# Improvement of Recycled Aggregates Properties by Means of CO\({}_{2}\) [PERSON]
## 1 Introduction  
Increasing urban growth worldwide consumes natural resources and increases the amount of deconstruction waste. According to the literature, the European Union and China produced 860 million tons and 1.5 billion tons of deconstruction waste in 2015, respectively [1]. Recycling concrete in the form of recycled concrete aggregates (RCAs) is described as the most effective way of reducing global demand for natural aggregates. Each country has its own recommendations for their use. In France, the rate of RCA substitution in concrete is currently governed by the standard NF EN 206/CN. It depends on the exposure conditions [2], and the fine fraction (sand) of the RCAs is not used. The stated values were determined by studying the properties of RCAs, considered to be weaker than the ones in natural aggregates due to the presence of bonded cement paste. In particular, RCAs do exhibit high porosity and a high water absorption coefficient [3; 4; 5; 6; 7; 8]. Their use in concrete is therefore limited, because this old cement paste affects the properties of the new concrete in both the fresh and hardened states [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. To increase the substitution rate, the properties of RCA must therefore be improved by mechanical, thermal or chemical treatments. Such treatments have two aims: to separate the cement paste from the RCA or to strengthen this cement paste [22; 23]. In addition, the production of cement is responsible for 5-7% of global carbon dioxide (CO\({}_{2}\)) emissions [24], with up to 540 kg of CO\({}_{2}\) emitted per ton of clinker as a result of the decarbonation of limestone. The 2016 Paris Agreement, which is a universal agreement on climate change, committed the world's countries to limiting global warming to between 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C by reducing their emissions of greenhouse gases, including CO\({}_{2}\)[25]. To reduce the ecological impact of concrete, the storage of CO2 is a possible option to contribute to the goal of carbon neutrality. Some techniques in the cement industry are used to remove CO2 from CH4 in membranes or to capture CO2 in mesoporous silica materials [26; 27]. In the literature, the uptake of CO2 is also achieved by accelerated carbonation of RCAs. With regard to the parametric study of the storage of CO2 in RCAs, an optimum water content was found in [28; 29; 30; 31], which led to a maximum level of carbonation. This optimum amount occurs due to a balance between a lack of water, which limits the dissolution of CO2, and an excess of water that limits its diffusion. This optimum amount is not described in the literature for RCAs. Moreover, this optimum amount is not applied by a process of impregnation and drying to obtain the saturated surface dry state of RCA, as described for the measure of the water absorption coefficient in [32]. The experimental results also showed that the initial carbonation status can affect the diffusion of CO2 due to the formation of calcium carbonates [33], but no data has been presented about CO2 uptake. CO2 uptake can also be affected by the type and size of the RCA, by a high cement paste content and by the diffusion of CO2 in the total volume of the RCA. Fine RCA can take up more CO2 than coarse RCA [29; 34; 35; 36]. An increase in the concentration of CO2 increases its uptake, but this increase is limited by the quantity of calcium carbonates that are formed during the carbonation tests [37]. The accelerated carbonation of old cement pastes is also increased by the temperature, as described in [38; 39]. A higher temperature affects the solubility of the hydrates and the diffusion of CO2, which increases the contact between the gas and the cement matrix and enhances CO2 uptake. With regard to the improvement of the properties of RCAs by accelerated carbonation, it has been found that the storage of CO2 reduces the water absorption coefficient (WAC) [37], but these values are affected by the size and source of the RCAs.