Effect of β-Hemihydrate Gypsum on the Properties of γ-C2S Slurry Before and After Carbonation

γ-dicalcium silicate (γ-C2S) is a carbonatable binder with excellent carbonation reactivity. However, its extremely low hydration activity prevents the paste from setting and hardening properly, making it difficult to be directly cast and molded. This study introduces β-hemihydrate gypsum as an early-strength regulating agent, which utilizes its rapid hydration to form dihydrate gypsum, thereby imparting early-age strength to the composite system and enabling subsequent carbonation curing after demolding. The experimental investigation examined the effects of β-hemihydrate gypsum content on the fluidity, setting time, and carbonation performance of γ-C2S paste. The results indicate that when the β-hemihydrate gypsum content is not less than 10%, the paste can obtain sufficient early-age strength to achieve smooth demolding. At a β-hemihydrate gypsum content of 10%, after pre-drying treatment, samples subjected to carbonation curing for 24 h under a CO2 partial pressure of 0.3 MPa achieved a peak absolute dry compressive strength of 117.29 MPa with a softening coefficient of 0.92. Further increases in β-hemihydrate gypsum content lead to reductions in both strength and water resistance. Microstructural analysis reveals that at a gypsum content of 10%, the crystalline network of dihydrate gypsum interlocks and coexists with calcium carbonate and silica gel generated from γ-C2S carbonation, forming a compact structure. In contrast, the framework formed by excessive gypsum before carbonation restricts the continuous interlocking and overall development of calcium carbonate products generated from γ-C2S, resulting in deterioration of structural integrity and mechanical properties. This study provides theoretical foundation and technical support for the engineering application of γ-C2S-based carbon mineralization materials.