Waste materials are abundant and often act as slow environmental contaminants, creating severe ecological challenges. With rapid industrialization, electricity demand has increased substantially, and in India, coal-based thermal power plants (TPPs) remain the dominant source of power generation. Coal combustion produces two major by-products: fly ash and bottom ash (BA). While fly ash is widely utilized in blended cements due to its pozzolanic nature, BA has received comparatively limited attention despite having similar chemical characteristics. Owing to its coarser particle size, BA shows strong potential as a substitute for natural river sand, the excessive extraction of which has led to severe resource depletion and sustainability concerns. Unlike previous studies that focused on single-source BA or limited performance evaluation, this study investigates the use of BA from multiple sources to develop resource-efficient bottom ash concrete (BAC). Concrete mixes containing 0%, 20%, 35%, and 50% BA as volumetric replacements of river sand were evaluated for their fresh, mechanical, durability, and microstructural properties. The results indicate that BA significantly influences concrete performance due to its porous structure. Among the investigated mixes, 35% river sand replacement with BA showed the most favorable performance for the specific materials and sources used in this study, achieving up to 17.46% higher compressive strength and up to 16.14% higher resistance to transport-related properties at 90 days. Microstructural analysis confirmed the formation of secondary C–S–H gel, which enhanced matrix densification. However, 50% replacement resulted in reduced performance. The findings demonstrate that BA can be effectively utilized in concrete at replacement levels of up to 35% as a sustainable substitute for river sand under the investigated material conditions.

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.

As one of the primary building components exposed to fire, mortar plays a crucial role in the initial thermal response of building envelopes; therefore, its thermal stability and residual mechanical performance under elevated temperatures are significant. This study investigates the performance of mortar incorporating supplementary cementitious materials (SCMs) under elevated temperature exposure, aiming to improve thermal resistance, promote sustainability, and reduce CO\(_2\) emissions. Mortar mixtures were prepared with a constant water-to-binder ratio of 0.485 and a binder-to-sand ratio of 1:2.75. A control mix (C1) consisting of 100% Portland pozzolanic cement (PPC) was compared with fly ash (FA)-based mixtures (30–70% cement replacement) and FA–ground granulated blast furnace slag (GGBS) blended mixtures (10–50% FA with a constant 20% GGBS replacement). The specimens were cured for 7 and 28 days and evaluated for compressive strength before and after exposure to elevated temperatures of 400, 600, and 800 \(^\circ \)C. The results reveal that increasing FA content reduces ambient compressive strength by up to \(\sim \)54%; however, it enhances high-temperature performance, resulting in a strength gain of up to \(\sim \)33.7% after exposure to 400 \(^\circ \)C. The incorporation of 20% GGBS improves early-age strength development and mitigates strength deterioration at 400–600 \(^\circ \)C. Nevertheless, all mixtures undergo severe degradation at 800 \(^\circ \)C, with an average strength loss of \(\sim \)81.5%, attributed to extensive microcracking and the decomposition of major hydration products, as confirmed by microstructural analysis. The FA–GGBS blended mortars exhibit superior thermal stability and residual strength retention, demonstrating their potential as sustainable alternatives for fire resistant construction applications.