Impact of Carbonation on the Water Resistance and Mechanical Properties of Magnesium Oxychloride Cements

Magnesium oxychloride cements (MOC) and their composites have gathered significant attention in recent years as sustainable alternatives to traditional Portland cement, primarily due to their lower carbon footprint and superior mechanical properties [1]. The primary hydration products at ambient temperature are Phase 3 (3Mg(OH)₂·MgCl₂·8H₂O) and Phase 5 (5Mg(OH)₂·MgCl₂·8H₂O), which are responsible for the material’s strength and durability [2]. MOC exhibits several advantageous characteristics, including rapid setting, high early strength, fire resistance, and low alkalinity, making it suitable for a range of construction applications. However, its practical use is limited by poor water resistance, primarily due to the leaching of magnesium chloride from Phase 5, which leads to a significant reduction in mechanical strength [2]. To overcome this, various additives such as phosphoric acid, soluble phosphates, and fly ash have been explored to enhance water resistance of MOC [1]. Recent studies have also highlighted MOC’s capacity to undergo carbonation, forming surface magnesium chlorocarbonates like chlorartinite, which can improve both durability and environmental performance [2]. However, there are contradictory findings suggesting the metastable character of chlorartinite, which can further transform into other magnesium carbonates under certain conditions and as a consequence compromise the long-term stability of the superficial layer [3]. Given the dual benefit of CO₂ sequestration and improved material properties, this study investigates the phase transformations induced by carbonation and their effects on water resistance and mechanical strength of MOC. Therefore, multiple MOC formulations were developed: one using half-calcined dolomite as the sole MgO source, and another combining MgO from magnesite with half-calcined dolomite. After full hardening, the samples underwent forced carbonation. Water resistance (after 24 hours immersion) and compressive strength were evaluated before and after carbonation, with X-ray diffraction (XRD) used to identify phase changes and ion chromatography employed to quantify MgCl₂ leaching. The findings reveal that carbonation leads to a change in Phase 5 content and the formation of chlorartinite and other crystalline and amorphous phases, depending on the formulation. A clear correlation was observed between reduced MgCl₂ leaching and enhanced compressive strength. These results suggest that carbonation not only mitigates the water sensitivity of MOC but also contributes positively to its structural integrity and environmental sustainability.