Selected properties and life cycle assessment of mortars incorporating municipal solid waste bottom ash and silica fume for sustainable construction

Abstract

The use of municipal solid waste bottom ash (MSWBA) as a sustainable alternative construction material is an effective approach to reduce natural resource depletion and advance circular economy concepts. The combined effects of MSWBA as a partial replacement of natural fine aggregate and silica fume (SF) as a supplementary cementitious material on the engineering performance and environmental sustainability of mortar were investigated in this study. Mortar mixtures were prepared with different binder content (389.6–524.9 kg/m 3 ), water to binder (w/b) ratio (0.526–0.663), MSWBA replacement level (60%–90%) and silica fume content (2%–8%). Ten mortar mixtures were prepared, including one control and nine modified mixtures. Tests for workability, setting time, compressive strength, flexural strength, ultrasonic pulse velocity (UPV), unit weight and water absorption were carried out for fresh and hardened properties evaluation. Correlation analyses, contour analysis and life cycle assessment (LCA) were also performed. The results showed that optimizing the w/b ratio produced satisfactory workability for all the mixtures. Although the early-age mechanical performance was reduced with high MSWBA replacement, mixtures with higher binder contents and silica fume had higher strength, density and durability. Among the MSWBA-modified mixtures, T4, T7, and T8 exhibited the most favorable overall combination of mechanical strength, UPV, and water-absorption performance. These optimized mixtures achieved comparatively high compressive and flexural strengths and UPV while maintaining relatively low water absorption, suggesting that the incorporation of silica fume helped mitigate the adverse effects associated with the porous nature of MSWBA. Correlation and contour analyses indicated that lower w/b ratios, particularly approximately 0.53–0.55, were associated with more favorable compressive-strength performance within the investigated mixture range. Results of the environmental assessment showed that the replacement of natural sand by MSWBA reduced the cradle-to-gate embodied carbon by up to 21.0%. This suggests the potential of optimized MSWBA–SF mortars to produce sustainable and low-carbon cementitious materials without compromising engineering performance.