Improved macro-microscopic characteristic of gypsum-slag based cementitious materials by incorporating red mud/carbide slag binary alkaline waste-derived activator

The expensive cost and high carbon footprint of commercial alkaline activators hindered the promotion and industrialization of geopolymers. The rising demand for geopolymer as low carbon and excellent building materials will lead to enormous research works towards to low carbon alkaline activator. This study aimed to explore the impact of red mud (RM)/carbide slag (CS) binary alkaline wasted-derived activator (RCA) on the macroscopic and microscopic properties of RCA activated cementitious materials (RCM) prepared with titanium gypsum (TiG) and granulated blast furnace slag (GBFS). It is proposed to adjust the pH value of solid waste powder for calibrating the waste-derived activator, prepare RCM and explore the relationship between its mechanical properties and hydration products. The research showed that the highest flexural and compressive strengths at 28 days occurred with a dosage of 0.8 wt% CS, meeting the strength requirements of 52.5 R Portland cement. It is worth noting that RCM with binary alkaline waste-derived activators with appropriate pH values can exhibit excellent early mechanical properties. Raising the pH of RCA significantly improves the compressive and flexural strength, increasing them to 30.7 MPa and 6.3 MPa at 3 d, and to 60.7 MPa and 10.8 MPa at 28 d. The primary hydration products of RCM include C(N)-A-S-H gels, ettringite, and gypsum crystal phase, which contribute to the development of mechanical strength. Due to the difference in the alkaline environment required for the growth of C(N)-A-S-H gel and ettringite crystal, the increase in RCA pH value has been found to both inhibit the growth of ettringite to a certain extent and promote the precipitation of C(N)-A-S-H gels, improving the chemical structure of the hydration products and the pore distribution characteristics of the RCM. Furthermore, the research has been successfully applied in a pilot test in Jiaozuo, China. RCM introduces a novel concept for the efficient and industrial-scale utilization of solid waste. This study can provide a fundamental academic and industrialized support and a novel concept for the whole solid waste cementitious materials especially from alkaline industrial solid wastes such as RM and CS.