Investigation on high-magnesium nickel slag treated by phase-separated activation as cementitious material

Abstract

High-magnesium nickel slag is a hazardous waste from nickel smelting process with low pozzolanic activity. In order to improve the pozzolanic activity of high-magnesium nickel slag, this study produced a phase separation structure of high-magnesium nickel slag by activation treatment, and studied the effect of phase separation structure on the pozzolanic activity, engineering performance and hydration mechanism of high-magnesium nickel slag as cementitious material. The results showed that the phase separation structure was crucial for the pozzolanic activity of the high-magnesium nickel slag. It was confirmed by dissolution test that the phase separation structure significantly improved the pozzolanic activity of high-magnesium nickel slag. The main reason for the increase in slag activity is that the phase-separated glass consists of a silica-rich phase and an alkali-rich phase, which is rich in Mg, Fe, etc. (network modifiers), and the modifiers break Si–O–Si by producing "non-bridging" oxygen atoms (NBO), forming more Si-O-NBO and Si–O–2NBO, which disrupt the continuity of the glass network. Compared with the untreated high-magnesium nickel slag, the compressive strength of the geopolymer prepared from the high-magnesium nickel slag with a phase separation structure (W-HMNS) was increased by 69%. Its volume stability was also much better than that of geopolymers prepared from untreated high-magnesium nickel slag. The main hydration products of phosphoric acid activated W-HMNS based geopolymers were newberyite (MgHPO4·3H2O), and a small amount of aluminosilicates and hydrated calcium silicates. Together they make up the gel phase and the crystalline phase. A large number of gel phases such as newberyite wrapped unreacted quartz and crystallized newberyite form a dense gel-crystal structure, with crystal as the skeleton and colloidal as the bonding medium, increasing the density of geopolymer, forming a denser hardening body with higher strength. These results may provide some guidance for the practical application of high-magnesium nickel slag as a raw material to prepare geopolymer, and provide a new idea for improving the reactivity of slag.