Recovering acidic solid residues from wet chemical phosphorus recovery of sewage sludge ash in the production of cementitious composites

Phosphorus (P) recovery from sewage sludge ash (SSA) represents a promising solution to P supply concerns, with a key challenge being the management of the acidic solid residues (ASR) generated by wet chemical P recovery processes. This work explores the integration of ASR, obtained by SSA leaching with different extractants, as a partial substitute of sand in cementitious mortars. ASR samples were first physically and chemically characterized, and their disposability in landfills as non-hazardous or inert waste was assessed. Fe, Si and Ca were the main components identified in H2SO4- and HCl-based ASR (ASR/H2SO4 and ASR/HCl, respectively), while Zn and Cu were the principal detected heavy metals. Slight exceedances of Cu and Zn thresholds for ASR disposability in non-hazardous landfills were observed, with further optimisation of the leaching process potentially ensuring legal compliance. The residues were then ground and employed as a 25% volumetric sand replacement in cementitious mortars. Tailored water content adjustment need to be made to ensure workable mixes and cast the specimens. The mechanical performance of mortars was evaluated through flexural and compressive strength tests. By properly adjusting the water content, the reduction in compressive strength was maintained within acceptable levels when using ASR/HCl. Specifically, while the use of ASR/H2SO4 led to a minimum 42% decrease in 28th-day compressive strength and 47% decrease in 28th-day flexural strength, sand substitution with ASR/HCl limited the reduction to 12% for compressive strength and 26% for flexural strength, paving the way for effective use of these residues in the production of cement-based construction materials.