Pore structure evolution and state of pore water in hydrating cement paste at cryogenic temperatures

The development of microstructure in cement pastes and its resistance to harsh environments is of considerable current interest. In this work we have studied the effects of cryogenic temperatures on hydrating white cement pastes using Nuclear Magnetic Resonance (NMR) techniques. These methods are non-destructive and are applied in situ. NMR relaxation analysis has been used to interpret the evolution of the micropore structure in a cement paste during hydration, permitting a basic understanding of the state of water and change of pore structure undergoing wet-dry and freeze-thaw cycles. The pore structure evolution has been investigated by the suppression of the freezing temperature of water and compared with relaxation analysis performed at room temperature. Both methods consistently show that hydrating cement pastes have water contained in two principal and distinct components in their pore size distribution, capillary and gel water. Measurements have been made of the water consumption, the total specific surface area, and pore size distribution as a function of hydration times ranging from 2.5 hours to 6 months. The amount of evaporable water in the pore space can be determined from the magnitude of the NMR signal. The NMR relaxation times provide a measure of the characteristic pore sizes. Their interpretation is made in the context of a fast exchange model for which strong support can be obtained from systematic measurements with samples partially filled with water. The drying studies have been performed to determine the surface spin-spin relaxation time, which can be related to the pore structure. These experiments support a model of capillary and gel pores in the cement paste and provide strong evidence of a stable dense-gel structure. Supercooling and freezing point depression of confined water has been studied systematically. The depression of the freezing point of liquid water confined within a pore was found to be dependent on the pore size with capillary pore water freezing at 240 K and the remaining gel pore water freezing at temperatures near 160 K. Comparison with neutron scattering experiments is made on samples prepared with heavy water. Application of these non-destructive methods can be helpful to further understanding of cement hydration in extreme harsh conditions such as in the lunar environment. Under these circumstances the effects of drying and freeze-thaw cycles on microstructural stability must be determined. The NMR methods we have developed can provide useful tools to characterize cement pastes in simulated lunar conditions. The experimental results should allow us to make quantitative estimates and establish models of water loss during freezing at cryogenic temperatures taking into account transport through the complex structure of frozen capillary water and mobile gel water.