Using Belitic Calcium Sulfoaluminate Cement to Improve the Early-Age Strength of Limestone Calcined Clay Cement-Based Mixtures

Limestone calcined clay cement (LC3) is a sustainable binder for reducing the cement industry’s carbon footprint. However, the low unconfined compressive strength (UCS) at early ages significantly hinders the further application of LC3. This paper aims to improve the UCS of LC3-based mixtures at early ages by blending them with belitic calcium sulfoaluminate (BCSA) cement. The study investigated how blending LC3 with BCSA cement at different contents (0%, 20%, 40%, 60%, 80%, and 100%) affects the mechanical performance (i.e., setting and UCS) of mixtures. Additionally, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were conducted to investigate the hydration characteristics of mixtures. The results indicate that BCSA cement significantly improved the UCS of LC3-based mortar at early ages. For example, at 2 h, the UCS of LC3-BCSA mortar containing 20% BCSA cement was 2.5 MPa, whereas LC3-based mortar barely gained UCS. Additionally, a lower substitution ratio of LC3 was conducive to achieving a higher UCS at later ages. LC3-BCSA mortar containing 20% BCSA cement achieved the highest 28-d UCS (48.4 MPa), which was 116.3% of that of LC3-based mortar and 128.0% of that of BCSA cement-based mortar. By contrast, LC3-BCSA mortar containing 80% BCSA cement obtained the lowest 28-d UCS. Moreover, LC3-BCSA mortar containing 20% BCSA cement exhibited the lowest CO2 intensity, which was only 89.0% of that of LC3-based mortar and 67.6% of that of BCSA cement-based mortar, demonstrating the enormous potential for effectively reducing carbon emissions in the cement industry.