Sustainable production of lightweight geopolymer from mixed fly ash: Effects of alkali activators and surfactants

Upcycling fly ash is a key challenge in supporting the circular energy production of biomass power plants. Using a mixture of palm ash (PA) and rubberwood ash (RA) in the production of lightweight geopolymer offers an innovative pathway for converting industrial waste into a valuable construction material. The study demonstrates that up to 40% of mixed PA and RA can be utilized for metakaolin (MK) replacement in lightweight geopolymer production. The geopolymer formulation was examined at various mass ratios of MK:PA:RA using sodium hydroxide or potassium hydroxide combined with sodium silicate as alkaline activators. The suitable formulation was further applied for lightweight geopolymer production, comparing two types of surfactants—anionic and nonionic surfactants. Combining anionic and nonionic surfactants at a mass ratio of 1:3 resulted in a more stable foam than using a single surfactant alone. The formulated lightweight geopolymer has a density of 1,544 kg/m³, a compressive strength of 5.41 MPa, and a water absorption rate of 23.61% by weight, meeting the standard for lightweight non-load-bearing concrete, with a dry density below 1680 kg/m³ and an average net-area compressive strength above 4.14 MPa, as specified in ASTM C129. Its thermal conductivity was 0.741 W/m·K, making it suitable for thermal insulation.