## 1 Introduction  
Nowadays, the construction sector is confronted with two specific circumstances that influence its sustainability and will become increasingly important in the future. On the one hand, intensive construction activity requires larger quantities of concrete, which leads to an increased consumption of natural resources and energy. It is estimated that 14 billion m\({}^{3}\) of concrete was produced worldwide in 2020 [1]. In terms of natural resources, there is a particular focus on aggregates for concrete, the increasing use of which is leading to environmental degradation. Sand, gravel and crushed stone for construction alone account for almost 24% of the global extraction of non-metallic materials, and volumes are expected to double by 2060, reaching around 60 G [2]. In 2019, a total of around 3 Gt of aggregates was produced in EU countries, including UK and EFTA countries, of which 9.3% were recycled aggregates [3]. The production of concrete consumed around 5.5% of total primary energy demand in 2015 and was responsible for 8.5% of global greenhouse gas emissions [2]. On the other hand, the high intensity of construction means that more and more construction waste is being generated. For this reason, the recycling and reuse of construction waste as a new building product is becoming increasingly important in many industrialised countries around the world. Data show that around 30 to 50% of construction waste consists of concrete [4; 5]. It is estimated that 50 million tonnes of concrete waste are generated in China every year [6]. In the EU, concrete accounted for the largest share of construction and demolition waste (CDW) in 2020, estimated at around 56% or 74 million tonnes [7]. One of the sustainable ways to reuse concrete waste is to produce recycled concrete aggregate (RCA) and use it as a substitute for part of the aggregate in new concrete. In this way, several benefits are achieved, such as reducing the consumption of natural aggregates, reducing the amount of construction waste, and reducing the cost of transportation, energy consumption and overall CO\({}_{2}\) emissions [8; 9; 10]. Today, however, the majority of concrete waste is downcycled, by being used mainly for road construction and backfilling [7], or disposed of in landfills [6]. Whether this practice will change depends on national regulations. Numerous studies on RCA show that its quality can vary greatly, but also that its properties always fall short of those of natural aggregates. The density of RCA is up to 10% lower than that of natural aggregates, the water absorption of recycled aggregates is several times higher, the crushing coefficient according to the Los Angeles method is lower etc. [11; 12; 13]. In addition, recycled aggregates often contain microcracks due to the crushing process and are considerably more porous [14]. The results of experimental studies on concrete with RCA have shown that the incorporation of RCA into concrete reduces workability, reduces strength and stiffness, and increases porosity. The main reason for this is the poorer properties of the recycled aggregate due to the presence of old cement paste, which leads to higher absorption and the formation of several different interfaces (contact zones) in the concrete [15; 16; 17]. Since the matrix of self-compacting concrete (SCC) is adjusted to have better workability properties than normal concrete, the idea was brought up that the SCC paste matrix would serve as a better medium for RCA [18]. Compared to normal concrete, SCC contains more fine particles originating from cement, finest aggregate fraction and fillers, which affect the inter-particle forces in the fresh state and result in a refined microstructure and pore network in hardened concrete. Much research, particularly in the last twenty years, has focused on determining the effects of partial or complete replacement of natural coarse aggregate with RCA.