Sewage and drinking water sludge ash enhance low-slump concrete strength through distinct densification of the interfacial transition zone

This study investigated the effects of Fe-rich sewage sludge ash (FSA) and Al-rich drinking-water sludge ash (ASA) on the mechanical performance and interfacial transition zone (ITZ) of low-slump concrete. Ash replaced 5–15% of cement, and 28-day properties were evaluated by mechanical testing, SEM-BSE porosity mapping, EDS, microhardness, XRD, UPV, and AVPV. At 15% replacement, FSA and ASA increased compressive strength by 49% and 54%, respectively. ASA densified the ITZ uniformly across 10–100 μm, whereas FSA showed a threshold-dependent response with preferential outer-ITZ densification, possibly linked to its high P2O5 content, pending verification by hydration-kinetics measurements. Reduced ITZ porosity, higher microhardness, and lower Ca/Si ratios at high dosages supported improved interfacial quality. Both ashes reduced permeable voids without impairing internal quality. On a gross cement-substitution basis (excluding preexisting sludge-incineration emissions), 15% replacement could avoid approximately 40 kg CO2-eq/m3, demonstrating sludge ash’s potential as a low-carbon supplementary cementitious material.HighlightsFSA and ASA at 15% raise 28-day concrete strength by 49% and 54%FSA and ASA densify the ITZ via two distinct pathwaysASA decreases ITZ porosity consistently across the 10-100 μm regionFSA selectively densifies the outer ITZ possibly linked to its high P2O5 content15% cement substitution cuts gross emissions by ∼40 kg CO2-eq per m3 of concrete