Abstract

The application of palm oil fuel ash (POFA) and fly ash (FA) as an alumino-siliceous precursor to create a viable geopolymer binder for the manufacture of unfired lightweight masonry units, using soft soil (SS) as aggregate, was studied in this research. The POFA was a byproduct from a palm oil factory, while FA was a byproduct from a coal-fired electricity generation plant. Unconfined compressive strength (UCS) and the microstructure of POFA-FA-SS geopolymers were investigated. The optimal liquid alkaline activator (L) providing the maximum total unit weight was found to decrease with increases in the POFA replacement ratio for all Na2SiO3:NaOH ratios. The optimum Na2SiO3:NaOH ratio providing the highest strength for all heat conditions decreased with increasing POFA replacement because additional NaOH was required for leaching SiO2 from POFA for a geopolymerization reaction. The optimum Na2SiO3:NaOH ratios providing the highest UCS were found to be 90:10, 80:20, 70:30, and 60:40 for FA:POFA ratios of 100:0, 90:10, 80:20, and 70:30, respectively. The optimum heat condition was found to be 80°C for 48 h. The higher temperature of 90°C was not recommended because UCS gain was relatively small due to a substantial loss of moisture during heat curing. More time was required for the lower temperature of 70°C to accelerate the geopolymerization reaction. Microstructural analysis showed that the highest cementitious product, sodium aluminum silicate hydrate (N-A-S-H), was formed for the POFA-FA-SS geopolymers at the optimum Na2SiO3:NaOH ratios and heat conditions. The cementitious product decreased along with the FA:POFA ratio, which is associated with UCS reduction. Based on industrial standards in Thailand, the POFA-FA-SS geopolymer was found to be viable as an environmentally friendly nonbearing masonry unit at an optimum FA:POFA ratio of more than 80:20.