Effect of drying method on calcium silicate hydrate (C-S-H): Experiments and molecular dynamics simulations study

Terminate hydration and drying procedures were necessary before characterizing the cement-based materials. This study investigated the influence of drying method (-including freeze-drying and vacuum-drying) on the composition-structure of main hydration product (C-S-H) in cement-based materials. In addition, the effect of pH value was considered. The in-situ atomic scale change of C-S-H under different drying conditions was described using molecular dynamics simulations (MDS). The results reveal that freeze-drying has a stronger drying effect than vacuum-drying. Freeze-dried C-S-H samples have lower bound water content, smaller interlayer distance, longer mean chain lengths (MCL), denser pore structure and lower crystallinity than vacuum-dried samples. As the pH values increase, the bound water content, interlayer distance, MCL and cumulative pore volume decrease. In addition, although the micromorphology of freeze-dried samples is similar to vacuum-dried ones, the crystallinity of freeze-dried samples is lower than that of vacuum-dried samples. MDS provides evidence for the influencing mechanism of drying method on interlayer distance and chain length of C-S-H from the atomic scale. It also proves that the complexation between water molecules and interlayer Ca2+ is more accessible to be destroyed in freeze-drying than vacuum-drying, resulting in a higher reduction of interlayer distance after freezedrying. Furthermore, water loss due to drying action breaks the Ca-OH bond, enhances Ca2+ movement in the interlayer of C-S-H and facilitates the generation of Ca-O structure in the interlayer, leading to the increase in chain length of C-S-H.