Abstract
At 100-nanometer length scale, the mesoscopic structure of calcium silicate hydrate (C-S-H) plays a critical role in determining the macroscopic material properties, such as porosity. In order to explore the mesoscopic structure of C-S-H, we employ two effective techniques, nanoindentation test and molecular dynamics simulation. Grid nanoindentation tests find different porosity of C-S-H in cement paste specimens prepared at varied water-to-cement (w/c) ratios. The w/c-ratio-induced porosity difference can be ascribed to the aspect ratio (diameter-to-thickness ratio) of disk-like C-S-H building blocks. The molecular dynamics simulation, with a mesoscopic C-S-H model, reveals 3 typical packing patterns and relates the packing density to the aspect ratio. Illustrated with disk-like C-S-H building blocks, this study provides a description of C-S-H structures in complement to spherical-particle C-S-H models at the sub-micron scale.
Highlights
Calcium silicate hydrate (C-S-H) gel is a major hydration product in cement
Considering that the porosity could be related to the structure of calcium silicate hydrate (C-S-H), we develop a mesoscopic model and perform molecular dynamics simulations to investigate how the w/c ratio induces changes to the C-S-H structure and affects the gel porosity
Grid nanoindentation results show increasing amounts of low-density C-S-H phases and increasing porosity as w/c ratio increases from 0.3 to 0.66. This change of porosity could be related to the mesoscopic C-S-H structure, which is assumed to be composed of disk-like C-S-H building blocks
Summary
Calcium silicate hydrate (C-S-H) gel is a major hydration product in cement. In decades of studies, a lot of information about the microstructure of C-S-H has been collected and translated into theoretical descriptions and numerical models. Up to the sub-micron level (i.e. 10 to 1000 nanometers), it is generally accepted that C-S-H is made up of small building blocks with a characteristic length of around 4–5 nm[17,18] These basic units can flocculate to form high-density (HD) or low-density (LD) C-S-H depending on packing density[19,20,21]. A mesoscopic molecular dynamics model is developed using the concept of disk-like objects[11,30,31] Simulations using this model could embody the lamellar nature of C-S-H gel at the scale of around 100 nanometers. Grid nanoindentation tests are employed to measure the material properties and molecular dynamics simulations are performed to reproduce the structures of C-S-H. This study provides illustrations of the mesoscopic structure of C-S-H, links the C-S-H structure to the porosity and could enrich our understandings of cement-based materials at small scales
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