# The Effect of Curing Conditions on Selected Properties of Recycled Aggregate Concrete
## 1 Introduction
The authors of [33] focused on the effects of steam temperature and its application time, conducted on 28-day recycled aggregate concrete, indicating an upper temperature limit for low-pressure brewing at the level of 50 \({}^{\circ}\)C and a steam curing application time of no more than 1 h in order to avoid the reduction of compressive strength. The application for more than 2 h significantly reduced the strength. The strength and durability of concrete, as key parameters determining its quality, depend on the amount of cement, which should be optimized in terms of eco-efficiency. The proposal to optimize the cement content in accordance with the requirements of the designed concrete by taking into account two environmental impact factors when determining the composition of the concrete is related to strength [34]. The binder intensity index (\(b_{i}\)), which expresses the mass of cement per 1 m3 of concrete necessary to achieve the strength of 1 MPa (kg/m\({}^{3}\)/MPa), and the carbon dioxide index (\(c_{i}\))--being expressed as the mass of carbon dioxide emitted during the production process of such a quantity of cement--will allow the achievement of the strength of 1 MPa (kg/MPa) for the concrete. In both cases, the lowest possible values should be obtained. The optimal solution is to produce concretes of higher strength, because if the compressive strength is higher than 50 MPa, the \(b_{i}\) coefficient can reach 5 kg/m\({}^{3}\)/MPa, while in low-strength concretes (up to 20 MPa) \(b_{i}\) increases even up to 13 kg/m3/MPa. For the \(c_{i}\) coefficient, the minimum value is assumed at the level of 1.5 kg/MPa (in case of using mineral additives in the production of cements), whereas in pure clinker cements it is not possible to achieve a value lower than 4.3 kg/MPa. In relation to the above-mentioned approach, according to the authors of this study, the concept of sequestrational carbonation can be introduced, as at the same time the effect of absorbing CO\({}_{2}\) from the atmosphere is achieved, which also brings environmental benefits--carbonation closes the CO\({}_{2}\) cycle that began with the production of cement. According to [35], 1 m\({}^{3}\) of concrete can absorb even more than 100 kg of CO\({}_{2}\). In [36], as part of a life cycle assessment taking into consideration carbonation studies on recycled aggregate, it was stated that per one ton of concrete, from the moment of its crushing, contains 11 kg of absorbed CO\({}_{2}\), which corresponds to the absorption of approximately 25 kg CO\({}_{2}\) per 1 m\({}^{3}\) of concrete. This reduces CO\({}_{2}\) emission by 5.5% over the entire life cycle. The authors of this paper decided to focus on the impact of curing conditions on some properties of recycled aggregate concrete as there has not yet been much research in this area. The problem of water presence in RCA and its technological consequences was taken into account as well. The study was performed in laboratory conditions (at a temperature of 20 \({}^{\circ}\)C and at relative humidity of about 60%). The following curing conditions were selected in the laboratory: wet, dry, and protection against water evaporation. Dry conditions are not optimum for the hardening of cement matrix composites. However, they were chosen as the extreme and in order to identify potential differences between their impact and the effect of proper curing conditions. Such an approach was also inspired by comments in [37] on the impact of different hardening conditions on the characteristics of cement matrix composites produced with eco-friendly cements. Although studies have referred to cement mortars without fine recycled aggregate, [37] put such emphasis on the relationship between the hardening conditions and the type of cement which manifests even after long periods of hardening, that in the case of the experiment presented in this paper such assumptions of extreme curing conditions were made.