The effect of superhydrophobic nano-silica particles on the transport and mechanical properties of hardened cement pastes

The water permeability of hardened cement pastes (HCP) is closely related to the durability of cement-based materials since most deteriorations of cement-based materials result from the ingress of foreign ions. In this study, superhydrophobic nano-silica (SNS) particles with a dosage of 1%, 2%, and 4% (by cement mass) were added into cement pastes (water cement ratio is 0.3) to take advantage of the pozzolanic reactivity and hydrophobicity of SNS particles, and their influence on the water sorptivity and compressive strength of the HCP were investigated. In addition, the techniques of isothermal calorimetry, mercury intrusion porosimetry, and X-ray diffraction were combined to reveal the reinforcement mechanism of SNS particles. The results showed that the compressive strength was increased by 42%, 61%, and 71%, respectively, and that the water sorptivity was reduced by 4.2%, 13.8%, and 25.1%, respectively. Moreover, the increment of compressive strength and decrement of water sorptivity were positively proportional to the dosage of SNS particles. The improvement was attributed to the pozzolanic reaction between SNS and Portlandite produced by cement hydration, which can not only refine the pore structure of HCP, but also increase the hydrophobicity of calcium silicate hydrates. Furthermore, the dosage of superplasticizer also played a significant role in the development of HCP microstructure. The results indicated that addition of SNS particles is a promising method to enhance the durability of cement-based materials.