The feasibility of utilizing sewage sludge as a source of aluminosilicate to synthesise geopolymer cement
T. Charles Mahutjane, Léonel Tchadjié, Thandiwe Sithole
Abstract
The purpose of this study was to evaluate the feasibility of utilizing sewage sludge as a source of aluminosilicate to synthesise geopolymer cement. Sewage sludge was calcined at various temperatures to enhance its geopolymer reactivity. Sodium hydroxide and sodium silicate solution mixture was used as an alkaline activator. The geopolymer mortars' setting time, flow workability, unconfined compressive strength (UCS), pore volume, and water absorption characteristics, wet-dry cycles and TCLP characteristics were measured. X-ray diffraction (XRD), Fourier transforms infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were used for microstructural analysis. The results showed that the highest 7-day UCS of up to 18.9 MPa was obtained, corresponding to a 270.6% strength increase over the control. The microstructural analysis showed that the optimal mechanical and physical properties were obtained when the sewage sludge was calcined at 750 °C showing that calcining the sewage sludge increases the geopolymer reactivity. The UCS decreases with the increase in wet dry cycles but the geopolymer composites were still within the minimum strength requirement of 3.5 MPa. TCLP showed that the leachability of the produced geopolymers is below the permissible limit. Geopolymer mortar made from sewage sludge ash can be utilized for various concrete and building applications without the risk of short- or long-term environmental contamination.