Mechanical strength, hydration products, and microstructure of waste-derived composite-activated cementitious materials by varying titanium gypsum phase composition

Abstract

The high carbon footprint of commercial chemical activators has hindered the industrial application of alkali-activated cementitious materials as a substitute for Portland cement, leading to substantial pollution and carbon emissions. This study aims to investigate the effect of the Titanium Gypsum (TiG) phase composition on the mechanical strength, hydration products, and microstructure of Red Mud (RM)-Granulated Blast Furnace Slag (GBFS)-based waste-derived composite-activated cementitious Materials (GRGM). The properties were characterized through flexural and compressive strength measurements, XRD, FTIR, TG-DTG, MIP, and SEM-EDS. The results revealed that GRGM mortars, prepared by combining dihydrate gypsum and hemihydrate gypsum in specific proportions as a waste-derived sulphate activator, exhibited flexural and compressive strengths of 8.9 MPa and 56.5 MPa at 28 d, respectively. It was observed that the presence of hemihydrate gypsum played a crucial role in promoting the transformation of C(N)-A-S-H gel to C-S-H gel, enhancing the growth of ettringite and influencing its degree of polymerization. A hemihydrate gypsum content of at least 15 % significantly accelerated this transformation process, resulting in a broader range of hydration products. This diversification ultimately led to the densification of the pore structure in GRGM paste, thereby improving the mechanical strength of the specimens. Additionally, a hemihydrate gypsum content of less than 15 % resulted in a significant reduction in mechanical strength. Furthermore, the production energy consumption of GRGM was calculated to be 89.388 MJ/t, much lower than the energy consumption and carbon emissions typically associated with cement manufacturing. This paper elucidates the mechanisms by which various single-phase and double-phase waste-derived sulphate activators influence several critical physicochemical properties of waste-derived composite-activated cementitious materials. The study further explores the potential of TiG as a waste-derived activator for the design of waste-derived composite-activated cementitious materials, thereby strengthening the theoretical framework supporting their applications.