Abstract
The cement sector is a major emitter of CO2, highlighting the need for sustainable alternatives to ordinary Portland cement. At the same time, agricultural residues such as banana biomass and bamboo leaves are largely underutilised, creating environmental burdens. While supplementary cementitious materials (SCM) support low-carbon construction, conventional sources are declining, and agro-waste binders remain poorly understood, particularly in terms of pozzolanic activity, calcination requirements, and cementitious performance. This study addresses these gaps by valorizing calcined banana and bamboo leaf ashes as eco-friendly SCMs, advancing circular resource utilisation and sustainable binder development. Ordinary Portland cement was partially substituted with banana leaf ash (BALA) and bamboo leaf ash (BLA) at replacement levels of 5, 10, 15, and 20%. The resulting mortar mixtures were systematically evaluated for their mechanical performance and microstructural characteristics. The results reveal that 5% BLA achieved the highest compressive strength (63.33 MPa at 14 days and 79.35 MPa at 28 days), whereas higher BLA and all BALA replacement levels reduced strength, with 20% BALA exhibiting the lowest values (45.40 and 61.17 MPa), confirming 5% BLA as the optimum sustainable cement replacement level. This indicating 5% BLA as the optimum replacement level under the present experimental conditions when compressive strength and microstructural densification considered together. However, 15% BLA minimized water absorption, indicating a trade-off between mechanical strength and resistance to water ingress. Microstructural analysis confirmed the higher pozzolanic reactivity of BLA through greater portlandite consumption, enhanced secondary C-S-H formation, and improved thermal stability, while BALA showed slower and weaker reactions. Excessive replacement (≥ 20%) resulted in clinker dilution, increased carbonation, and reduced matrix compactness, highlighting the need for controlled ash dosage.