Elucidation of the optimum alkali activation condition for one-part electric furnace nickel slag geopolymers based on hydration reaction analysis

Electric furnace nickel slag (ENS) is a bulk industrial by-product generated during ferronickel smelting. Owing to its inherently low reactivity, long-term stockpiling not only consumes scarce land resources but also creates latent environmental hazards. Unlike the extensively studied high-calcium nickel slag and other high-calcium precursors, the geopolymerization behavior of Mg-Si-rich precursor, such as ENS, has received limited attention. Therefore, this study elucidates how activator chemistry fundamentally redirects the geopolymerization behavior of ENS, thereby revealing the geopolymerization mechanism of ENS. We systematically investigated its geopolymerization behavior under three typical alkali activators (Na₂SiO₃, NaOH, and Na₂SiO₃-Na₂CO₃) at varying alkali equivalents (4 %, 5 %, and 6 %). The macroscopic properties were evaluated through flowability and compressive strength tests, while the reaction mechanisms and microstructural evolution were elucidated using a suite of characterization techniques, including XRD, FTIR NMR, MIP, and SEM-EDS. In addition, ion leaching tests were conducted to assess environmental safety. The results demonstrate that the role of alkali activators in one-part nickel slag geopolymer (ONG) extends beyond simple alkalinity regulation; the type of activator determines the partitioning of dissolved species between network-forming gels (N-(M)-A-S-H) and harmful crystalline products. The Na₂SiO₃-activated system exhibited the most favorable workability and mechanical performance at an alkali equivalent of 5 % Na₂O, achieving a 28-day compressive strength of 44 MPa and forming a dense network of N-(M)-A-S-H and M-S-H gels. In contrast, the NaOH-activated system accelerated the dissolution of Si and Mg; however, the insufficient Si supply limited gel formation. Consequently, large amounts of brucite and layered hydrotalcite were generated, resulting in a porous structure and reduced strength. The Na₂SiO₃–Na₂CO₃ system showed intermediate performance, producing only a moderate quantity of gels. These findings provide theoretical insights and technical support for the sustainable and high-value utilization of electric furnace nickel slag.