Study of the effects of drinking water treatment sludge on the properties of Class F fly ash-based geopolymer

Drinking water treatment sludge (DWTS) generated from water treatment plants is a global issue because of the environmental risks it imposes. Managing the abundance of DWTS in landfills remains an important issue. The reuse of these sludges as a construction material could contribute to the development of a geopolymer and mitigate the harmful effects of the excessive production of these sludges on the environment. This study aims to evaluate the effect of DWTS on the properties of Class F fly ash (FFA) geopolymers. Seven geopolymer blends were made with the addition of DWTS in the total fly ash weight of 0%, 5%, 10%, 15%, 20%, 30%, and 40%, and with an alkaline solution composed of 12 M sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ) solution; the liquid/solid and (Na 2 SiO 3 )/NaOH weight ratios were set to 0.75 and 2.5 respectively. The polymerization temperature was set at 60 °C and different polymerization times such as 3, 7, 14, and 28 days were considered. The bulk density, apparent porosity, compressive strength, and microstructure of the geopolymer samples were tested. The experimental results revealed that the optimum percentage of DWTS incorporation is 20 wt%, which generates a dense and homogeneous microstructure. The addition of more than 20% DWTS decreased the compressive strength from 40.87 to 35.3 MPa and bulk density from 2.134 to 2.087 g/cm 3 due to the retention of air bubbles and evaporation of water during the polymerization process forming voids in the matrix, which results in increased apparent porosity from 19 to 22%. This investigation confirmed the feasibility of incorporating DWTS into FFA-based geopolymers. Graphical abstract