Viscoelastic nature of calcium silicate hydrate

The origin of the time-dependent response of cement-based materials to applied stress has not been clearly resolved. The role of interlayer water in the mechanical behavior of calcium silicate hydrate (C-S-H) is still debated. In order to better understand the pertinent mechanisms, the stress relaxation tests were conducted on thin rectangular beams of compacted synthetic C-S-H powder and hydrated Portland cement subjected to three-point bending. C-S-H specimens of variable composition (C/S = 0.8, 1.2 and 1.5) were prepared at various moisture content levels from saturation to the dry state. A special drying procedure was applied in order to remove the adsorbed and interlayer water incrementally from C-S-H conditioned at 11%RH. It was shown that a significant part of the relaxation at saturation is attributed to the hydrodynamic component associated with the pore water. It was demonstrated that the viscoelastic performance of C-S-H depends considerably on the presence of interlayer water. It was argued that the results support the validity of the theory of sliding of C-S-H sheets as a time-dependent deformation mechanism responsible for the creep and stress relaxation of cement-based materials. This concept was illustrated in a proposed model for the viscoelastic response of C-S-H. (C) 2010 Elsevier Ltd. All rights reserved.