A systematic review of agricultural ash supplementary cementitious materials as low-carbon alternatives to ordinary Portland cement from 2005 to 2025

The cement industry contributes almost 8% of global CO₂ emissions, creating an urgent need to develop scalable low-carbon substitutes for ordinary Portland cement (OPC). Supplementary cementitious materials (SCMs) based on agricultural ashes have emerged as potential candidates, but comparative evidence across the various types of ashes remains scattered and inconsistent. This systematic review synthesised 47 peer-reviewed experimental studies published between 2005 and 2025 using six agricultural ashes, including rice husk ash (RHA), sugarcane bagasse ash (SCBA), palm oil fuel ash (POFA), corn cob ash (CCA), bamboo leaf ash (BLA) and wood ash (WA). Quantitative synthesis revealed that there was a strong positive correlation between the content of SiO₂ and the compressive strength of 28 days ( r = 0.73, p = 0.001), and a significant negative correlation between loss on ignition and 28-day compressive strength ( r = -0.68, p = 0.001). Significant inter-ash differences were confirmed by one-way ANOVA (F (5,184) = 8.43, p < 0.001, η² = 0.186). The meta-regression was able to explain between-study variance (R²= 0.71), with silica content, fineness and curing age as the major predictors of performance. RHA was found to show the most consistent improvement with an average 28-day strength ratio of 1.16 compared to OPC and estimated CO₂ reductions of up to 22% at 20% replacement. A three-level reactivity classification framework is suggested that will inform material selection and standardisation. The lack of durability data in aggregated aggressive conditions, inconsistent activation procedures and little lifecycle cost data are, nonetheless, significant impediments to massive deployment.