Microstructural and mechanical characterisation of grout and semi-flexible pavements incorporating sewage sludge ash as a cement substitute

This study examines sewage sludge ash (SSA) as a partial cement replacement (0–20%) in Semi-flexible pavements (SFPs) grouts, focusing on microstructural, mechanical, and environmental aspects. Characterisation (XRD, XRF, BET, LPSA, SEM) confirmed SSA’s latent hydraulic reactivity and porous morphology. At up to 10% replacement, grout retained adequate strength (compressive >30 MPa, flexural >5 MPa at 28 days). At 5% SSA, indirect tensile strength rose by 13%, while fracture energy increased by 7% (Mode I) and 15% (mixed Mode I/II). Despite lower compressive strength, SSA-modified grouts enhanced crack resistance and grout–asphalt bonding, as shown by altered fracture paths. Workability decreased and setting times were delayed due to SSA porosity and phosphate content. Environmentally, SSA reduces cement demand, diverts sludge from landfilling, and supports low-carbon construction. This work provides the first systematic evidence of SSA as a functional additive for SFPs, with optimal performance at ∼5% replacement.