Strength and Microstructure of Palm Oil Fuel Ash–Fly Ash–Soft Soil Geopolymer Masonry Units

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.