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Limestone (calcite, CaCO₃) is an abundant and cost-effective source of calcium oxide (CaO) for cement and lime production. However, the thermochemical decomposition of limestone (~800 °C, 1 bar) to produce lime (CaO) results in substantial carbon dioxide (CO_2(g)) emissions and energy use, i.e., ~1 tonne [t] of CO₂ and ~1.4 MWh per t of CaO produced. Here, we describe a new pathway to use CaCO₃ as a Ca source to make hydrated lime (portlandite, Ca(OH)₂) at ambient conditions (p, T) while nearly eliminating process CO_2(g) emissions (as low as 1.5 mol. % of the CO₂ in the precursor CaCO₃, equivalentmore » to 9 kg of CO_2(g) per t of Ca(OH)₂) within an aqueous flowelectrolysis/ pH-swing process that coproduces hydrogen (H_2(g)) and oxygen (O_2(g)). Because Ca(OH)₂ is a zero-carbon precursor for cement and lime production, this approach represents a significant advancement in the production of zero-carbon cement. The Zero CArbon Lime (ZeroCAL) process includes dissolution, separation/recovery, and electrolysis stages according to the following steps: (Step 1) chelator (e.g., ethylenediaminetetraacetic acid, EDTA)-promoted dissolution of CaCO₃ and complexation of Ca²⁺ under basic (>pH 9) conditions, (Step 2a) Ca enrichment and separation using nanofiltration (NF), which allows separation of the Ca-EDTA complex from the accompanying bicarbonate (HCO₃^–) species, (Step 2b) acidity-promoted decomplexation of Ca from EDTA, which allows near-complete chelator recovery and the formation of a Ca-enriched stream, and (Step 3) rapid precipitation of Ca(OH)₂ from the Ca-enriched stream using electrolytically produced alkalinity. These reactions can be conducted in a seawater matrix yielding coproducts including hydrochloric acid (HCl) and sodium bicarbonate (NaHCO₃), resulting from electrolysis and limestone dissolution, respectively. Careful analysis of the reaction stoichiometries and energy balances indicates that approximately 1.35 t of CaCO₃, 1.09 t of water, 0.79 t of sodium chloride (NaCl), and ~2 MWh of electrical energy are required to produce 1 t of Ca(OH)₂, with significant opportunity for process intensification. This approach has major implications for decarbonizing cement production within a paradigm that emphasizes the use of existing cement plants and electrification of industrial operations, while also creating approaches for alkalinity production that enable cost-effective and scalable CO₂ mineralization via Ca(OH)₂ carbonation.« less

Binders containing portlandite (Ca(OH)₂) can take up carbon dioxide (CO₂) from dilute flue gas streams (<15% CO₂, v/v), thereby forming carbonate compounds with binding attributes. While the carbonation of portlandite particulates is straightforward, it remains unclear how CO₂ transport into monoliths is affected by microstructure and pore moisture content. Therefore, this study elucidates the influences of pore saturation and CO₂ diffusivity on the carbonation kinetics and strength evolution of portlanditeenriched composites (“mortars”). To assess the influences of microstructure, composites hydrated to different extents and conditioned to different pore saturation levels (S_w) were exposed to dilute CO₂. First, reducing saturation increasesmore » the gas diffusivity and carbonation kinetics so long as saturation exceeds a critical value (S_w,c ≈ 0.10) independent of microstructural attributes. Second, 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. Furthermore, these insights are foundational to create new “low-CO₂” cementation agents via in situ CO₂ mineralization (utilization) using dilute CO₂ waste streams.« less