Effects of silica micropowder on the long-term water stability of MKPC paste

Magnesium potassium phosphate cement (MKPC) are future materials for the corrosion protection of steel–reinforced concrete structures owing to their environmental friendliness, strong adhesion to the substrate, and excellent resistance to salt corrosion. In this study, an MKPC coating paste satisfying the design specifications of self-compacting concrete was prepared by increasing the water-cement (W/C) ratio and adding Na₂SiO₃ and silica micropowder (SMP) to regulate the fluidity and compatibility. The mechanical strength, deformation, water absorption, and long-term water stability were compared for MKPC coating pastes with and without SMP. Replacing MgO powder with 10% SMP could improve the physical and mechanical properties of MKPC paste under long-term water immersion. After 540 d of water immersion, the flexural and compressive strength residuals of the reference MKPC without SMP (0SMP10M) were 84.4% and 74.6%, respectively. In contrast, those of the MKPC with 10% SMP (1SMP9M) were 105.6% and 97.9%, respectively. The volume of 0SMP10M expanded during 540 d of water immersion, and the final volume deformation was 10.1 × 10⁻⁴, while the volume expansion of 1SMP9M was 7.46 × 10⁻⁴. After 540 d of immersion, the water absorption levels of 0SMP10M and 1SMP9M were 1.45% and 1.17%, respectively.