Influence of water re-curing on microstructure of heat-damaged cement mortar characterized by low-field NMR and MIP

Water re-curing has been recognized as a potential method to recover the mechanical strength and durability of heat-damaged cement-based materials, which benefits from the improvement of its microstructure due to the rehydration and carbonation of dehydration products. In this contribution, the influence of water re-curing on the microstructure of heat-damaged cement mortar was evaluated by combining Mercury Intrusion Porosimetry (MIP) with low-field Nuclear Magnetic Resonance (LF-NMR). The heat-damaged cement mortar specimens were prepared through exposing to a series of ascending temperatures (105 °C, 200 °C, 400 °C, 600 °C, 800 °C). First, the influence of water re-curing on the porosity and pore size of the heat-damaged cement mortar was analysed with the results of MIP tests. Then, the microstructural evolution of the heat-damaged cement mortar was monitored by means of LF-NMR in the process of water re-curing. At last, a conversion of T2 spectra into pore size distribution (PSD) curves was performed. Causes for the difference in the PSD curves determined by LF-NMR and MIP were discussed. Results demonstrated that LF-NMR was a powerful tool to follow the microstructural evolution of heat-damaged mortar specimens during water re-curing. When combined with MIP tests, the LF-NMR technique can provide quantitative information with respect to the evolution of PSD in a nondestructive way. Capillary pores can be segmented into smaller pores due to the filling of products generated by the rehydration of dehydrated products. The microdamage was partially healed after 10.5 days of water re-curing. The original exposure temperature has a noteworthy impact on the recovery rate and level of the heat-damaged mortar during water re-curing.