Matrix relaxation and change of water state during hydration of peat

In this study, hydration of a peat sample was investigated with differential scanning calorimetry (DSC) in terms of glass transition behavior and the formation of freezable and unfreezable water. Special attention was drawn to the development of these characteristics in the course of hydration and to plasticizing and antiplasticizing effects of water. Freezable water was formed above a water content threshold of Θcrit = (23 ± 7)% and revealed structured melting peaks indicating freezable bulk-like water and freezable bound water. The freezable bound water revealed a broad, kinetically controlled melting endotherm, and the melting barrier increased with increasing hydration time. Glass transitions were found in between 43 °C and 68 °C with a change in heat capacity of ΔC = (0.13 ± 0.08) J g−1 K−1. Glass transition behavior does not fully match the theoretical expectations and is linked with water binding. Water reveals a short-term plasticizing function in the range of days as well as a slow antiplasticizing function in the range of weeks or even months. The findings are consistent with the hydrogen bond based cross-linking model (HBCL) suggested in a previous study. Non-equilibrium and matrix relaxation are considered the rule rather than the exception in nature and underline the ecological relevance of hydration, the relevance for sorption and transport phenomena was well as possibly for soil development.