Use of waste sludge generated from water defluoridation plant as partial substitute for fine aggregates in mortars: microstructural characterization and compressive strength

In the present study, sludge produced from the continuous defluoridation set-up was used as partial replacement for fine aggregates in mortar making. Aluminium sulphate (alum) and polyaluminium chloride (PACl) were used as coagulants for fluoride removal and mortar cubes with size of 50 mm x 50 mm x 50 mm were prepared with alum and PACl sludge in different proportions, i.e. 0%, 1%, 2%, 3%, 4% and 5%. The compressive strength tests were conducted and it was observed that the strength of mortar with 1% replacement increased by 2.5% for alum sludge and 6% for PACl sludge and the compressive strength showed satisfactory values up to 3% sludge replacement for alum and PACl sludge. The impact of partially replacing alum and PACl sludge on the strength of mortars regarding calcium silicate hydrate gel, silica and other hydration products was studied. Toxic Characteristics Leaching Procedure was performed for the mortars and aluminium concentration was within the permissible limits of 0.2 ppm. The results suggested that the reuse of alum and PACl sludges can lead to new paths in reducing the deterioration of the ecosystem because of waste disposal.