Investigation on the suitability of aluminium-based water treatment sludge as a sustainable soil replacement for road construction

•50/50 ratio of aluminium-based sludge (AS) and clay (CL) indicated promising opportunity.•CLAS to help increase the optimal moisture content and decrease the dry density.•The shear strength of CLAS increased by 50% on average compared to CL.•In the UCS tests, the strength of CLAS improved to 126 kPa from 74 kPa.•CLAS mixture improved the CBR of the mixture from 0.95% (CL only) to 1.92%.