With the continuous expansion of infrastructure construction in alpine regions, a demand for concrete construction under sub-zero temperatures increases significantly. However, conventional low-temperature construction methods suffer from high energy consumption and complex operational processes. Although sulfoaluminate cement (SAC) exhibits excellent low-temperature adaptability, and calcium chloride (as an antifreeze agent) can effectively lower the freezing point and maintain a liquid-phase environment, the hydration kinetics of the simple SAC-calcium chloride composite system under fully cold construction conditions (≤-15 ℃) are severely inhibited at sub-zero temperatures, leading to an insufficient early-age strength development, which fails to meet the rapid construction requirements of engineering projects. Consequently, there is an urgent need to optimize this composite system to fully exploit its rapid hardening and early-strength characteristics under fully cold conditions at -15 ℃.

Anhydrite as a key component of SAC critically regulates the composition of hydration products, microstructure, and macro-scale performance of the cementitious system. However, the existing studies on the effect of anhydrite dosage primarily focus on ambient temperatures or relatively mild sub-zero conditions (e.g., >-5 ℃). Systematic investigations into how anhydrite dosage governs the hydration product evolution, microstructural formation, and mechanical-deformation properties (particularly early-age performance) of the SAC-calcium chloride composite system under extreme low-temperature conditions (-15 ℃) and fully cold construction processes remain largely unexplored.

This study was to systematically reveal the critical regulatory effects and intrinsic mechanisms of anhydrite dosage on the low-temperature hydration and hardening behavior of the SAC-calcium chloride composite system under fully cold construction conditions at -15 ℃, simulating the harsh environments encountered in real-world engineering. The findings could be expected to expand the application potential of SAC in extreme environments, particularly in remote regions lacking thermal energy supply.

Under fully cold construction conditions (chilled materials, mixing, and curing) at -15 ℃, a composite system was designed with SAC clinker and anhydrite as binders and a 20% calcium chloride solution as a mixing medium. Anhydrite dosage gradients were set at 0%, 5%, 10%, 15%, 20%, 25%, and 30%, respectively.

The mortar specimens (40 mm × 40 mm × 160 mm) were prepared at a water-to-binder ratio of 0.37 based on the standard GB/T 17671—2021 Test Method for Cement Mortar Strength (ISO Method) for strength evaluation. The shrinkage specimens (40 mm × 40 mm × 160 mm) were prepared based on the standard JGJ/T70—2009 for Test Methods of Basic Properties of Construction Mortar.

Cement paste specimens were formulated at a water-to-binder ratio of 0.32. The calcium chloride solution and cement were sequentially poured into a mixer, stirred at a low speed for 120 s, paused for 15 s, and then stirred at a high speed for 120 s. The resultant mixture was cast into molds and compacted by a vibrating table for 60 cycles. All the specimens were cured on racks under uncovered conditions at -15 ℃.

After reaching specified curing ages, the mortar specimens underwent macroscopic tests. Cement paste specimens were crushed on-site, submerged in anhydrous ethanol to terminate hydration at room temperature for ≥7 d, and then vacuum-dried at 40 ℃ and -0.08 MPa for ≥24 h. The dried samples were subjected to the microstructural analyses to investigate the phase composition and hydration mechanisms.

Under fully cold construction conditions at -15 ℃, the incorporation of 10% anhydrite dosage enables the SAC-calcium chloride composite system to achieve the 1-d and 3-d compressive strengths of 20.1 MPa and 38.0 MPa, respectively, representing approximately 100% enhancement, compared to the pure clinker group. At an anhydrite dosage of 15%, the peak hydration temperature increase reaches 18.9 ℃ (33.9 ℃ relative to ambient temperature), effectively addressing the challenges of delayed early hydration kinetics and insufficient strength in sub-zero environments.

Anhydrite significantly suppresses a shrinkage via promoting the formation of expansive AFt phases. At the dosage of 5%, the 3 d shrinkage rate reduces to 0.100% (i.e., 47.4% reduction), while at the dosage of 25%, the 28 d shrinkage rate decreases to 0.237% (i.e., 35.1% reduction), offering a graded solution for a low-temperature shrinkage control.

The microstructural analyses reveals that anhydrite regulates the types and morphology of hydration products. In the 28 d hydrated pure SAC clinker, unstable petal-like Friedel's salt crystals occur. Anhydrite optimizes the Cl/S ratio of the system, promoting AFt formation, while suppressing Friedel's salt generation. At the optimal dosage of 10%, needle/rod-shaped AFt crystals form a dense interlocking network, which is pivotal for early-age strength enhancement. However, the excessive dosage (>10%) leads to unreacted anhydrite enrichment and short rod-shaped AFt aggregates with localized bridging, resulting in the strength regression at later ages.

Under fully cold construction conditions at -15 ℃, anhydrite significantly enhanced the hydration activity and early-age strength of the SAC-calcium chloride composite system. An appropriate anhydrite dosage of 10% promoted the hydration process of calcium sulfoaluminate in the system and accelerated the nucleation and growth rate of AFt, thereby forming a denser microstructure of early hydration products. However, the excessive anhydrite disrupted the sulfate phase equilibrium in the reaction system, inhibited the continuous formation of AFt, and induced anhydrite enrichment, ultimately degrading the mechanical properties due to structural loosening.

At early ages, the anhydrite dosage of 5% approached the optimal sulfate balance required for hydration, effectively compensating for plastic shrinkage and chemical shrinkage. In contrast, the dosage of 25% provided sufficient SO₄^2- ions during later hydration stages, ensuring sustained AFt formation and suppressing drying shrinkage over time.

Anhydrite optimized the Cl/S molar ratio in the system, facilitating the formation of a dense network of needle-bar Aft, while inhibiting the formation of the unstable Friedel's salt phase. The excessive anhydrite, however, led to interconnected short-rod AFt and gypsum enrichment, further compromising the structural integrity.