Simultaneous utilization of mine tailings and steel slag for producing geopolymers: Alkali-hydrothermal activation, workability, strength, and hydration mechanism

Utilization of aluminosilicate-rich solid waste via geopolymerization, such as mine tailings (MTs), possesses a dual advantage of reducing CO2 release from cement manufacturing and increasing tailings utilization. However, a major challenge for this technology is the low reactivity of MTs. In this study, an alkali-hydrothermal method at 200 °C was developed for MTs activation, and MTs were converted into an active alkaline aluminosilicate precursor that could play an alkaline activator-like role. Geopolymers were subsequently prepared from activated MTs and slag by simply adding water, and to improve the performance of geopolymers, steel slag (SS) was added as a supplementary alkali source. Results showed that the advantage of SS as a calcium-based alkaline material could be fully exerted in this system. The Ca(OH)2 phase derived from SS hydration increased the pH of the pore solution, on the one hand, and complexation reactions between Ca2+ ionized from Ca(OH)2 and Si and Al monomers dissolved from activated MTs might occur and generate calcium aluminosilicate hydrate gel, on the other hand. As a result, the addition of SS accelerated the hydration process, prompted the production of hydration products, and thus enhanced the strength of geopolymers. The 3-day and 28-day strengths of geopolymers achieved as high as 31.10 and 44.79 MPa, respectively, when SS content was kept at 10 wt%. In addition, SS-doped geopolymers had satisfactory soundness because the fast consumption of Ca2+ prevented the growth of Ca(OH)2 crystals and the resulting expansion effect.