Mechanistic insight of white carbon black in regulating hydration and improving water resistance of magnesium oxysulfate (MOS) cement

Abstract

As a relatively new type of eco-friendly material, magnesium oxysulfate (MOS) cement offers a sustainable alternative to Portland cement. However, its poor water resistance poses a significant challenge to its wider applications. White carbon black (FS) predominantly consists of SiO₂ and exhibits a low degree of crystallinity. This study investigated the mechanism by which FS enhances the strengths and water resistance of MOS cement, using a range of characterisation techniques (NMR, HR-TEM, XPS, etc.). FS actively participated in the hydration of MOS cement by consuming the OH- released by MgO hydration or Mg(OH)₂, therefore transforming them into magnesium silicate hydrate gel, while also influencing the content of the 5Mg(OH)₂ ·MgSO₄·7H₂O phase, which exhibited adsorption of magnesium silicate hydrate gel on its surface, resulting in an intertwinement at the nanoscale between it and the magnesium silicate hydrate gel and increased in both compressive and flexural strength, as well as demonstrating compatibility. Additionally, the magnesium silicate hydrate gel provided partial protection against the decomposition of 5Mg(OH)₂·MgSO₄·7H2O phase upon water immersion. As the proportion of MgO/Mg(OH)₂ participating in the reaction varied, the resulting magnesium silicate hydrate exhibited corresponding shifts in its Mg/Si composition. The ongoing production of magnesium silicate hydrate gel under water immersion led to a 75.76% increase in the compressive strength of MOS cement, with the resulting gel's structure progressively approaching that of talc (T:O:T). A mechanism model of synergetic hydration of MOS cement with FS is proposed to elaborate on the findings.