Structure, dynamics and mechanical properties evolution of calcium silicate hydrate induced by dehydration and dehydroxylation

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The dehydration and dehydroxylation of C-S-H gel ubiquitously occurs in the thermal destruction or recycle of the concrete materials, and the latter is closely related to the sustainability in the construction industry. Utilizing reactive molecular dynamics simulation, this paper presents an investigation on the structure, dynamics and mechanical properties evolution of the C-S-H with decreasing water content, at the molecular level. Dynamically, it is found that the dehydration and dehydroxylation are successively occurred as the hydration degree of C-S-H decreases. At the dehydration stage, water molecules are dissociated to maintain the number of hydroxyls. The C-S-H structure does not change much at this stage. At the dehydroxylation stage, the number of hydroxyl groups sharply decreases, along with large structural transformation in the C-S-H, including fast decreasing interlayer spacing, merging of interlayer calcium layers and disordering of the primary calcium silicate layers. With respect to the mechanical properties, the interlayer spacing collapse significantly increases the stiffness and toughness of the C-S-H structure. On the other hand, drastic volume shrinkage of the C-S-H structure due to dehydroxylation can lead to decreasing contact points between C-S-H nano-globules. This means an increase in the mechanical properties of C-S-H matrix but a decrease in the packing density of C-S-H gel. Eventually, the indentation modulus of C-S-H gel monotonously decreases during dehydration and dehydroxylation.