The influence of defects on the mechanical behaviors of tobermorite and C-S-H—A comparative study

Calcium silicate hydrate (C-S-H), as the main binding agent in concrete, significantly influences its strength and toughness. However, the influence of its defective structures and inevitable nanoscale defects are still not clear. With molecular dynamics simulations, we have investigated the impact of nanoscale defects and portlandite inclusions on the mechanical properties of C-S-H and its crystal analog, tobermorite. It is found that the peak stress in the z-direction (perpendicular to the layered structures) tends to decrease with increasing porosity for both C-S-H and tobermorite, and the effect of random pore distribution is more obvious. The portlandite inclusions positively increase the strength of the tobermorite but have less effect on C-S-H, indicating that the defective sub-nano structures in the C-S-H matrix mainly determine the strength of C-S-H. In the y-direction (parallel to the layered structure), C-S-H presents different post-peak mechanical behavior from tobermorite, which shows hardening behavior. The influence of spatial pore distributions and portlandite inclusions is more significant on tobermorite than C-S-H. Besides, the portlandite inclusions positively improve the toughness of the structure, especially in the presence of random pores with the same diameter. The results show that spatial pore distribution and inclusion have complicated effects on C-S-H and toberomite. The findings provide new physical insights for the design of stronger and tougher cementitious materials.