Performance evaluation of wood ash-based geopolymer bricks activated with different sodium hydroxide concentrations and Si/Al ratios

Wood ash is a potentially useful aluminosilicate precursor for alkali-activated masonry products, although its variable composition and limited reactivity can restrict engineering performance. This study investigated wood ash-based geopolymer bricks prepared with theoretical precursor Si/Al molar ratios of 1, 2 and 3 and sodium hydroxide concentrations of 8, 12 and 16 M. The target precursor ratios were established before mixing through calculated additions of SiO₂ or Al₂O₃. Compressive strength, bulk density, porosity, water absorption, sulphuric acid resistance and microstructural characteristics were evaluated. Increasing NaOH molarity generally improved the measured properties, and mixture S2W16 exhibited the best overall performance, attaining a compressive strength of approximately 14.6 MPa, bulk density above 1650 kg/m³ and water absorption below 12%. SEM observations indicated a comparatively dense matrix at higher molarity, while localised EDS results were treated as semi-quantitative compositional indicators. The results demonstrate the technical potential of wood ash for producing geopolymer masonry units under the investigated laboratory conditions. However, life-cycle, embodied-carbon, energy and techno-economic assessments are required before environmental or commercial superiority can be claimed.