David Bastos

CO₂ emissions, a significant contributor to climate change, have spurred the exploration of sustainable solutions. One putative solution involves using recycled aggregates (RAs) from construction and demolition waste (CDW) to substitute natural sand in construction materials. This not only extends the life cycle of the waste but also reduces the use of natural resources. The potential to capture CO₂ in RAs presents a promising route to mitigate the environmental impact of the construction industry and contribute to its much anticipated decarbonization. This research takes a unique approach by investigating the incorporation of an amine-based additive—specifically 2-amino-2-methyl-1,3-propanediol (AMPD)—to enhance CO₂ capture into a real-case RA from recycling plants, transforming CDW with low carbon-capture potential into a highly reactive CO₂ capture material. Through TG analysis, FTIR-ATR and the combination of both (TG-FTIR), we were able to validate the use of RA materials as a support medium and quantify the CO₂ capture potential (12%) of the AMPD amine; a dual valorization was achieved: new value was added to low-quality CDW and we enhanced CO₂ sequestration, offering hope for a more sustainable future.

This research presents a sensitivity analysis of various parameters that affect the carbonation of recycled aggregates (RAs), namely CO₂ concentration, temperature, and relative humidity. The range of parameter values is close to that found in cement plant chimneys with regard to the forced carbonation of RAs. With this purpose, the main characteristics of flue gas streams (CO₂ concentration, temperature, and relative humidity) from two Portuguese cement plants were identified and used in this research. The results indicated that temperatures around 60 °C and CO₂ concentrations around 25% accelerate the carbonation reaction and increase CO₂ absorption in mixed recycled aggregates (MRAs). CO₂ absorption consistently decreased as the relative humidity was reduced from 60% to 40%. The highest amount of CO₂ captured was by a recycled concrete aggregate (RCA) in the conditions of 23 °C, 60% RH, and 25% CO₂. Overall, the RAs were able to capture a significant amount of CO₂, ranging from 52 to 348 kg of CO₂ per tonne of cement paste, depending on the nature of the RA. These findings drawn from a parametric campaign provide valuable insights into the potential enforcement of carbonation for recycled aggregates under conditions that closely reflect those found in cement plants.

Global warming is one of the most important issues that the world is currently facing. The cement industry accounts for around 7% of total global CO₂ emissions. According to the 13th United Nations Sustainable Development Goals, cement plants must become carbon neutral by 2050. This neutrality may be achieved by a reduction in CO₂ emissions complemented with carbon capture, utilization and storage (CCUS) technologies. In accordance with these sustainable goals, several approaches have been studied. This paper investigates life cycle carbon of mortars produced with carbonated recycled aggregates. In previous works, the carbon dioxide capture capacity of construction and demolition waste (CDW) was analysed, and mortars with CDW recycled aggregates submitted to high levels of CO₂ were evaluated in terms of their mechanical performance. This paper focus on the life cycle carbon impact assessment (LCCA) of industrial mortar formulations in a cradle-to-gate boundary. This assessment is carried out through a global warming potential environment impact assessment, since it represents the amount of CO₂ equivalent that is sent to the atmosphere and contributes to the “greenhouse effect”. This LCCA includes the impacts associated with the treatment and additional transportation routes of the recycled aggregates. With this work, it was found that mortars with carbonated recycled aggregates have a considerably lower global warming potential impact than mortars without recycled aggregates. The mortars with recycled aggregates presented lower CO₂ emissions of up to 6.31% for 100% incorporation of non-carbonated recycled aggregates. These values were incremented with the carbonation of the recycled aggregates, achieving a reduction of CO₂ emissions of up to 36.75% for 100% of incorporation.

Cementitious materials can capture CO₂ through carbonation reaction during their service life and post-demolition. Indeed, construction and demolition waste (CDW) still have some potential for carbonation as they contain concrete and cement-based mortars. This research consists of an experimental programme to evaluate the CO₂ capture of recycling aggregates (RAs) from CDW. Two types of CDW were studied, namely mixed recycled aggregates (MRAs) and recycled concrete aggregates (RCAs). The recycled aggregates were submitted to forced and accelerated carbonation at 23 °C, 60% relative humidity and 25% of CO₂ concentration. This study contributes to the existing literature by investigating more realistic RA sources that have already absorbed atmospheric CO₂ during their service life. From the experimental campaign, the results show that RCAs have higher carbonation potential when compared to MRAs due to the higher cementitious material content (Rc) and to the degree of natural carbonation. The recycled aggregates’ maximum CO₂ capture was assessed by thermogravimetric analysis (TGA) at different CO₂ exposure times. It was verified that the maximum CO₂ capture, respectively, for MRAs and RCAs, occurred after 5 h and 12 h of exposition. In short, CDW captured from 5 wt.% to 35 wt.% of CO₂ per tonne of cement paste, which corresponds to 0.6% to 4.1% per tonne of aggregate. It was concluded that the carbonation process of CDW has the potential to sequester from 123 kg to 225 kg of CO₂ per tonne of cement paste for MRAs and 52 up to 491 kg of CO₂ per tonne of cement paste for RCAs.

Construction and demolition wastes (CDW) are generated at a large scale and have a diversified potential in the construction sector. The replacement of natural aggregates (NA) with CDW recycled aggregates (RA) in construction materials, such as mortars, has several environmental benefits, such as the reduction in the natural resources used in these products and simultaneous prevention of waste landfill. Complementarily, CDW have the potential to capture CO₂ since some of their components may carbonate, which also contributes to a decrease in global warming potential. The main objective of this research is to evaluate the influence of the exposure of CDW RA to CO₂ produced in cement factories and its effect on mortars. Several mortars were developed with a volumetric ratio of 1:4 (cement: aggregate), with NA (reference mortar), CDW RA and CDW RA exposed to high levels of CO₂ (CRA). The two types of waste aggregate were incorporated, replacing NA at 50% and 100% (in volume). The mortars with NA and non-carbonated RA and CRA from CDW were analysed, accounting for their performance in the fresh and hardened states in terms of workability, mechanical behaviour and water absorption by capillarity. It was concluded that mortars with CDW (both CRA and non-carbonated RA) generally present a good performance for non-structural purposes, although they suffer a moderate decrease in mechanical performance when NA is replaced with RA. Additionally, small improvements were found in the performance of the aggregates and mortars with CRA subjected to a CO₂ curing for a short period (5 h), while a long carbonation period (5 d) led to a decrease in performance, contrary to the results obtained in the literature that indicate a significant increase in such characteristics. This difference could be because the literature focused on made-in-laboratory CDW aggregates, while, in this research, the wastes came from real demolition activities, and were thus older and more heterogeneous.