Mechanical and microstructural investigations on the low-reactive copper mine tailing-based geopolymer activated by phosphoric acid

This paper presents an experimental study on low-reactive copper mine tailings (MT)-based silicate-aluminate-phosphate (SAP) geopolymer activated by phosphoric acid. First, the effect of liquid to solid ratio (L/S) (0.4, 0.5, and 0.6), acid concentration (30, 40, and 50 wt%), curing temperature (45, 60, 75, and 90 °C), and curing time (3, 7 and 28 days) on the unconfined compression strength (UCS) of the MT-based SAP geopolymer was investigated. Then, to better understand the effect of the different factors, microscale analyses using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD) techniques were performed on the MT raw material and the tested SAP geopolymer samples. Finally, durability and leaching tests were carried out on cylindrical samples to investigate the integrity of the SAP geopolymer samples in exposure to harsh environments and the leaching of heavy metals from the SAP geopolymer samples. The results show that the optimum L/S ratio and acid concentration are 0.4 and 40 wt%, respectively, giving an optimum Al/P ratio of 2.21, for the MT-based SAP geopolymer. The geopolymerization of MT at the low curing temperature 45 °C is slow but continuous with time; too high a curing temperature (≥75 °C) adversely affects the geopolymerization process and even induces depolymerization and cracking after a long curing time. Therefore, a curing temperature of 60 °C can be used for the production of the MT-based SAP geopolymer. Moreover, the MT-based SAP geopolymer shows good durability and effective solidification/stabilization of heavy metals within the MT in both neutral and acidic environments.