2023 Volume 77 Issue 1 Pages 35-43
During the first drying, microstructural change in hardened cement paste （HCP） accompanies irreversible alteration, even subsequent re-humidification. This study applied T1-T2 correlation measurements using 1H NMR relaxometry to explore microstructural reorganization in HCP after re-humidifying. Based on T1-T2 correlations, diagonal features along T1/T2≈1.7, including 1H in interlayer space and gel pore in C-S-H, and capillary pore, were observed for sealed HCP. This is the fact that the observed relaxation time of 1H in the pores of HCP is due to pore size, described by the fast exchange between the adsorbed pore surface water and bulk pore liquid. After the re-humidification, increasing in water contents of interlayer space and gel pore with increasing in exposed relative humidity was confirmed in diagonal peaks at T1/T2≈1.7, while the new off-diagonal peak at T2≈131us with T1/T2≈12.3 was consistently detected at all re-humidify conditions. Comparing T1-T2 correlations between sealed or re-saturated HCP and re-humidified HCPs, this off-diagonal relaxation component is likely derived from a part of irreversible C-S-H alteration under the first drying. This irreversibility cannot be recovered by re-wetting even at 95％RH other than re-saturation under water. Presented two-dimensional T1-T2 interpretation of microstructure of HCP indicated the new insight on unique pore structure change under re-wetting, including the part of irreversible change.
