High temperature performance of high magnesium nickel slag based geopolymers with different P/Al molar ratios prepared by acidic activator

This study uses high magnesium nickel slag (HMNS) directly as a raw material, to prepare high-temperature-resistant HMNS silico-aluminophosphate geopolymer (HMNSGP) activated by different types of acid activators including Al(H2PO4)3, H3PO4 and KH2PO4. By using characterization techniques such as XRD, FTIR, SEM-EDS, and MIP, the micro-properties, pore structure, and reaction mechanism of HMNSGP were investigated. The high temperature resistance of HMNSGP was evaluated by comparing compressive experiments before and after high temperature and TG-DSC. The results showed that the best samples were prepared using aluminium dihydrogen phosphate (ADP) at a P/Al molar ratio of 0.6. At room temperature, the main phases geopolymerized of the sample are crystalline products (Newberyite) and amorphous hydrated products (-Si-O-P-O-Si-, -Al-O-P-O-, -Si-O-Al-O-Si- and -P-O-Mg-). At 1200 °C, HMNSGP can still maintain its integrity, and its compressive strength can be increased to 48.4 MPa. The HMNSGP prepared using H3PO4 has the same hydration products as the optimal samples, but the lower amount of gel phase causes spallation at high temperatures. Besides primary phases geopolymerized with ADP as activator, the KH2PO4 sample includes crystalline products (K-struvite). However, this phase decomposes at 207 °C, affecting its high-temperature resistance while a significant volume shrinkage occurs at 1200 °C. In summary, using ADP with a P/Al molar ratio of 0.6 can directly prepare HMNS into an environmentally friendly refractory geopolymer material, the compressive strength shows stable growth after being exposed to 1200 °C. This results suggested that HMNSGP prepared with ADP can be used for structural applications, laying the foundation for future replacement of refractory cement.