Effects of low pressure and low humidity on hydration and freeze-thaw resistance of air-entrained concrete with fly ash and GGBS

The hydration and freeze-thaw resistance of air-entrained cement-based materials incorporating supplementary cementitious materials (SCMs) under combined low pressure and low humidity remain poorly understood. This study investigates the effects of fly ash (FA) and ground granulated blast furnace slag (GGBS) in the hydration, air void stability, and freeze-thaw durability of air-entrained concrete through isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, proton nuclear magnetic resonance, air void analysis, and freeze-thaw testing. GGBS specimens, despite high moisture sensitivity in strength development, maintain superior air void stability with spacing factors below 200 μm. In contrast, FA specimens demonstrate strength stability through carbon-induced densification but suffer air void deterioration with spacing exceeding 235 μm. After 200 freeze-thaw cycles at 60 kPa and 60% relative humidity, GGBS-blended concrete achieves 85.9% modulus retention and 89% strength retention, significantly outperforming FA-blended concrete with 64.9% and 64% retention, respectively. Low humidity dominates environmental deterioration, contributing 80-95% of strength loss, while low pressure primarily affects air bubble stability. Recommended mix proportions for plateau environments are 65-85% cement with 20-30% GGBS replacement and FA content below 5%, achieving both adequate compressive strength and superior freeze-thaw resistance.