Molecular Dynamics Simulation of the Precipitation of Calcium Silicate Hydrate Nanostructures under Two-Dimensional Confinement by TiO₂: Implications for Advanced Concretes

Given the environmental footprint of concrete, the demand for new concrete with higher macroscopic performances is continuously increasing to build structures with less concrete while extending the service life. With the advances in nanotechnology, comprehensive investigations have shown that nanomodification of concrete can significantly improve the macroscopic performances. However, many questions regarding the interaction between nanomaterials and cement hydrates at the nanoscale are still unclear, which greatly limits the further development of nanoengineered concrete in construction. Herein, we use reactive molecular dynamics simulation to investigate the precipitation of calcium silicate hydrate gel (C-S-H gel, the principal binding phase of conventional concrete) in the TiO2 nanochannel with different spacings. Results show that the polymerization kinetics as well as the final degree of polymerization of C-S-H can be enhanced under the TiO2 nanoconfinement. Up to 15% Q4 units form in the more polymerized C-S-H under TiO2 nanoconfinement. Because of the difference in adsorption capacity for Ca and Si ions, the chemical composition of C-S-H precipitated on the surface of TiO2 with oxygen dangling bonds will result in the nanosegregation of Ca-rich and Si-rich regions, while the highly connected Qn units (Q3 and Q4 units) are formed in the Si-rich regions. We also show that the hydroxylation of TiO2 surface drives the polymerization process of C-S-H. On the basis of the water mobility in the C-S-H gels, we demonstrate the C-S-H growth in a limited spacing and precipitated on the TiO2 surface, resulting in a more compact nanostructure.