Water Redistribution within the Microstructure of Cementitious Materials due to Temperature Changes Studied with 1H NMR

Changes of water state within the pore structure of cement paste due to temperature changes are followed by means of H-1 nuclear magnetic resonance (NMR) relaxation analysis. The study shows that, with increasing temperature, the signal due to water contained in the smallest C-S-H interlayer spaces decreases while that from the larger gel pores, and to a lesser extent from the capillary pores, increases. On cooling, the opposite behavior is observed with complete reversibility. The observed changes in water populations appear to be instantaneous compared to the rate of temperature change in the samples. These changes are postulated to be responsible for macroscopically observed changes of relative humidity in pores during heating/cooling and are therefore key in understanding thermal deformations of cement-based materials. It is evident that the previous hypothesis of microstructural delayed water transport being responsible for macrostructural delayed thermal deformations can be rejected. Different microstructural mechanisms are discussed that could explain the redistribution in water signals, namely, water migration and pore rearrangement mechanisms.