Effect of low volume coal bottom ash as partial sand replacement on the strength, durability, and microstructural properties of PPC based concrete

Abstract India’s continued reliance on coal-fired power generation produces nearly 60 million tonnes of coal bottom ash (CBA) annually due to the high ash content of domestic coal, creating significant disposal and environmental challenges. Concurrently, excessive extraction of natural river sand has resulted in resource depletion and ecological degradation, highlighting the need for alternative fine aggregates. Although coal fly ash has been widely utilized in the manufacture of Portland pozzolana cement (PPC), the use of CBA as a fine aggregate replacement in PPC-based concrete remains comparatively underexplored. This study evaluates the feasibility of incorporating CBA as a partial replacement for natural sand in PPC-based concrete. Sand was replaced with CBA at 0%, 10%, 20%, 30%, 40%, and 50% by volume, and the effects on workability, strength, durability, and microstructural properties were investigated. Increasing the CBA content reduced workability owing to its porous texture and higher water absorption. Within the investigated replacement levels, concrete containing 10–20% CBA exhibited the most favourable overall performance. At 28 days, compressive strength increased by 11.30% and 10.87%, while splitting tensile strength improved by 5.17% and 8.62% for the 10% and 20% replacement levels, respectively, compared with the conventional concrete (CC). Microstructural observations indicated a denser cementitious matrix and refined pore structure at moderate CBA replacement levels. In contrast, higher replacement levels resulted in a weaker interfacial transition zone and reduced mechanical performance. The results demonstrate that replacing natural sand with up to 20% CBA can produce PPC-based concrete with satisfactory engineering performance while promoting the beneficial utilization of an industrial by-product. Quantitative sustainability assessments remain a subject for future investigation.