Type: Process Essays
Sample donated: Alma Schwartz
Last updated: September 21, 2019
1. Resultand Discussion1.1 Properties of fresh concreteFromthe result obtained, it was observed that the foamed concrete containing 60%POFA content achieved the lowest slump reading of all the mixes, which only165mm, even though having the highest water content of 284.51kg/m3.Meanwhile, foamed concrete containing 20%, 30%, 40% and 50% POFA obtained thesame slump value of 220mm.
This indicates that the increasing amount of POFAcontent added in concrete mixture will reduce the workability and significantlyincreased water demand (Figure-4). The increased water required to achieve therequired spread is due to porous nature and the angular and irregular in shapedof 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 contenthas higher water demand than POFA foamed concrete up to 50%, the slump valuewas lower.
The low slump reading of the control is considered caused by theabsence of superplasticizer as water reducer. With additional ofsuperplasticizer, concrete which having the same workability can be mixed and combinedat lower w/c ratio, hence, the strength of the concrete is improved due to thereduction water demand (Li, 2011). Inthe foamed concrete mixture studied, the amount of foam dropped with theincreasing the POFA content. The foam content decreased from 0.065m3for C100 to 0.
054m3 for LFC-60. This happened due to the lowerspecific gravity and low density of POFA compared to that of cement. Hence,when replacing cement by POFA, the actual mortar density drops, leads to theless foam required to achieve the targeted plastic density. This also agreed byAwang and Al-Mulali (2016)who observed the effect of sieved only POFA as cement replacement in foamedconcrete. It found that the amount of required foam content to achieve thetargeted plastic density was reduced with the increasing POFA content.
1.2 Compressive strengthThecompressive strength development of foamed concrete with various percentage ofPOFA for curing duration between 7 days up to 56 days are presented inFigure-5. The results of control specimens are concrete with 100% cementcontent.
By referring to the results obtained, it was found that foamedconcrete containing up to 40% POFA obtained the higher compressive strength atall the ages compared to the control specimen, where the magnificent resultobtained by foamed concrete with 20% POFA content, LFC-20. At 28 days, theLFC-20, PFC-LFC-30, LFC-40, LFC-50 and LFC-60 obtained compressive strength of3.21MPa, 2.
30MPa, 2.14MPa, 1.19MPa and 0.61MPa, respectively, or about 241%,173%, 161%, 89% and 46% of the control C100 which having compressive strength of1.33MPa at the same age (Table-3).
These result also agreed by Liu et al., (2014)who mentioned that compressive strength increased with increases in POFAcontent up to 20%; however, a higher replacement of POFA caused in asignificant reduction of strength. Moreover, additional superplasticizerand silica fume in the mixture also contributes to the higher strengthdevelopment of foamed concrete containing POFA.
Superplasticizer reduces waterdemand, hence, avoids the segregation experienced during mixing caused by highwater/cement ratio. Meanwhile, additional silica fume or microsilica contributes to making cement paste denser becausesilica fume provides finer particle than cement, hence, it can pack thewater-filled pockets among the cement particles (Li, 2011). The compressive strength of foamedconcrete containing POFA increases with curing time but decreases with anincrease of POFA replacement level, this attributed to the pozzolanic reactionoccurred when POFA introduced. In cement hydration process, the hydrationreaction occurs due to the chemical constituent in cement and water, while thepozzolanic reaction occurs due to the calcium hydroxide (Ca(OH)2)from cement hydration process reacts with silicon dioxide (SiO2)and aluminium oxide (Al2O3) from POFA, hence, produced anincrease in calcium silicate hydrate or C-S-H. The low initial strength of POFA mixesmight be related to the high silicon/aluminum (Si/Al) ratio in the POFA matrix, which isresponsible for less Al(OH)44- availability forcondensation in the early age of strength development (Salami et al., 2016).Moreover, the particle size of POFA also gave a contribution to the strengthbehavior of foamed concrete. The fineness of POFA had a greater filler effecton the void and increased the SiO2 content which then reacts withCa(OH)2 from cement hydration, hence, increasing the C-S-H content.
Additional silica fume in the mixture also strengthen the POFA foamed concrete,because it increases the C-S-H content. 1.3 Flexural StrengthTheaverage values of the flexural strength of all the specimens are shown inFigure-6. The flexural strength of foamed concrete is much lower than thecompressive strength which also decreasing linearly in line with the additionof POFA but increasing throughout the ages.
Generally, the flexural strengthdevelopment has the similar trend with compressive strength development, wherefoamed concrete with 20%POFA content obtained the highest strength of all themixes. At 28 days, foamed concrete with up to 50% POFA as cement replacementobtained the flexural strength of 0.97MPa, 0.96MPa, 0.
90MPa and 0.88MPa,respectively, higher than the control specimen C100 which only gained 0.59MPaat the same age. Meanwhile, foamed concrete containing 60% obtained 0.432MPa,the lowest flexural strength of all the mixture. 1.
4 Ultrasonic pulse velocity (UPV)Theultrasonic pulse velocity readings of foamed concrete with various percentageof POFA for curing duration between 7 days up to 56 days are presented inFigure-7. The result reveals that the UPV readings tend to decrease withincreasing POFA replacement levels; however, the foamed concrete with 100%cement as binder obtained so much lower readings compared to the foamedconcrete with POFA as cement replacement beyond 20%. At 28 days, the controlC100 obtained the UPV readings of 1630m/s while LFC-20, LFC-30, LFC-40, LFC-50and LFC-60 achieved 2090m/s, 2060m/s, 2015m/s, 1940m/s and 1075m/srespectively. This highest UPV reading of LFC-20 dueto the reducing water/cement ratio and the foam quantity during mixing, hencecreating a denser, stronger and more refined microstructure compared to thecontrol specimen. This condition contributes to the similar trend with thestrength development of POFA foamed concrete.The deterioration in UPV readings withincreasing POFA replacement level is due to the increased amount of micro-poreswithin the paste. However, the UPV readings also increase over time. Theenhancement happened because of the reduction of pores volume due to the pozzolanicreaction creating more C-S-H which makes the paste denser.
The higher readingof the pulse’s speed relates to the higher quality of the foamed concrete.