Unravelling depth-dependent carbonation pathways of reactive magnesium cement produced from different MgO sources

This study investigates the effects of three salt-lake by-product–derived and two magnesite-derived MgOs on the compressive strength and depth-dependent carbonation behavior of reactive magnesium cement (RMC). Mineralization products, morphology, and pore structure were characterized to reveal reaction mechanisms. The results show that salt-lake by-product–derived MgO possessed a high specific surface area and low to medium reactivity, forming abundant plate-like Mg(OH) 2 and amorphous hydration products that filled pores and disrupted connectivity. Although substantial surface carbonation was observed with nesquehonite as the dominant phase, CO 2 diffusion toward the interior was limited. Carbon analysis confirmed this strong carbonation gradient, with CO 2 uptake decreasing from 5.62–8.82 wt% near the surface to only 0.70–1.37 wt% at a depth of 16–20 mm. In contrast, magnesite-derived MgO exhibited higher reactivity and lower specific surface area. XCT analysis further revealed a pronounced surface-to-interior porosity gradient in salt-lake-derived RMC, whereas magnesite-derived RMC exhibited a uniformly dense microstructure throughout the specimen. These results demonstrate that MgO source governs carbonation pathways, pore evolution and strength development.