Carbon Dioxide Upcycling to Improve Ready-Mixed Concrete

Environmental awareness has prompted research into the use of carbon dioxide as a feedstock in the production of concrete. Industrial trials have examined the use of CO₂ in ready-mix production. A CO₂ gas injection system delivered carbon dioxide into a ready-mix truck during batching and mixing of concrete. The carbon dioxide was thereby absorbed into the concrete and formed stable carbonate reaction products. The impacts of carbon dioxide dose, injection method and time of injection were examined. The concrete slump and temperature was measured. Compressive strength and resistivity were measured at ages from 24 hours to 58 days. Strength improvements up to 14% at 24 hours and 26% at 28 days were observed. The resistivity was not affected by the carbonation treatment. The strength increase is attributable to the reaction of carbon dioxide with the cement very early after hydration started. A combination of isothermal calorimetry and microstructural analysis suggests that nano-scale carbonates seed the hydration and contribute to the development of a stronger microstructure. The strength benefit suggests that carbonation is a possible tool to provide acceleration that could allow the greater use of supplementary cementitious materials in concrete mixtures. The use of waste carbon dioxide in the process offers a means to upcycle the CO₂ as a beneficial additive to concrete that could be widely employed throughout the built environment.