The effect of slag and fly ash content on the properties of electric furnace nickel slag-based geopolymer used for repair materials

Large quantities of electric furnace nickel slag (EFNS) are annually generated through the smelting of ferronickel in industries. EFNS contains a large amount of glass phase SiO2 and MgO, with certain volcanic ash activity. Nevertheless, the lower early activity of EFNS due to the ultra-low Ca and Al greatly limits its application. To descend the consumption of raw materials and mitigate environmental pollution, utilizing such wastes as a binding agent in the domain of repairing mortars is a suitable solution. To address the lack of Ca and Al and adjust the ratios of Ca/Al and Si/Al in EFNS-based mortars, the purpose of the research was to incorporate different contents of slag (SL) and fly ash (FA) from 25% to 40%, respectively, into EFNS-based mortars activated with Na2SiO3. The study involved an investigation of the workability, mechanical properties, bond strength, and dry shrinkage of the mortars, employing different percentages of SL and FA. At the same time, TGA, XRD, MIP, and SEM were applied to investigate the chemical composition and microstructure of the binder gel phase. The results show that when the SL content reached 40% and the FA content was 30%, the repair performance of mortar is much better than numerous repair materials, with an excellent compressive strength (94.0 MPa), flexural strength (11.0 MPa), and bond strength (6.33 MPa). As such, increasing the SL and FA contents seemed to have a positive influence on dry shrinkage and mechanical strengths. Additionally, when the FA content was 30%, the incorporation of SL was found to promote EFNS hydrolysis, and the hydration could directly generate hydrotalcite, which is beneficial to decreasing drying shrinkage at 28 d. The present study provides a theoretical foundation for the application of EFNS-based geopolymers in repair materials.