Effects of Na₂CO₃/NaOH on properties of titanium gypsum-ground granulated blast furnace slag based composite cementitious materials

Abstract

This study highlights the novelty of utilizing titanium gypsum (RG), an industrial solid waste with limited current applications, as the primary component in alkali-activated composite cementitious materials. The binary alkaline activation system (Na₂CO₃/NaOH) with different alkalinity (mass percentage of Na₂O) and molar ratios of Na₂CO₃ and NaOH was adopted, and its influence on the properties of titanium gypsum-ground granulated blast furnace slag (RG-GGBS) composite cementitious materials was explored in depth. The results show that increasing the Na₂CO₃/NaOH molar ratio improves fluidity, setting times, mechanical strength, and significantly reduces autogenous expansion strain. This improvement is mainly due to the binary activator suppressing ettringite formation and promoting the generation of Calcium (Alumino) Silicate Hydrate (C-(A)-S-H) by providing more nucleation sites, thereby reducing harmful porosity. However, higher alkalinity leads to concentrated heat release, resulting in more microcracks. Excessively high pH levels also hinder precursor dissolution, limiting hydration reactions and reducing aluminosilicate formation, which further degrades the pore structure. In addition, compared with single NaOH activation, the binary activator system offers better volume stability and environmental performance, with the optimal conditions—Na₂CO₃/NaOH molar ratio of 4:6 and an alkalinity of 4 %—achieving the best combination of fluidity, strength, and stability. In summary, these findings provide valuable technical support for the effective utilization of titanium gypsum as building materials.