Influence of Nano C-S-H on Cement Hydration, Pore Structure of Hardened Cement Pastes and Strength of Concrete

Effects of synthesized nanohydrated calcium silicate (n-C-S-H) on cement hydration, pore structure of hardened cement pastes and concrete strength were investigated using isothermal calorimetry, non-evaporative water test, X-ray diffraction, mercury intrusion test and compressive strength tests. The results show that n-C-S-H accelerates the early hydration of cement, significantly increases the hydration degree of cement in the age of 12-24 h, and thereby significantly enhances the compressive strength of concrete in the age of 12-24 h. The strength enhancement effect gradually reduces after 24 h. After 3 days, the acceleration effect on cement hydration and the enhancement effect on compressive strength of concrete become unobvious and the addition of 1.5% n-C-S-H even leads to lower compressive strength in comparison with the blank concrete sample. Compared with blank cement paste, the capillary porosity and the threshold pore size of the hardened cement pastes with addition of n-C-S-H decreases in the age of 12 h-3 d, while the gel porosity increases. Meanwhile, the incorporation of n-C-S-H in cement pastes changes the generation mode of C-S-H gel, which makes a large amount of the hydration product of C-S-H growing from the added n-C-S-H particles, instead of the typical growth from the surface of cement grains due to the so-called heterogeneous nucleation. The massive growth of C-S-H from the added n-C-S-H nuclei leads to the decrease of capillary porosity and the increase of inter-gel porosity. This multi-cores growth model causes a looser structure of hydration product and, thus reduces the strength of the C-S-H matrix. Because of this reason, at the same hydration degree, the compressive strength of concrete mixed with n-C-S-H is slightly lower than that of the blank concrete. © 2019, Editorial Department of Journal of the Chinese Ceramic Society. All right reserved.