Thermal stability performance of sustainable mortar incorporating construction and demolition waste as fine aggregate

The construction industry faces increasing environmental challenges due to the overexploitation of natural sand and the growing volume of construction and demolition (C&D) waste. Utilizing recycled sand (RS) derived from C&D waste offers a sustainable alternative, contributing to resource conservation and circular economy practices. This study examines the feasibility of using RS as a substitute for natural sand (NS) in cement mortar, with a focus on thermal stability at elevated temperatures. Five mortar mixes were prepared by substituting NS with RS at 0%, 25%, 50%, 75%, and 100% by weight. Comprehensive experimental assessments were conducted, including compressive strength testing at ambient and elevated temperatures (100-700 °C), water demand measurements, mass loss analysis, and microstructural evaluations using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Results reveal a linear increase in water demand, rising by 46.33% from RS0 to RS100, attributed to the high porosity and angularity of RS particles. At ambient temperature, the compressive strength of mortar specimens decreased progressively with increasing RS content, showing a 55.23% reduction at RS100. Upon exposure to 100 °C, all mixes showed a marginal improvement in strength due to secondary hydration, with RS0 exhibiting a peak increase of 7.95%. Beyond 100 °C, thermal degradation intensified, with RS100 showing a 94.25% reduction in strength at 700 °C. SEM-EDX analyses demonstrated the formation of a more extensive calcium silicate hydrate (C–S–H) gel at 100 °C, correlating with improved microstructural densification. Mass loss increased with temperature and RS content, with RS100 experiencing 16% loss at 700 °C, compared to 11% in RS0. Overall, the study concludes that partial replacement of natural sand with up to 25% RS is technically feasible for producing thermally stable and eco-friendly mortar, whereas higher replacement levels significantly compromise mechanical and thermal performance. The statistical analysis of experimental data was performed using ANOVA. The summary of the results showed that temperature is a more sensitive parameter than the mix ID. These findings highlight the potential of C&D waste utilization in promoting sustainability in the construction sector.