Influences of thermal damage on water transport in heat-treated cement mortar: Experimental and theoretical analyses

Abstract

Water transport in the heat-damaged cement mortar is closely related to the evaluation of durability for structures exposed to fire and the recycling of thermally damaged cement-based materials. In this study, the influence of microstructure on water transport in unsaturated and saturated heat-treated cement mortar was analyzed based on experimental results from neutron radiography, water permeability, mercury intrusion porosimetry (MIP) and X-ray computed tomography (CT) tests. Neutron radiography was employed to track the dynamic wetting front in real time during water imbibition in the heat-treated mortar specimens exposed to a series of temperatures (105°C, 200°C, 400°C, 600°C, 800°C). The evolution of the wetting front was analyzed according to the square root of time based on neutron images. A deviation from the standard t1/2 pattern of the wetting front was found when water imbibition in the cement mortar specimens treated by a temperature ≥400°C. The absorptivities of different imbibition stages were determined for heat-damaged cement mortar. Mechanisms for the change of typical absorptivities with the evolution of thermal damage were discussed from the perspective of microstructure. In addition, causes for the occurrence of anomalous water absorption were explored by considering the role of thermal cracks and potential physicochemical reactions. Methods were proposed to estimate the effective tortuosity of water channels in the cement mortar damaged by different temperatures. The representative size of water channels was estimated with the calculated tortuosity. The estimated water channels size was compared with the pore aperture determined by MIP. At last, the estimated water channels size and tortuosity from imbibition data were utilized to predict the water permeability of the heat-treated mortar using a capillary model. By comparing the predicted and measured permeabilities, the correlation and difference between unsaturated and saturated water transport in the heat-treated cement mortar were analysed. Future work for modeling water transport in heat-damaged cement mortar was discussed.