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An important part of improving the embodied carbon of the built environment is reducing carbon emissions associated with concrete. The long-term limitations around the availability of supplementary cementitious materials (SCMs) to replace portland cement have driven the search for additional innovative approaches. The beneficial use of carbon dioxide (CO2) in ready mixed concrete production has been developed and installed as retrofit technology with industrial users. An optimum dose of CO2 added to concrete as an admixture leads to the in-place formation of mineralized calcium carbonate (CaCO3) and can increase the concrete compressive strength. The improved performance can be leveraged to design concrete mixture proportions for a more efficient use of portland cement, along with the use of CO2 to reduce the carbon footprint of concrete. One producer has used the technology, starting in 2016, at over 50 plants. More than 3 million m3 of concrete have been shipped with an estimated net savings of 35,000 tonnes of CO2. The concrete produced with carbon dioxide is discussed in terms of the fresh and hardened performance, durability performance, and life cycle impacts. [ABSTRACT FROM AUTHOR]

Wash water is generated as a by-product of ready mixed concrete production. The water can be reclaimed to be used as mix water in new concrete. The properties, durability and sustainability of concrete produced using high solids reclaimed water, subjected to a novel CO2 (carbon dioxide) treatment, as mix water was examined. A simulated reclaimed water with a specific gravity of 1.10 was used as a partial (44%) replacement for mix water in concrete production. A comparison was made between reference concrete, batches made with untreated reclaimed water, and reclaimed water treated with CO2 to either a low or high CO2 uptake. The treatment mineralized CO2 at 5 and 20% by weight of the cement in the reclaimed water solids. The mix designs either included the reclaimed water slurry as a replacement for the mix water or further incorporated a reduction to the virgin binder in proportion to the added reclaimed water solids. The compressive strength was improved with the use of both untreated and treated wash water; it is suggested that the improvements are attributable to cementitious properties of the solids, improved particle packing in the binder phase and an effective reduction of the water to binder ratio. A suite of testing (including RCPT, bulk resistivity, bulk diffusion, abrasion, freeze/thaw, salt scaling, drying shrinkage, carbonation, and corrosion) demonstrated that the durability was not affected by the incorporation of the treated reclaimed water. The treated slurry batches had a 10–11% improvement in the carbon index (kg CO2 embodied/MPa of strength) for the low treatment and 13–14% for the high treatment. [ABSTRACT FROM AUTHOR]

Carbon dioxide was investigated for use as a beneficial admixture to concrete as it was truck mixed. The reaction between the CO 2 and the hydrating cement creates finely distributed calcium carbonate reaction products that thereby influence the subsequent hydration. Comparisons of the fresh, hardened and durability properties were made between a reference concrete batch, a batch that contained a conventional accelerating admixture, and three batches subjected to a carbon dioxide addition. The optimum dose of carbon dioxide was found to reduce the time to initial set by 40% and increase the one and three day compressive strengths by 14% and 10% respectively. In comparison to the CO 2 batch, the conventional accelerator provided greater reductions in set time but lower early strength. Concrete durability test results indicated that the carbon dioxide process did not compromise the expected durability performance of the treated concrete. Carbon dioxide is a viable admixture to improve concrete performance. [ABSTRACT FROM AUTHOR]

Early age carbonation curing of slag-cement concrete was investigated to assess the feasibility of binding CO2 in slag-cement building products while improving their short-term and long-term performances. Four binder types were compared: an ordinary portland cement; an 85/15 slag cement; a 75/25 slag blend; and a 50/50 slag blend. A 2-h carbonation-curing treatment allowed concretes to bind 8–10% CO2 by mass of binder and attain as much as 82% of the 24-h hydration strength. The subsequent strength development of carbonated concrete was slower in the first 24 h possibly due to the carbonate buildup, but it was comparable to the conventionally hydrated concrete after 28 days. The carbonated concrete was shown to have a fracture toughness comparable to that of the hydrated concrete. The freeze/thaw durability of the concrete in deicing salt solution was vastly improved by the carbonation treatment. The pH of the carbonated concrete was reduced but was still above the threshold level required for the passivation of iron. The use of slag in carbonation curing is beneficial to strength gain, shrinkage reduction, and deicing salt resistance. [ABSTRACT FROM AUTHOR]