Influence of Water-to-Powder Ratio on the Frost Resistance of Tuff Powder-Modified Self-Compacting Concrete

Self-compacting concrete (SCC) shows considerable potential for infrastructure applications on the Qinghai–Tibet Plateau, which is characterized by a hypoxic environment. However, the quantitative effects of the volumetric water-to-powder ratio (VW/VP) on the freeze–thaw durability of SCC remain poorly understood, particularly when tuff powder is incorporated as a filler. This research explores the effects of VW/VP on the frost resistance of SCC incorporating tuff powder. Three SCC mixtures with VW/VP values of 1.0, 1.1, and 1.2 were prepared to systematically evaluate their mechanical performance and freeze–thaw durability. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were employed to characterize the microstructure of the corresponding hardened pastes. The findings indicate that both compressive and splitting tensile strengths decrease markedly as VW/VP rises. Under the same number of freeze–thaw cycles (FTCs), higher VW/VP gives rise to an increased mass loss rate and accelerated degradation of the relative dynamic elastic modulus (RDEM). Over the course of 250 FTCs, the RDEM decreased from 75.5% to 67.3%, accompanied by an increase in mass loss from 1.34% to 5.20%. Microstructural analyses reveal that the proportions of harmful pores (100–200 nm) and multi-harmful pores (>200 nm) increase as VW/VP rises, indicating that a reduced VW/VP enhances pore structure densification and the frost resistance of SCC. The forecasted trajectories of mass loss and RDEM derived from our established damage model are in close accordance with the measured laboratory data, confirming that lower VW/VP effectively prolongs the operational life of SCC under harsh freeze–thaw environments. Furthermore, an improved comprehensive performance evaluation considering mechanical performance, freeze–thaw resistance, cost, and environmental impact was proposed, demonstrating that SCC with lower VW/VP exhibits superior overall performance.