Understanding the effect of pre-water-saturated pottery sand on the microstructure and water resistance of the slag-blended MgO-SiO2-K2HPO4 system

Abstract

The poor water resistance has long limited the widespread application of magnesium phosphate cement (MPC). This study utilized the high-alkaline of the MgO-SiO₂-K₂HPO₄ system to activate slag and incorporated pre-water-saturated pottery sand to prepare a water-resistant magnesium-based cement (SP-MSPC). The research primarily examined the effect of pottery sand on the water resistance and microstructure of SP-MSPC with different slag substitution rates. The results indicated that an increase in pottery sand content prolonged the setting time and reduced the fluidity of the paste. The incorporation of higher amounts of pottery sand decreased the mechanical properties of the matrix, but the addition of pottery sand effectively promoted paste hydration, particularly slag hydration, significantly enhancing the water resistance of the matrix. The water resistance coefficient of the S20-P6 matrix at 56 days was 72.42 %, a 61.86 % increase compared to the control group S0-P0. This can be attributed to the internal curing effect of the pottery sand, which promotes paste hydration and the continuous formation of K-struvite crystals and the water-resistant M-S-H and C-(A)-S-H gels. The crystal and gel phases were bonded to each other and work synergistically with the dense filling effect of ultrafine slag particles, effectively filling the matrix pores and microcracks, blocking moisture transport channels, slowing the penetration of external moisture, and reducing the dissolution of K-struvite crystals and other hydration products aqueous environments. The incorporation of pottery sand promoted slag hydration enhanced the average polymerization degree of the C-(A)-S-H gel, and reduced the volume of macropores within the matrix.