Experimental and Predictive Modelling Assessment of Spear Grass Ash as a Partial Cement Replacement in Concrete

The global construction industry is under increasing pressure to minimize its environmental impact, especially that of cement manufacture, which contributes approximately 5 to 8% of anthropogenic CO2 emissions. This study investigates Spear Grass Ash (SGA) as a partial replacement for Ordinary Portland Cement (OPC), focusing on workability, compressive strength, and split tensile strength. OPC was replaced with SGA at 0% to 30% by weight in 5% increments, at a constant water-to-cement ratio of 0.50. The ash was characterised by X-ray fluorescence (XRF), and both strengths were tested at 28 and 56 days. XRF confirmed a combined SiO2, Al2O3 and Fe2O3 content of 75.85%, satisfying the ASTM C618 requirement for a natural pozzolan. Workability fell markedly with SGA content, slump reducing from 75 mm to 5mm at 30% SGA. Compressive strength declined with SGA content; the control achieved 23.85 N/mm2 at 28 days, and no blended mix exceeded it, the 5% mix retaining about 90% of the control and strength falling to 12.65 N/mm2 at 30% SGA. Split tensile strength peaked at 10% SGA, reaching 2.21 N/mm2 at 28 days and 2.40 N/mm2 at 56 days, modestly exceeding the control. Quadratic response surface models fitted both properties well, achieving R2 of 94.78% for compressive and 94.05% for tensile strength. Scanning electron microscopy of the 20% SGA concrete revealed a dense matrix with well-developed calcium silicate hydrate gel, validating the strength results. Overall, SGA offers no compressive advantage but maximises split tensile strength at about 10%, suiting tension-sensitive applications such as pavements and slabs.