Nano-scale mechanical properties investigation of C-S-H from hydrated tri-calcium silicate by nano-indentation and molecular dynamics simulation

Calcium silicate hydrate (C-S-H) is the most important hydration product in cementitious material. Due to its complexity, the investigation on its mechanical properties is still in progress. In this study, synthesized C3S was utilized to produce relatively pure C-S-H gels with two different water/solid ratios. Then mechanical properties of the C-S-H were measured and characterized by nano-indentation and SEM/EDX/BSE. Two types of C-S-H with Ca/Si ratio around 1.8 were observed through statistical analysis of the indentation modulus (Er) and hardness (H). The looser one possessing lower Er around 25-30 GPa while the denser one possesses higher Er of 35.9 GPa. Both phases have higher stiffness than those of high density (HD) and low density (LD) phase obtained from cement pastes. Furthermore, molecular dynamics provides mechanical understanding of C-S-H gel at nano-scale. Based on the crystal structure of tobermorite 11 angstrom, 64 models with Ca/Si ratios from 1.3 to 2.0 were constructed for uniaxial tension test. The stiffness and cohesive strength of the C-S-H gel are greatly influenced by Ca/Si ratio, water content and nano-porosity. With increasing Ca/Si ratio from 1.3 to 2.0, long silicate chains were broken to shorter ones and more water molecules penetrated into defect regions. The destroyed silicate skeleton and the "hydrolytic weakening" effect resulted in the reduction of the mechanical properties of C-S-H gels. Concurrently considering the intrinsic porosity and chemical compositions, the appropriate micromechanical models are utilized to upscale the molecular properties to higher level. (C) 2018 Elsevier Ltd. All rights reserved.