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

There is a potential for leveraging the synergy of carbonationof portlandite and Mg-based binders to improve the CO2 uptake,mechanical property, and thermodynamic stability of Mg- and Ca-basedbinders and enable its application. This work reports insights intothe carbonation behavior of Mg(OH)(2)-Ca(OH)(2) (MH-CH) mixtures including the precipitation of differentcarbonates, degree of reaction of each phase, mechanical properties,microstructure characterization, and thermodynamic stability. Resultsindicate that the compressive strength and degree of carbonation (DOC)of compacts increase first and then decrease with the increase ofthe Ca(OH)(2) content. The compact containing 25% Ca(OH)(2) yields the highest compressive strength of 91.4 MPa, whilethe compact containing 75% Ca(OH)(2) has the highest DOCof 91.2%. The formation of nesquehonite with a lower density in compactswith lower contents of Ca(OH)(2) (0 and 25%) results in ahigher volume increase even at a very low DOC (14.2%), leading toa more densified structure in the compacts. Compacts with higher contentsof Ca(OH)(2) (50, 75, and 100%) precipitate calcite and magnesiumcalcite with higher thermodynamic stability than nesquehonite andimproved water stability. Meanwhile, the high temperature in the initialstage of Ca(OH)(2) carbonation causes severe water evaporationand insufficient water in compacts, thus restricting the continuouscarbonation at the center having a lower DOC than the surface. Themixing of Mg(OH)(2) and Ca(OH)(2) significantlyenhances the carbonation degree of both hydroxides, which may be explainedby 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. This work reports insights into the carbonationbehaviorof Mg(OH)(2)-Ca(OH)(2) mixtures, which havebeen proposed to be sustainable binders as potential CO2 sinkers.