Influence of sewage sludge ashes on the performance of cementitious composites

Particular attention has been paid to replacing cement with sewage sludge ash (SSA) in order to improve circular economy practices and reduce carbon emissions linked to cement production. This work explored the effect of SSA on the hydration kinetics, phase assemblage and early properties SSA-blended composites. Four SSA were used, originating from a unique sewage sludge incineration process, with limestone injection for toxic gas treatment. The findings showed that SSA contained residual limestone. The lime content varied between SSA, creating a more or less exothermic hydration reaction at early age due to its dissolution. The flue gas desulphurisation by limestone led to the formation of calcium sulfate wich delayed the sulfate depletion peak. The introduction of SSA in cement delayed also the alite peak reaction but the delay in the setting times were <30 min. The mineralogic analyses of 30%SSA-blended cement paste highlighted the formation of monocarbonate due to the residual calcite content brought by SSA. Moreover, SSA insoluble phases (whitlockite, hematite, quartz, biotite, and illite) were still detected and quantified in the pastes. The hydration degree of cement paste with SSA was comparable to that of the reference OPC, showing that SSA did not hinder the hydration reaction. Finally, SSA-blended mortars exhibited a sligth reduction in compressive strength, which increased with the substitution rate but remained below 14% for 30% SSA (58 MPa) compared to the reference mortar (68 MPa).