Effect of CaO/Na2SiO3 ratio on mechanical properties, microstructures and environmental leaching characteristics of neutralization slag based geopolymers

Neutralization slag (NS) of copper smelting is a waste residue mainly composed of calcium sulfate, which is produced during the process of metal smelting. The large accumulation and storage of NS poses a significant threat to the environment. Therefore, it is imperative to explore a new economically and environmentally friendly approach for recycling NS. This study proposes a method for preparing a new eco-friendly geopolymer material by utilizing NS. A geopolymer with satisfactory mechanical properties was created by employing a hybrid solid activator consisting of CaO-Na2SiO3. The effects of hybrid solid alkali activators calcium oxide (CaO)/sodium silicate (Na2SiO3) ratio on the hydration properties of the NS-based geopolymer were studied, and its curing mechanism was discussed. The results show that, when the hybrid solid activator CaO/Na2SiO3 ratio was 3:1, the maximum 28 d compressive strength of the geopolymer reached 27.8 MPa. The solid activator at a suitable ratio can accelerate the hydration reaction, form higher amounts of C-A-S-H gel and ettringite, reduce the proportion of harmful macropores in the geopolymer, and improve the microstructure of the geopolymer. Toxicity characteristic leaching procedure (TCLP) test confirmed that the NS-based geopolymer had a solidification effect on heavy metals. The leaching concentration of elemental Pb from the neutralised slag feedstock was 7.241 mg/L, while the lowest concentration was 0.286 mg/L in the NS based geopolymer. As a green cementitious material, the NS-based geopolymer is expected to be applied in engineering practice.