Alkaline fused phosphate mine tailings for geopolymer mortar synthesis: Thermal stability, mechanical and microstructural properties

The present study investigates the efficiency of alkaline fusion method to enhance the geopolymeric reactivity of phosphate mine tailings. The effect of sodium hydroxide content and fusion temperature on the mineralogical composition of the fused phosphate sludge was assessed by XRD measurement. The microstructure as well as the mechanical behavior of the prepared geopolymer mortars were investigated using compressive strength test and EDS/SEM analysis. Furthermore, the thermal stability of samples was studied by measuring the mechanical properties after exposure to several cycles of heat treatment at different elevated temperatures (350, 500, 650 and 800 °C). The obtained results showed that the structure of phosphate sludge underwent a significant variation after fusion. Alkali thermal treatment led mainly to the decomposition of illite, palygorskite and dolomite, and the formation of Na-rich crystalline phases. NaOH content as well as temperature of fusion has been proved to be essential factors controlling the transformations of the material structure and hence the development of the geopolymer strength. The optimum conditions of fusion were set at 10 wt% of NaOH and a temperature of 550 °C, providing geopolymer mortars with high compressive strength (40 MPa).