Effects of Ambient Humidity on Water Migration and Hydrate Change in Early-Age Hardened Cement Paste

Ultra-low humidity environments will lead to changes in the microstructure of C–S–H, which will reduce the mechanical properties and service life of cement-based concrete. Thus, to further explore the mechanism on the microscale, this paper studied the water migration and the changes in the hydration products in white cement that was cured for 7 days at 20 °C and at different ambient relative humidities (RHs). The migration and transformation of different types of water in cement paste were studied by low-field nuclear magnetic resonance (NMR). At the same time, Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were used to analyze semi-quantitatively the crystal phase in the hydration products. The results showed that in the first 7 days of the curing process, the content of the different types of water and the hydration products in the cement samples were influenced by the ambient RH. The total water content of the samples will decrease with the decrease in the RH; when the RH decreases to 54% or below, the chemically bound water in the samples will increase with the decline in the RH. Additionally, when the ambient RH is lower than 54%, the grossular will gradually transform into hydrogrossular crystals with the decrease in the RH, and the hibschite with less chemically bound water will transform into katoite with more chemically bound water. In future research, the water migration and hydrate changes under different curing ages, drying processes, and coupling effects should be explored.