Development of sustainable geopolymer paving blocks from palm oil fuel ash and fly ash: Mechanical and microstructural evaluation

This research investigates the development of geopolymer pavement blocks using fly ash (FA) and palm oil fuel ash (PA) as aluminosilicate source materials, activated with 15 M sodium hydroxide (NaOH). FA, obtained from a thermal power plant, and ground PA, collected from a biomass power plant, were mixed at varying ratios to determine optimal mechanical performance. The compressive strength of geopolymer pastes was first evaluated, with the 75FA25PA mix achieving the highest strength and selected for block production. Pavement blocks were produced using different binder-to-aggregate ratios, including quarry dust and sand. Mechanical properties including compressive strength, flexural strength, and water absorption were tested at 7 and 28 days. The optimal mix, 75FA25PA with a quarry dust to sand ratio of 3:1, achieved a flexural strength of 2.6 MPa, meeting the Thai Industrial Standard TIS 378-2531 for outdoor concrete tiles. Microstructural characterization by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) revealed effective bonding between the geopolymer matrix and aggregate particles. Elemental mapping confirmed the formation and distribution of sodium aluminosilicate (NASH), calcium aluminosilicate hydrate (CASH), and calcium silicate hydrate (CSH) gels, associated with the presence of Si, Al, Ca, and Na. The results confirm that PA can be used to partially replace FA in geopolymer formulations for non-structural outdoor applications. The study contributes to sustainable construction practices by utilizing industrial and agricultural waste in value-added building materials.