# The Effect of Curing Conditions on Selected Properties of Recycled Aggregate Concrete
## 1 Introduction  
The annual production of concrete --as the most popular construction material--amounts to nearly 10 billion tons per year. Undoubtedly, significant consumption concerns mineral natural and crushed aggregates resulting from the crushing of rock raw materials. This leads, among other effects, to the depletion of natural resources, the violation of ecosystems, and carbon dioxide emission, the latter also resulting from the production of cement (in 2017 with a 4% share in global anthropogenic CO\({}_{2}\) emission [1]), which, as well as aggregate, is a key component of concrete and despite a smaller share in the quantity of concrete compared to the share of aggregate, exceeds the emission from the exploitation of natural aggregates, the production of crushed aggregates, and the transport of aggregates [2]. Moreover, the construction industry generates large amounts of waste through the reconstruction and demolition of buildings, which means that it takes up space and engenders high costs for waste disposal. The mitigation of the above-mentioned factors detrimental to the environment by using recycled concrete aggregate (RCA) is obvious. However, RCA is a material of a poorer quality (e.g., large porosity, water absorption, content of irregularly sharper grains, lack of uniformness, higher water demand) in comparison with the quality of natural aggregates. Special attention should be paid to the use of fine recycled aggregate (FRA) for its pernicious effect on the properties of cement matrix composites. The presence of FRA decreases compressive strength and water absorbability [3], more so with highershares of FRA [3; 4] and more significantly when fine recycled aggregate is soaked in comparison to its application in dry state. The pores of FRA are unable to fill with water completely during soaking [5]. Therefore, [6] proposes realizing the saturation process of FRA under vacuum, reaching a degree of saturation of 99.7%. However, such an approach does not yet seem possible from an economic viewpoint. Thus, the use of FRA in concrete technology is still controversial and studies on it need to be developed. The properties of RCA can be modified in different ways, among others by heating and rubbing [7; 8], ultrasonic cleaning [9], mechanical grinding, acid cleaning [10], and even biodeposition [11; 12]. An effective method to improve the quality of recycled aggregate is to carbonate it [13; 14; 15]. A significant disadvantage of the recycled concrete aggregate-influenced durability of recycled aggregate concrete (RAC) is the moisture content in RCA [16]. One of the difficulties of manufacturing recycled aggregate concrete mixes is the apparent density of aggregate combined with its porosity [17], responsible for water absorbability, which is in the range of 3-10% [18]. The water absorbability of recycled aggregates does not have to be a technological problem in the concrete manufacturing process, but only on condition that it is measured correctly. If it is underestimated, the workability of the concrete mix deteriorates, and the hydration processes are disturbed. Therefore, two ways of adding water to concrete mix are proposed, resulting from the water absorption of recycled aggregate [19]: compensation (the use of additional water resulting from the absorbability of recyclable aggregates) and the pre-soaking of RCA with water. The water absorption time of RCA is also essential. However, the authors of publications are not in agreement on this aspect. It is rare to achieve full saturation after 24 h or more, sometimes even taking up to 120 h [19]. According to [20], the absorption time should be equal to 10 min. Other authors [21; 22] have pointed to a longer time (20-30 min). The choice of a not very long period is justified by the study in [23], which concluded that the use of both dry recycled aggregate and aggregate in the state of full saturation worsens the frost resistance of concrete.