially
when it is crushed to ultrafine size.
(ii) Density Test
As illustrated in Fig. 4, the densities of compressed ultrafine POFA brick samples
were higher than the compressed unground POFA bricks at all curing ages. However,
the density of compressed ultrafine POFA brick was slightly lower than the control
brick at 24 days curing when 30 and 40% of ultrafine POFA were used in the mix.
With highest value in the chart, this indicated that the compressed ultrafine POFA
brick has a good pozzolanic reaction within the sample. The pozzolanic reaction
produced a secondary gel to interlock the bonding between the particles and enabled
a production of denser brick which was high in compressive strength. With finer size,
the ultrafine POFA provides a better filler effect, which filled up pore voids between
the particles and decreases the internal pores. The increment of the density is also due
to the fact that the finer POFA boosted the pozzolanic reaction with the by-product of
hydration to produce secondary CSH gel (Kroehong et al. 2011). The large amount of
CSH gel provides a path for the densification of brick, thus assisting pore refinement
to produce higher density brick as well as the brick strength. However, with an
increased in the amount of POFA, both ultrafine and unground POFA compressed
brick had shown a reduction in the density. This is probably due to the dilution effect,
which resulted from the excess amount of the POFA at fixed cement content. The
degree and the rate of hydration is reduced due to the increased amount of POFA
in total weight of paste, resulting in gradual reduction of the CH content and finally
leads to the arising issue of the reduction of CSH gel (Altwair et al. 2013).
(iii) Water Absorption
The results shown in Fig. 5 reveal that all compressed brick made with ultrafine
POFA (5.8–15.4%) has lower water absorption than those bricks made from