Effect of sewage sludge ash on high-temperature resistance, carbon emissions and economic performance of eco-friendly geopolymer mortar

The incineration and landfill of sewage sludge have raised significant environmental concerns. Simultaneously, the production of ordinary Portland cement (OPC) has caused high carbon emissions which hinder the sustainable development of the construction industry. To address these issues, this study proposes a green, low-carbon geopolymer mortar (GPM) that utilizes sewage sludge ash (SSA) as a partial precursor to replace OPC-based materials. The research systematically investigates the effects of varying SSA content (0 %, 5 %, 10 %, 20 %, and 30 % substituting ground granulated blast-furnace slag, GGBFS) on the durability, microstructure, and carbon emission performance of SSA-GGBFS based GPM after exposure to elevated temperatures (200°C, 400°C, 600°C, and 800°C). A comparative analysis with ordinary Portland cement mortar (OPCM) control samples was conducted to evaluate relative performance. High-temperature resistance was assessed through changes in surface morphology, mass loss and compressive strength. Mineralogical and microstructural evolution was characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). Results demonstrate that an optimal SSA proportion (10 %) enhances the high-temperature resistance of GPM, manifested as reduced mass loss and higher residual compressive strength. Additionally, characterizations such as SEM and MIP reveal that GPM improved the densification of the matrix due to the formation of C-A-S-H and N-A-S-H gels. Furthermore, the carbon emission assessment based on per-unit compressive strength confirms that GPM outperforms OPCM in environmental sustainability. This study provides a feasible approach for developing low-carbon cementitious materials with excellent fire resistance, achieving dual benefits of solid waste recycling and performance enhancement.