Emergence of an Isolated Pore within Calcium-Silicate-Hydrate Gel after Primary Desorption: Detection by 2D 1H NMR T1–T2 Correlation Relaxometry

Calcium silicate hydrate (C-S-H) is the primary hydration product of modern Portland cement pastes and concrete. Concrete is inevitably dried and rehumidified when it is hardened with water, for operation under ambient conditions. This drying and rehumidification induces a change in the microstructure of the C-S-H agglomerate and is considered the driving factor of anomalous moisture transport in cement pastes. To obtain further insights into the microstructural changes in C-S-H in response to drying/rehumidification, hardened ordinary Portland cement pastes were subjected to a first drying and rehumidification process for >6 months in this study. The relative humidities (RHs) at 20 °C were 23%–75% for first drying and 11%–95% for rehumidification after drying at 105 °C. Two-dimensional ¹H NMR T₁–T₂ relaxation correlation measurements, rather than conventional T₂ measurements, were conducted on the conditioned samples. Under sealed conditions, all of the pore-water-related features appeared on the T₁–T₂ correlation map on a diagonal at a unique T₁/T₂ ratio. Once the paste was first-dried at RH < 75% or rehumidified at RH < 95%, ¹Hs corresponding to water in the interlayer–gel pores appeared on a diagonal with a T₁/T₂ ratio similar to that of the sealed state, whereas an off-diagonal component was newly identified at T₁/T₂ > 10 with a T₂ value between those of interlayer and gel pores for all the first-dried/rehumidified samples. Although a major change in the water content during drying/rehumidification was observed on the diagonal, the off-diagonal peak likely emanated from the microstructural change in the C-S-H agglomerate upon first drying at RH < 75%. Additionally, the off-diagonal component was consistently observed upon drying/rehumidification, except after the resaturation of the dried paste. Therefore, the appearance of an off-diagonal relaxation component during first drying could be an irreversible feature of the C-S-H microstructural rearrangement.