When subjected to fires, cementitious composites can be seriously damaged. However, the mechanical behavior of nanoscale calcium silicate hydrate (C–S–H) grains, which are the main binder of cementitious composites, exposed to elevated temperatures under shear deformations remain poorly investigated. In this paper, considering different calcium/silicate (C/S) molar ratios (i.e., C/S = 1.10, 1.33, and 1.64), the shear behavior of the C–S–H grain after exposure to different high-temperature levels (i.e., 300 K, 500 K, 700 K, 900 K, and 1000 K) is studied by conducting series of reactive molecular dynamics simulations. Results reveal that the C–S–H grain exhibits good plasticity under shear deformation. Furthermore, the shear modulus of the C–S–H grain is between 14 GPa and 17 GPa and exhibits a decrease with the calcium/silicon (C/S) molar ratio at ambient temperature. While the shear strength is around 1.0 GPa and reaches the lowest value at C/S = 1.64. Interestingly, we report that heating can lead to the increase of the shear modulus and shear strength due to the sevaporation of the interlayer water which generally acts as the lubricant. Additionally, we show that heating has no clear influence on the shear strain corresponding to the onset of yielding when the C/S ratio is high but can indeed improve the shear strain corresponding to the shear strength. Furthermore, the yielding area of the C–S–H grain under the shear deformation can be enlarged. Finally, factors affecting the strength degradation of cementitious composites after being heated to different temperature levels are further discussed based on the simulation results.

当遭受火灾时，水泥复合材料可能会受到严重损坏。然而，作为水泥复合材料的主要粘合剂的纳米级水合硅酸钙（C-S-H）颗粒在剪切变形下暴露于高温下的力学行为仍然很少研究。在本文中，考虑不同的钙/硅酸盐 (C/S) 摩尔比（即 C/S = 1.10、1.33 和 1.64），C-S-H 晶粒在暴露于不同高温水平后的剪切行为（即，300 K、500 K、700 K、900 K 和 1000 K）通过进行一系列反应分子动力学模拟来研究。结果表明，C-S-H晶粒在剪切变形下表现出良好的塑性。此外，C-S-H 晶粒的剪切模量在 14 GPa 和 17 GPa 之间，并且在环境温度下随着钙/硅 (C/S) 摩尔比的增加而降低。而剪切强度约为 1.0 GPa，在 C/S = 1.64 时达到最低值。有趣的是，我们报告说，由于通常作为润滑剂的层间水的蒸发，加热会导致剪切模量和剪切强度的增加。此外，我们表明，当 C/S 比高时，加热对与屈服开始相对应的剪切应变没有明显影响，但确实可以提高与剪切强度相对应的剪切应变。此外，C-S-H晶粒在剪切变形下的屈服面积可以扩大。最后，