Strength and Micro-Structural Characteristics of Concrete Incorporating Rice Husk Ash and Locust Bean Waste Ash as Partial Replacements for Cement

Abstract

This study evaluates the potential of Rice Husk Ash (RHA) and Locust Beans Waste Ash (LBWA) as sustainable partial replacements for cement in concrete. Emphasis was placed on material properties, mechanical performance, optimal blend ratios, and microstructural behaviour. Oxide composition analysis showed that RHA contains 86.59% combined silica, Alumina, iron oxide surpassing ASTM C618 minimum standards of 70% for class F pozzolanic materials, while LBWA is rich in potassium oxide (K₂O) 42.98% and magnesium oxide (MgO) 21.63%, both of which contribute to early hydration. A total of 56 concrete mixtures were developed using Scheffé’s simplex lattice optimization technique and tested, with a ternary blend of 7% LBWA and 20% RHA (Mix R246) exhibiting the highest compressive (16.25 N/mm²) and flexural (1.71 N/mm²) strengths among the modified mixes, indicating a synergistic interaction between the ashes. Microstructural analysis using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) revealed that RHA promotes matrix densification via secondary calcium silicate hydrate (C-S-H) formation, whereas LBWA contributes to pore refinement but remains limited by its coarse particle size. The study’s key contributions include: validating RHA and LBWA as effective supplementary cementitious materials (SCMs), identifying an optimal blend ratio that balances strength and sustainability, and demonstrating the superior pozzolanic reactivity of RHA compared to LBWA. These findings support the reduction of cement content in low- to mid-strength concrete applications, such as pavements and masonry, while promoting the valorisation of agricultural waste. Future research should explore durability performance, chemical activation of LBWA, and feasibility of large-scale implementation.