Probing the time-dependent behavior of calcium-silicate-hydrate gel by nanoindentation continuous stiffness measurement

Abstract

Calcium-silicate-hydrate (C–S–H) is the most important phase in concrete materials, and plays a dominant role in the time-dependent properties of concrete. However, the evolution of mechanical behavior of C–S–H gels over time remains unclear, leading to a lack of fundamental understanding of the mechanical behavior of cementitious materials over time. In this study, nanoindentation continuous stiffness measurement (CSM) is adopted to probe the time-dependent behavior of C–S–H gel. Furthermore, the corresponding mechanism was analyzed and discussed. The results show that C–S–H structure units would occur preferred orientation arrangement under external load, and the process of completing the preferred orientation arrangement is expedited under greater load, resulting in decreased fluctuations in storage modulus and hardness. For different types of C–S–H, when the same load function is applied, the C–S–H gel with higher packing density first completes the preferred orientation arrangement. Besides, the hardness of C–S–H measured by the CSM was more sensitive to the indention time or depth than the storage modulus. Overall, this research provides direct evidence for C–S–H re-orientation from a micromechanical perspective.

Keywords

• C-S-H gel
• Continuous stiffness measurement
• Nanoindentation
• Microstructure
• Storage modulus
• Hardness