Sustainable strength enhancement of cement-clay composites through partial replacement with sewage sludge and sewage sludge ash

The increasing generation of solid waste due to rapid urbanization and population growth poses significant environmental challenges, particularly in developing countries. Among these, sewage sludge (SS) and its thermally treated form, sewage sludge ash (SSA), are critical concerns. Concurrently, the stabilization of problematic soils, such as expansive clays, is vital in geotechnical engineering. This study explores the use of SS and SSA as partial replacements for Portland cement in cement-clay composites. Samples were prepared with varying cement contents (5 %, 7 %, 10 %) and SS/SSA replacement levels (3 %, 5 %, 7 %, and 10 %), compacted at dry densities of 1.5 and 1.7 g/cm3 , and cured for 7, 28, and 60 days. Unconfined compressive strength (UCS) tests demonstrated that optimal replacement levels (3-5 %) of SS and SSA improved strength, with fresh SS and SSA incinerated at 600 degrees C showing the highest pozzolanic activity. Microstructural analyses using X-ray diffraction (XRD) confirmed the formation of cementitious phases such as C-S-H and C-A-H, while scanning electron microscopy (SEM) revealed denser and more compact matrix structures with increased cement and SS/SSA content. These findings highlight the potential of SS and SSA in enhancing the mechanical properties of clay soils while promoting sustainable waste management and reducing reliance on conventional cement.