Armoring of MgO by a Passivation Layer Impedes Direct Air Capture of CO2

Exploring the Potential of Nonclassical Crystallization Pathways to Advance Cementitious Materials

Understanding the crystallization of cement-binding phases, from basic units to macroscopic structures, can enhance cement performance, reduce clinker use, and lower CO2 emissions in the construction sector. This review examines the crystallization pathways of C-S-H (the main phase in PC cement) and other alternative binding phases, particularly as cement formulations evolve toward increasing SCMs and alternative binders as clinker replacements. We adopt a nonclassical crystallization perspective, which recognizes the existence of critical intermediate steps between ions in solution and the final crystalline phases, such as solute ion associates, dense liquid phases, amorphous intermediates, and nanoparticles. These multistep pathways uncover innovative strategies for controlling the crystallization of binding phases through additive use, potentially leading to highly optimized cement matrices. An outstanding example of additive-controlled crystallization in cementitious materials is the synthetically produced mesocrystalline C-S-H, renowned for its remarkable flexural strength. This highly ordered microstructure, which intercalates soft matter between inorganic and brittle C-S-H, was obtained by controlling the assembly of individual C-S-H subunits. While large-scale production of cementitious materials by a bottom-up self-assembly method is not yet feasible, the fundamental insights into the crystallization mechanism of cement binding phases presented here provide a foundation for developing advanced cement-based materials.

Reaction Layer Formation on MgO in the Presence of Humidity

Mineralization by MgO is an attractive potential strategy for direct air capture (DAC) of CO2 due to its tendency to form carbonate phases upon exposure to water and CO2. Hydration of MgO during this process is typically assumed to not be rate limiting, even at ambient temperatures. However, surface passivation by hydrated phases likely reduces the CO2 capture capacity. Here, we examine the initial hydration reactions that occur on MgO(100) surfaces to determine whether they could potentially impact CO2 uptake. We first used atomic force microscopy (AFM) to explore changes in reaction layers in water (pH = 6 and 12) and MgO-saturated solution (pH = 11) and found the reaction layers on MgO are heterogeneous and nonuniform. To determine how relative humidity (R.H.) affects reactivity, we reacted samples at room temperature in nominally dry N2 (∼11-12% R.H.) for up to 12 h, in humid (>95% R.H.) N2 for 5, 10, and 15 min, and in air at 33 and 75% R.H. for 8 days. X-ray reflectivity and electron microscopy analysis of the samples reveal that hydrated phases form rapidly upon exposure to humid air, but the growth of the hydrated reaction layer slows after its initial formation. Reaction layer thickness is strongly correlated with R.H., with denser reaction layers forming in 75% R.H. compared with 33% R.H. or nominally dry N2. The reaction layers are likely amorphous or poorly crystalline based on grazing incidence X-ray diffraction measurements. After exposure to 75% R.H. in air for 8 days, the reaction layer increases in density as compared to the sample reacted in humid N2 for 5-15 min. This may represent an initial step toward the crystallization of the reaction layer. Overall, high R.H. favors the formation of a hydrated, disordered layer on MgO. Based on our results, DAC in a location with a higher R.H. will be favorable, but growth may slow significantly from initial rates even on short timescales, presumably due to surface passivation.