| Summary: | In this paper, we prepared permeable protective coatings composed of lithium silicate (Li<sub>2</sub>SiO<sub>3</sub>), where the coating was modified by colloidal nano-silica (CNS). Three levels of lithium silicate (i.e., 30 wt. %; 40 wt. %; 50 wt. %), sodium silicate (i.e., 5 wt. %; 10 wt. %; 15 wt. %), and surfactant (i.e., 0.05 wt. %; 0.1 wt. %; 0.15 wt. %) were involved in this study. An orthogonal experiment design selected the optimal proportion basedon thestrength and water absorption requirements of mortar. The effects of CNS-Li<sub>2</sub>SiO<sub>3</sub> coating on the resistance to permeability of chloride ions and carbonation of specimens were also studied. The outcomes were interpreted using scanning electron microscopy (SEM), X-ray diffraction (XRD), and mercury intrusion porosimetry (MIP) techniques. The results showed that the optimum mix formulation consisted of 40 wt. % of lithium silicate, 10 wt. % of sodium silicate and 0.1 wt. % of surfactant within the mixtures investigated. Meanwhile, compared tothe control group, after the specimens were coated at 21 days curing age of mortar, the strength development, 48-h water absorption, resistance to chloride ions penetration, and carbonation of CNS-Li<sub>2</sub>SiO<sub>3</sub> coated specimenswere improved. This could be attributed to the second hydration, leading to a reduction of the content of Ca(OH)<sub>2</sub> and an increase of the amount of C–S–H gel within specimens. Thus, the microstructure of mortar matrix was improved after coated with CNS-Li<sub>2</sub>SiO<sub>3</sub> permeable protective coatings.
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