Phase separation mechanism of high-magnesium nickel slag and its applicability as magnesium phosphate cement

Abstract

The focus of this work was on the process conditions of phase separation activation treatment of high-magnesium nickel slag (HMNS) and the mechanism of formation of HMNS phase separation structure, as well as the potential of HMNS as a precursor for magnesium phosphate cement (MPC). The results indicated that the phase separation structure played a crucial role in improving the pozzolanic activity of HMNS. The pozzolanic activity of the M4 sample obtained by doping with 8 wt% MgO and water quenching at 1550 °C remelting was substantially increased compared to the untreated HMNS, the dissolution of Mg element increased from 94.55 to 206.71 mg/g, Fe from 33.25 to 62.85 mg/g, Si from 24.85 to 93.48 mg/g, and Al from 10.57 to 27.93 mg/g, they were increased by 118.60%, 89.00%, 276.20% and 164.20% respectively. The main reason for the increase in HMNS pozzolanic activity was that the alkali-rich phase of the phase separation structure caused Si-O-Si to break by producing more "non-bridging" oxygen atoms (NBO), subsequently disrupting the continuity of the glass network. The obtained optimal process conditions existed for the water-quenched HMNS: water quenching at 1550 °C for HMNS with 32 wt% of MgO produced a phase separation structure in turn possesses high pozzolanic activity. Correspondingly, the compressive strength of the HMNS-based geopolymers (cured for 28 d) activated by ammonium dihydrogen phosphate reached 45 MPa, an increase of 433%, which meeted the 42.5 R grade of ordinary Portland cement (OPC) (GB175–2007). Moreover, the CO2 emissions was reduced by more than 74.4% compared to MPC.