Thermal resistance of alkaline fused phosphate sludge-based geopolymer mortar

The present study investigates the thermal behaviour of phosphate sludge-based geopolymers. Phosphate sludge which is a by-product from phosphate beneficiation processes was activated using the alkaline fusion method to improve the geopolymer activity of this material. Then, the mechanical properties as well as the thermal behaviour of the resulted geopolymer mortar were studied. The effect of sodium hydroxide addition and fusion temperature on the mechanical properties and the thermal behaviour of the geopolymers were assessed using compressive strength tests before and after thermal shock cycles at different temperatures (350, 500, 650 °C). The mineralogical composition of the fused materials was investigated using XRD measurement and the microstructure of the geopolymer mortars was studied using FTIR. The obtained results showed that NaOH content as well as the temperature of fusion are essential parameters controlling the structure and the strength of the developed geopolymeric gel. Exposure to elevated temperatures up to 650 °C induces a decrease in compressive strength between 58 and 71%. However, a further increase in the temperature of exposure till 800 °C induces a development of compressive strength. Overall, geopolymers with good compressive strength up to 40 MPa could be synthesized in the optimum conditions of fusion: 10% of NaOH and 550 °C.