Elucidating the Interaction Mechanism of Mg(OH)₂ and Ca(OH)₂ under Enforced Carbonation

There is a potential for leveraging the synergy of carbonation of portlandite and Mg-based binders to improve the CO2 uptake, mechanical property, and thermodynamic stability of Mg- and Ca-based binders and enable its application. This work reports insights into the carbonation behavior of Mg(OH)2–Ca(OH)2 (MH–CH) mixtures including the precipitation of different carbonates, degree of reaction of each phase, mechanical properties, microstructure characterization, and thermodynamic stability. Results indicate that the compressive strength and degree of carbonation (DOC) of compacts increase first and then decrease with the increase of the Ca(OH)2 content. The compact containing 25% Ca(OH)2 yields the highest compressive strength of 91.4 MPa, while the compact containing 75% Ca(OH)2 has the highest DOC of 91.2%. The formation of nesquehonite with a lower density in compacts with lower contents of Ca(OH)2 (0 and 25%) results in a higher volume increase even at a very low DOC (14.2%), leading to a more densified structure in the compacts. Compacts with higher contents of Ca(OH)2 (50, 75, and 100%) precipitate calcite and magnesium calcite with higher thermodynamic stability than nesquehonite and improved water stability. Meanwhile, the high temperature in the initial stage of Ca(OH)2 carbonation causes severe water evaporation and insufficient water in compacts, thus restricting the continuous carbonation at the center having a lower DOC than the surface. The mixing of Mg(OH)2 and Ca(OH)2 significantly enhances the carbonation degree of both hydroxides, which may be explained by the complementation of Ca(OH)2 and Mg(OH)2 carbonation, in which the water released by the carbonation of Ca(OH)2 is bound to the crystal structure of hydrated magnesium carbonates (HMCs) from Mg(OH)2 carbonation.