G. Falzone, I. Mehdipour, G. Sant
University of California, Los Angeles,
Keywords: concrete, mineralization, carbonation, flue gas, kinetics
Summary:Carbon capture and utilization processes that exploit “CO2 mineralization” reactions to produce concrete products offer a transformative platform for gigaton-scale CO2 utilization, globally, due to the vast market for concrete (> 20 billion tons, ~ $1 trillion annually). The reaction of portlandite (Ca(OH)2),1 an archetypal alkaline solid, with carbon dioxide (CO2) is one of such reactions that can be exploited to produce cementation agents with a low embodied-carbon intensity. This process is the core of the CO2Concrete technology, the development of which is supported by analysis of the effects of reaction temperature, relative humidity (RH), and CO2 concentration on the carbonation of portlandite in the form of finely divided particulates and monolithic compacts, and of the influences of pore saturation and CO2 diffusivity on the carbonation kinetics and strength evolution of portlandite-enriched composites (“mortars").2 Special focus is paid to uncover the factors affecting the extent of reactant (i.e., Ca(OH)2) conversion in relation to the process conditions, moisture state (i.e., the presence of molecular or condensed water), and the size and surface area-to-volume ratio (SA/V, mm-1) of monoliths. Notably, the carbonation of portlandite is not limited by surface passivation of reactants with products; rather, reaction progress is limited by the mobility of adsorbed water. The carbonation of portlandite particulates is broadly straightforward, however, it has remained unclear how CO2 transport into monoliths is affected by microstructure and pore moisture content. Scaling from particulates to monolithic components, the imposition of a porous microstructure (through which CO2 must diffuse prior to reaction) induces secondary conversion limits related to blocking of pore networks by condensed water. Further, the carbonation kinetics of monoliths are strongly linked to the rates of moisture transport and vaporization/condensation. In cementitious portlandite-enriched composites the influences of microstructure and pore water saturation (Sw) are evaluated by controlling degrees of cement hydration and drying prior to exposure to dilute CO2. Reducing saturation increases the gas diffusivity, and carbonation kinetics, so long as saturation exceeds a critical value (Sw,c ≈ 0.10); independent of microstructural attributes. Careful analysis reveals that both traditional cement hydration and carbonation offer similar levels of strengthening, the magnitude of which can be estimated from the extent of each reaction. As a result, portlandite-enriched binders offer cementation performance that is similar to traditional materials while offering an embodied CO2 footprint that is more than 50 % smaller. The outcomes provide new insights into the mechanisms of the water-mediated mineralization of CO2 by portlandite, and other alkaline solids (e.g., fly ash)3 in particulates and composite structures. Such knowledge is foundational to create scalable pathways for the production of new cementation agents, the synthesis of which is based on the direct utilization of CO2 from flue gas streams.