Influences of silica porous shell microcapsules on the impermeability of hardened cement mortar

By using tetraethyl orthosilicate (TEOS), epoxide resin (E51), styrene (St), et al. as the main materials, a kind of porous silica shell microcapsules (PSSM) were synthesized through in-polymerization in the core and hydrolysis-polycondensation for forming the shell. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to characterize the morphology, chemical composition and core shell ratio of the synthesized PSSM. Influences of PSSM on the impermeability and pore structure of hardened cement mortar via alternating current impedance spectroscopy technique (ACI) and mercury intrusion porosimetry (MIP) were studied after the hardened cement mortar samples were preloaded by 80% their compressive strength loads and cured including water curing or dry-wet cycling curing. The characterization results prove that the obtained product presents core-shell structured microcapsules with size of 10-100 μm, in which poly-siloxane forms porous shell and liquid form epoxy resin held by a polystyrene network composes the core. The mass ratio of the core to shell is 1.54. Compare with the blank samples without preloading-curing treatment, connected pore solution resistance R_CH and diffusion resistance coefficient σ of the preloading-curing treated blank samples decrease and their porosities increase, which suggests that the preloading treatment leads to the formation of micro-cracks in the specimen and the following curing does not result in full healing of the formed micro-cracks. On the other hand, for those sample containing 8% PSSM, R_CH and σ of the preloading-curing treated samples are even higher than those of the non-treated samples. This phenomenon is attributed to the micro-cracks formation due to the preloading treatment allows invasion of water into the body of the mortar specimens during the following water immersion. Thus, the PSSM that is incorporated into the mortar samples contributes to crack healing by leakage of the epoxy resin from the core through the intact or broken porous shell and the subsequent curing reaction with the curing agent located in cement matrix. The above mentioned results provide a clear proof of concept for such self-healing microcapsules.