1. Materials and Experimental details
1.1 Concrete materials
Portland composite cement conforming to MS 522-1:2007 from a single source was used as the binder. POFA used was collected from a local palm oil mill located in Nibong Tebal, Penang, Malaysia. The collected ashes were oven dried in the temperature of 105±50C for 24hours before then sieved passing 300µm sieve to remove larger particle as well as to reduce carbon content. The sieved POFA was then passed through the grinding process to produce finer ash particle.
Saturated surface dry river sand passing through 600µm (Figure-1) sieve with a fineness modulus of 1.35 and specific gravity of 2.74 was used as fine aggregate. The larger particles of sand were removed to avoid it settle in a lightweight mix resulted the foam will collapse during mixing (Brady et al., 2001). A protein based foaming agent was used with the dilution ratio of 1:30 and was then aerated to produce a stable foam having a density of 65kg/m3 using a portafoam machine with the air pressure of 70-75Psi. Tap water complies with BS EN 1008, polycarboxylate based superplasticizer (SP) and silica fume were used as the chemical admixture with a fixed dosage of 1% and 5% by weight of the binder, respectively, to improve workability and strength of concrete.
1.2 Characterization of POFA
POFA used in this study was a by-product of incinerated palm oil biomass at the temperature exceeding 10000C. The high burning temperature can remove the POFA’s unburned carbon. The unburned carbon particles in POFA result in the increasing water requirement and dosage of superplasticizer (SP) because carbon particles absorb SP (Sata et al., 2007).
After the sieving, drying and grinding process, the median particle size (d50) of ground POFA is 4.03µm, indicating that POFA has finer particle size than cement with d50 of 4.92µm (Figure-2). Based on the chemical composition (Table-1), POFA contains 62.16% of pozzolanic materials (SiO2 + Al2O3 + Fe2O3) and LOI reading at 5.66%, hence, the POFA can be classified as between Class C and Class F pozzolana (ASTM C618). The X-ray diffraction (XRD) analysis of POFA showed that the major crystalline phase is ?-Quartz (SiO2) which having the highest peak and it was identified there a minor crystalline phase of Gehlenite (Ca2Al(AlSiO7)), as shown in Figure-3.
Figure-3: X-ray diffraction patterns of POFA
1.3 Mixture Proportions and casting
Table-2 shows the proportions of the foamed concrete mixes where POFA used as a partial cement replacement in the level of 20% to 60% by the weight of the total binder, with the control of 100% cement content. The foamed concrete mixes having a target density of 900kg/m3 and a mix ratio of (1:1.5).
The fine sand and binder (cement and POFA) are mixed in a concrete mixer until completely mixed. Water is added gradually together with superplasticizer to the homogeneous mix until the spread achieved 160 to 240mm). The actual mortar density is measured by weighing a 1-liter cup of mortar to calculate the amount of foam before added to the mix. The actual wet density of the resultant mix is checked which should be equal to the targeted wet density of 1033kg/m3. The mix then poured into the moulds. After casting for 24hrs, the specimens were removed from the moulds and wrapped with plastic cling (film) until the testing age. This curing regime is called as sealed curing which typically practices followed by industry of foamed concrete (Jones and McCarthy, 2005).
1.4 Assessment of POFA foamed concrete properties
Cubes specimens of 100 x 100 x 100mm were cast to test for the compressive strength of foamed concrete. The specimen was dried in an oven at temperature 105±5ºC for 24 hours prior to testing day until constant weight was achieved. The compressive strength was determined according to BS EN 12390-3 (BS, 2009). The prism specimens with the dimension of 100mm x 100mm x 500mm were cast for the flexural test. The test conducted according to BS EN 12390-5 (BS, 2009). Cylinder specimens of 45mm in diameter and 50mm in height were used for porosity test. Each test has been conducted at the ages of 7, 14, 28 and 56 days. For microstructure analysis, the foamed concrete specimens were analyzed by SEM and EDX analyses at the age of 28 days. The SEM analysis was performed to investigate the micro-pore characteristic while EDX to investigate their chemical composition. The sample for investigation was obtained by cutting the specimens into smaller pieces.
2. Result and Discussion
2.1 Properties of fresh concrete
From the result obtained, it was observed that the foamed concrete containing 60% POFA content achieved the lowest slump reading of all the mixes, which only 165mm, even though having the highest water content of 284.51kg/m3. Meanwhile, foamed concrete containing 20%, 30%, 40% and 50% POFA obtained the same slump value of 220mm. This indicates that the increasing amount of POFA content added in concrete mixture will reduce the workability and significantly increased water demand (Figure-4). The increased water required to achieve the required spread is due to porous nature and the angular and irregular in shaped of POFA particle causing the absorption of the higher amount of water (Awang and Al-Mulali, 2016). However, even though the control foamed concrete containing 100% cement content has higher water demand than POFA foamed concrete up to 50%, the slump value was lower. The low slump reading of the control is considered caused by the absence of superplasticizer as water reducer. With additional of superplasticizer, concrete which having the same workability can be mixed and combined at lower w/c ratio, hence, the strength of the concrete is improved due to the reduction water demand (Li, 2011).
In the foamed concrete mixture studied, the amount of foam dropped with the increasing the POFA content. The foam content decreased from 0.065m3 for C100 to 0.054m3 for LFC-60. This happened due to the lower specific gravity and low density of POFA compared to that of cement. Hence, when replacing cement by POFA, the actual mortar density drops, leads to the less foam required to achieve the targeted plastic density. This also agreed by Awang and Al-Mulali (2016) who observed the effect of sieved only POFA as cement replacement in foamed concrete. It found that the amount of required foam content to achieve the targeted plastic density was reduced with the increasing POFA content.