Analysis of the Compressive Strength of CO₂ Concrete While Eliminating Overshadowing Concrete Variables

1 Extended Abstract

Although other technologies for the improvement of recycled aggregate, such as removal of adhered cement, improvement of aggregate by methods including nanomaterials, sequential mixing and precoating, the ideology of sequestration of CO₂ into cementitious materials provides great potential for both the enhancement and utilisation of recycled concrete materials and sequestration of CO₂. However, a great deal of study has focused on the complete carbonation of recycled aggregate without consideration of a practical timeframe for use with ready-mix concrete. Furthermore, the carbonation of recycled aggregate concrete does provide strength increases that can be overshadowed by other concrete mix design variables such as the water to cement (w/c) ratio and aggregate replacement percentage. Consequently, here, we focus on practical carbonation timeframes with a maximum of 2 h where the carbonation reaction is most prevalent, and keep other major concrete variables consistent to ensure the effect of carbonation can clearly be observed. The injection of CO₂ into recycled aggregate sequesters the undesirable gas while reducing the porosity and water absorption of the recycled aggregate. When CO₂ is injected into recycled aggregate, a chemical reaction occurs, converting the CO₂ into a mineral, calcium carbonate. The converted calcium carbonate fills air voids within the aggregate and consequently improves its mechanical performance. The carbonated aggregate concrete is also known as CO₂ concrete. Many researchers have investigated this mechanism, but on many occasions the true effect of carbonation variables (e.g., pressure and duration) on the compressive strength of the concrete cannot be observed due to other concrete variables, such as the w/c ratio and aggregate replacement percentage, which can overshadow the performance compressive increase provided by the carbonated recycled aggregate. In our study, we provided a constant w/c ratio and separate recycled aggregate percentages so that the effect of the carbonation variables could be truly observed. The longer carbonation duration of 120 min surpasses the lesser durations of 60 and 30 min. Furthermore, generally the lower carbonation pressure of 25 kPa provided greater results over the 75 and 200 kPa treated samples. Our results show that practical carbonation variables can help to increase the mechanical properties of recycled aggregate concrete. Furthermore, without other mix design variables a longer carbonation duration is of great importance to improving the properties of recycled aggregate concrete. The carbonation of recycled aggregate can help to make the greener product mainstream.