Paper Sludge Ash as a Sustainable Additive in Cement Mortars: Assessment of Performance and Environmental Impact

The development of sustainable construction materials is becoming increasingly crucial, given that the building sector seeks to mitigate its environmental impact both in terms of energy and non-renewable natural raw material consumption. This work evaluates the feasibility of valorising industrial Paper Sludge Ash (PSA), which are rich in calcium and siliceous compounds, as a supplementary cementitious material (SCM), addressing both PSA waste management and partial cement replacement, to manufacture eco-friendly cement mortars for the construction industry. Cement mortar specimens were prepared by incorporating PSA at varying cement substitution rates of 5, 10, 15, and 20 wt.%. The methodology applied includes the assessment of the hazardous character of PSA through leaching test for waste characterisation containing heavy metals, according to European Standard, followed by comprehensive physicochemical analysis (ICP-OES, TGA/DSC, XRD); and physicochemical, mechanical, thermal, and environmental assessment of the resulting composites. The results indicate that incorporating PSA improved thermal performance, achieving a reduction in thermal conductivity by up to 14.0%, thus potentially contributing to energy sustainability in buildings, while maintaining acceptable mechanical strength for masonry applications. On the other hand, the Cradle-to-Gate Life Cycle Assessment (LCA) revealed a substantial percentage decrease in all environmental impact indicators analysed: GWP (18.6%), EP (16.0%), ADP (15.6%), POCP (13.5%), ODP (5%) and AP (5.0%). This work demonstrates that the partial replacement of cement with PSA not only yields viable, energy-efficient mortars but also significantly reduces the environmental effects associated with construction materials, offering an optimal pathway toward circular economy practices in the building industry.