Co₂ Upcycling Via Mineralization of a Carbonate-Based Construction Material – Processing-Property Relationships of Co₂Ncrete™

To address the issue of anthropogenic CO₂ emissions in a manner that is immediately economically viable in today’s markets, it is necessary to develop routes for large-scale CO₂ utilization (rather than sequestration). CO₂ mineralization (i.e., carbonation) into stable carbonate compounds (e.g., CaCO₃ polymorphs) is an attractive method of CO₂ utilization, due to its cost-effectiveness and the high abundance of magnesium/calcium-bearing precursors. Under appropriate reaction conditions, the formation of carbonates by CO₂ mineralization can provide cementation (i.e., strength between aggregate grains), acting as a functional replacement of portland cement-based binders within concrete. The resulting material, CO₂NCRETE™, exhibits a high specific CO₂ uptake, and therefore has significant CO₂ avoidance potential as a replacement for portland cement concrete. CO₂NCRETE™ is derived from a paste containing hydrated lime (the reactant for the CO₂ mineralization/uptake reaction), sand/aggregates, water, and suitable performance modifiers, e.g., dispersants and supplementary binders. The CO₂NCRETE™ manufacturing process has been architected to efficiently utilize vapor phase CO₂ from a wide variety of flue gas streams, including emissions from cement kilns, petroleum refineries, and fossil fuel power plants. While the feasibility of CO₂NCRETE™ fabrication has been demonstrated in a laboratory scale, further efforts are yet required to develop this material to a maturity level suitable for market adoption.

As a critical step in this direction, we investigate the relationships between the composition of CO₂NCRETE™ formulations, their processing conditions (e.g., CO₂ pressure/concentration, reaction time/temperature), and their mechanical properties in the hardened state (e.g., compressive strength). First, relationships between the extent of CO₂ mineralization reaction as characterized via thermogravimetric analysis (TGA), and microstructural changes (e.g., porosity reduction) of model slurry-based hydrated lime specimens are evaluated. The CO₂ mineralization reaction is observed to manifest in an approximately 10 MPa improvement in compressive strength, depending on reaction extent (i.e., CO₂ uptake). Next, the influence of reaction temperature on the rate of CO₂ mineralization, and the compressive strength of the resulting CO₂NCRETE™ specimens is examined. Finally, supplementary strength-enhancing additives (e.g., calcium sulfoaluminate cement, polymeric adhesives) are evaluated for their ability to augment the compressive strength afforded by carbonation, with efficiency described in terms of the embodied CO₂ footprint (i.e., MPa/kg CO₂e) of the Ca(OH)₂-binder composite. The outcomes indicate that the CO₂ uptake/strength gain that accompanies carbonation may be exploited synergistically with properly selection/dosage of supplementary binders, to produce CO₂-efficient alternative binder systems with performance equivalent to that of portland cement concrete.