Feasibility of high-magnesium nickel slag-fly ash as precursor of magnesium phosphate cement and its hydration mechanism

Abstract

The aim of this study was to investigate the process conditions, hydration reaction products and mechanism of high-magnesium nickel slag (HMNS)-fly ash (FA) as a precursor of magnesium phosphate cement (MPC). The relationships between properties, hydration products, hydration processes, and micromorphology were investigated by engineering properties, XRD, TG-DSC, FT-IR, Raman, and SEM/EDS. The results showed that the optimal process (HMNS/FA = 1.0/0, NH4H2PO4 dosing of 25 wt%, water-cement ratio of 0.26, the maintenance condition is air maintenance, and 200 mesh particle size of raw materials) prepared the cementitious material with 28 d compressive strength of 58.00 MPa and linear expansion rate of only 2.5 %. The hydration process was a multi-stage process, including the dissolution of NH4H2PO4 and pre-treated HMNS (WHMNS), the precipitation of intermediate hydration products and the formation of struvite, and the formation of gelling phases such as -Si-O-P-, -Al-O-P-, and -Si-O-Al-O-P-. Eventually, a high-strength gel-crystal phase structure consisting of gelling phases such as struvite wrapped around unreacted raw materials and struvite crystals was formed. FA was harmful for cementitious material, and impurities and high levels of Ca in FA reacted with phosphate to form hydration products with a loose porous structure (e.g., brushite). The optimal process sample complies with the 52.5R grade of ordinary Portland cement (GB175-2007); its embodied energy was reduced by more than 73.4 % compared to magnesium phosphate cement (MPC), and the corresponding CO2 emission was reduced by more than 74.2 %. Therefore, it has the potential to partially replace MPC from the environmental and feasibility point of view.