Microstructural Controls on Mineral Carbonation Reactions

Abstract:

Mineral carbonation (a.k.a. “carbon mineralization”) describes a suite of chemical and geochemical reactions wherein various metal substrates (typically calcium or magnesium-based) react with gaseous carbon dioxide (CO₂) to produce carbonate minerals. Mineral carbonation processes have been receiving increasing interest as a potential option to reduce carbon dioxide buildup in the atmosphere, from both “disposal” and “utilization” perspectives. For mineral carbonation to serve as a permanent and effective sink for CO₂ (and further, to be used as a process to create useful materials for construction or industrial applications) the carbonation reactions and products need to be accurately characterized in terms of (1) reaction kinetics, (2) reaction products (and product stability), and (3) 3D structural influences on both kinetics and products.

Herein we present analysis of magnesium-based mineral carbonation experiments in idealized porous columns. The temporal evolution and interplay of 3D microstructure and mineralogy was quantified using a novel combination of X-ray computerized tomography (CT), and mineralogical (diffuse reflectance Fourier transform infrared spectroscopy, FTIR, and X-ray diffraction, XRD) analyses, conducted at five timepoints over 108 days. A new method of re-scaling X-ray CT attenuation intensity values was used to provide a proxy measurement for the evolving density of the cement phase; when combined with digital volume correlation techniques and mineralogical observations, this analysis allows for 3D observations of reaction progress on a microscopic level. The results demonstrate how 3D structural characteristics impact reaction progress; e.g. regions within samples with reduced access to connected void volume (i.e. reduced access to elevated CO₂ concentrations) exhibit slower reaction, while enhanced access to connected void promotes reaction speed and carbonate formation. Some suggestions for potential methodological improvements for future mineral carbonation studies are highlighted.