Sustainable concrete production with recycled concrete wash water beneficiated with CO2

A significant amount of wastewater is generated through the cleaning of equipment utilized within the ready mixed concrete production cycle. The reuse of concrete wash water as mix water is limited by the negative material performance impacts associated with the suspended solids; the effects are exacerbated with increasing solids contents and water aging. A novel carbon dioxide treatment to allow the use of high solids wash water (specific gravity 1.10) as mix water was examined. Seven batches of concrete were produced and compared: a reference mix, two batches with untreated wash water and four batches with CO2 treated wash water. The carbon dioxide treatment mineralized CO2 at 28% by mass of the treated solids. Acceptable concrete was produced through adjusting admixtures for workability. The compressive strength at 1, 7, 28 and 56 days was improved relative to both the reference and the concrete produced with untreated wash water. The suspended solids containing mineralized CO2 served as a viable cement replacement. The avoided cement and bound carbon dioxide served to lower the carbon impact of the concrete by about 14%. The approach allows three waste streams (CO2, wash water and wash water solids) to be reused to produce more sustainable concrete

Maximizing carbon uptake and performance gain in slag-containing concretes through early carbonation

Carbon dioxide (CO2) emissions have been identified as a major contributor to climate change. Current CO2 mitigation efforts focus on the removal, recovery and disposal of CO2 at point sources. Finding beneficial uses of as-captured or recovered CO2 is a critical challenge in greenhouse gas mitigation. This thesis investigates the possibility of the beneficial use of carbon dioxide in precast concrete production and the performance, both short-term and long-term, of the concretes so produced. The calcium compounds in cementitious materials react readily with carbon dioxide to convert CO2 to thermodynamically stable carbonates. The reaction accelerates strength development and makes the technology appropriate for early age curing. Paste, mortar and concrete samples were examined to quantify such aspects as the carbon dioxide uptake, strength development, and durability of carbonated concrete. It was found that the uptake by the cementitious binders was significant. Compared to their theoretical capacity, cement could reach a carbonation degree of over 25% when treated as pastes and about 20% when used as a part of concrete. The study compared carbonation-cured and hydrated Portland cement concrete and slag cement concretes in terms of their early strength, late strength, weathering carbonation shrinkage, freeze/thaw durability, water absorption, and pH. The carbonated concrete was generally comparable, or superior, to the hydrated concrete except for the case of a 50% GGBF slag blend which had a slower strength development due to reduced secondary cementitious reaction. A second method of binding carbon into concrete was considered by carbonating ladle slag fines and usLes émissions de dioxyde de carbone (CO2) contribuent de façon importante aux changements climatiques. Les efforts actuels de mitigation du CO2 se concentrent principalement sur la saisie, la récupération et le débarras du CO2 aux sources ponctuelles. Trouver des usages bénéfiques au CO2 tel que la capturé ou récupération représente un défi critique pour la mitigation des gaz à effet de serre. Cette thèse examine la possibilité d'utiliser le CO2 de façon bénéfique dans la production du béton préfabriqué, ainsi que la performance à court et à long termes du béton ainsi produit. Les composés calcaires dans le ciment réagissent avec le dioxyde de carbone pour convertir le CO2 en carbonates qui sont stables du point de vue thermodynamique. Cette réaction accélère le développement de la résistance du béton, ce qui rend la technologie appropriée pour des bétons à cure rapide. Des échantillons de pâte, de mortier et de béton ont été examinés pour tenter de quantifier certains aspects tels l'absorption du dioxyde de carbone, le développement de la résistance et la durabilité du béton carbonaté. L'absorption du CO2 par les liants dans le ciment fut importante. Comparé à leur capacité théorique, le ciment pourrait atteindre un degré de carbonation de plus de 25% quand il est préparé sous forme de pâte et de 20% quand il est utilisé dans le béton. L'étude a considéré des bétons composés de ciment Portland et de ciment de scories, curés de façon traditionnelle (hydratation) et curés par carbonatation, et a comparé leurs résistance après 7 jours, résistance après 56 jours, résistance à la contraction lors de la

Beneficiation of concrete wash water with carbon dioxide

Wash water is generated as a by-product of ready mixed concrete production. Reuse of the water as mixture water is limited, in practice, by the negative material performance impacts associated with the water chemistry and suspended solids (i.e. hydrating binder phases and very fine aggregate); the effects are intensified with increasing content of suspended solids and water age. A novel carbon dioxide treatment to allow the use of high solids wash water as mixture water was examined through mortar and concrete testing. An industrially sourced wash water with a specific gravity of over 1.20 was treated with CO2. Seven batches of concrete were produced and compared: a reference mixture at three different w/b, and four batches with a 6% reduction in cement where wash water was used as mixture water (diluted to a specific gravity of 1.08, comparing untreated versus CO2 treated conditions, aged either 1 day or 5 days). The treatment mineralized CO2 at 27% by weight of the cement in the solids and reduced or eliminated negative aspects associated with the untreated water (set acceleration, workability loss, strength reduction with water age). The CO2-treated solids displayed latent hydraulicity. The approach allows three waste streams (CO2, wash water and wash water solids) to be reused to produce more sustainable concrete