1.1 Water absorptionWater absorption increases proportionately with the POFA inclusion in foamed concrete mixes. Figure-8 shows that foamed concrete with POFA content beyond 20% absorbed more water compared to the control specimen.
The water absorption readings of LFC-20, LFC-30, LFC-40, LFC-50 and LFC-60 were 19.96%, 24.50%, 30.
36%, 32.55% and 46.88%, respectively. These values are higher than the control specimen which only obtained 8.63% of water absorbed.
The proportion of POFA as cement replacement contributes to the increasing water absorption readings due to the increasing the number of micro-pores because of the POFA particle which has the tendency of taking in water when submerged. 1.2 PorosityPorosity is a major component of the microstructure that is mainly caused by loss of water. Porosity value gives a strong influence on strength and durability of concrete (Li, 2011). The result of porosity of hardened foamed concrete with various content of POFA for curing duration between 7 days up to 56 days is presented in Figure-9. Based on the porosity result obtained, it was observed that all the mixes experienced lower porosity readings with increasing age. The control specimen C100 has higher porosity readings compared to foamed concrete containing up to 50% POFA as the cement replacement, however, the porosity increase with the POFA content beyond 50% at the same age.
At 7 days, the control specimen C100 obtained 60.13% and reduces to 53.85% at 28 days. This reduction in porosity is occurring regardless of the hydration process of cement paste and the existence of POFA content in the foamed concrete mix. The decrease in porosity readings is a result of the reduced amount of pores within the paste of foamed concrete due to the pozzolanic reaction occurring between SiO2 and Ca(OH)2. The high fineness POFA also contributes to the faster pozzolanic reaction in.
The small particles showed a good filler effect in reducing the voids of cement paste (Isaia et al., 2003). These results suggest that the addition of fine particles of POFA makes the blended cement paste denser.
Altwair et al., (2011) found that by increasing the amount of fine POFA in cement paste, small pores form in the structure, and C-S-H covers almost the entire fractured surface. The majority of the large spaces were filled with C-S-H gel, thus creating a dense structure.
Therefore, the microstructure of the paste became denser. 1.3 Microstructural and elemental content analysisFigure-10 presents the micrograph of foamed concrete containing POFA up to 60% at 28 days curing age. From the SEM image, it can be seen that the major hydration and reaction products of foamed concrete containing POFA are calcium silicate hydrate (C-S-H), calcium hydroxide (C-H) and ettringite (AFt).
Li (2011) stated that the C-S-H occupies about 50-70% of the structural component of cement past while C-H only occupies about 25%. From the SEM image, it can be seen that the C-S-H component is introduced in all foamed concrete mixtures, but only the control mix C100 shows the existence of ettringite. The ettringite here contributes to the early strength development of concrete since the needle-shaped crystals can work as the reinforcement for surrounding C-S-H before the paste has hardened (Li, 2011).The EDX result on foamed concrete at 10.000x magnifications (Table-4) shows the various ratios of silicon/alumina (Si/Al), calcium/silicon (Ca/Si) and calcium/alumina (Ca/Al). The aforementioned ratios also contribute to the properties of foamed concrete. As can be seen, the Si/Al ratio increases by increasing the POFA replacement level. The LFC-20 obtained Si/Al ratio of 3.
78, higher than the control sample C100 which only obtained 3.37. However, the ratio decreased by increasing POFA to 30-40%, which is obtained 3.
37 and 3.04, respectively. Even so, the ratio values increase when POFA added in the level of 50-60% which is 4.42 and 7.08, respectively. The high Si/Al ratio responsible for the low initial strength of foamed concrete in the early age of strength development (Salami et al., 2016).
Hence, it proves the reason why the LFC-50 and LFC-60 having a lower compressive strength compared to the control specimen.The LFC-20 obtained Ca/Si ratio of 5.81, a slightly higher compared to the control mix C100 which is 5.71, however, the Ca/Si ratio decreases until up to 60% of POFA replacement level.
The Ca/Si ratio having a similar trend with the compressive strength of POFA foamed concrete, where the LFC-20 having the highest compressive strength of all the mixtures and decrease with the increasing POFA replacement level. The Ca/Si ratio probably varying from one area to another, and influenced by the composition and reaction in the material, especially when additives are used (Li, 2011). Additionally, the high amount of C-S-H and its small size is believed in provides the major strength and mechanical properties and also improve the porosity value of cement based concrete. 2.
ConclusionsThe conclusions are drawn from this study and summarized as below:· Foamed concrete with 20% POFA content gained the highest compressive strength of all the mixes which gained compressive strength of 3.20MPa at the ages of 28 days· Due to the lower specific gravity and its porous particle, the increasing amount of POFA content reduces the foam content in the mixes due to the low specific gravity of POFA compared to that of cement· Replacing cement up to 40% by POFA contributes to the pore structure refinement and mechanical strength compared to the control specimen. However, the inclusion of POFA beyond 40%, the performances of foamed concrete tend to be weakened.· The SEM micrograph of the mix with 20% POFA exhibited a dense microstructure.· The Ca/Si and Si/Al ratio of foamed concrete containing 20% POFA is higher compared to the control specimen.
However, the ratio value is decrease with the increasing amount of POFA replacement level.· POFA with only grinding treatment has potential to be used as partial cement replacement in foamed concrete· The inclusion of POFA in foamed concrete could be an improvement of building technology in reducing cement demand and at the same time, reducing environmental deterioration caused by ash waste