Synergistic Effects of C-S-H Seeds and PCE on the Performance of Fluoroaluminate-Based Shotcrete: Hydration Kinetics and Microstructural Evolution

To address the trade-off between the rapid setting time and low mechanical strength of fluoroaluminate-based liquid accelerating agents, this study investigated the combined effects of synthetic C-S-H seeds and a polycarboxylate superplasticizer (PCE) on their performance. The underlying mechanisms were explored using isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM). The results indicated that the incorporation of C-S-H further accelerated the setting process and significantly improved the 1 d and 3 d compressive strengths of the mortar, with a 1 d strength increase exceeding 30% at a dosage of 1.2%. When combined with PCE, the 1 d strength was further enhanced to 31.5% above the control. Microstructural analysis revealed that C-S-H provided additional nucleation sites for ettringite (AFt), accelerating the early hydration of C3A and increasing the volume of hydration products in the paste. Furthermore, the hydration of the C-S-H seeds themselves contributed to the formation of additional C-S-H gel. This helped alleviate the retarding effect of fluoride ions on C3S hydration, promoted the formation of more hydration products, filled structural pores, and ultimately enhanced the compactness of the paste. The synergistic effect of PCE promoted more complete hydration of C3S, further amplifying the benefits of C-S-H, and leading to a 30.9% increase in C-S-H gel content (reaching 46.1% at 3 days) compared to the control. Consequently, the shotcrete achieved higher mechanical properties and a denser internal structure. The findings demonstrate that a suspension containing 0.9% CSH-PCE significantly improves the performance of a fluoroaluminate-based liquid accelerator by enhancing the early hydration rate and mechanical properties. This provides a theoretical basis for the development of high-performance fluoroaluminate-based liquid accelerators.