Environmental concerns have 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 gas injection system provided CO2 into a ready-mix truck during batching and mixing of a concrete with a binder containing 20% slag.The impacts of carbon dioxide dose, injection method and time of injection were examined.The slump and concrete temperature were 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 benefit is attributable to the influence of carbon dioxide on the very early hydration of the cement.A combination of isothermal calorimetry and microstructural analysis suggests that nano-scale carbonation reaction products 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.