## 1 Introduction  
Due to the rising need for urbanization, concrete usage is expected to be doubled by 2050 [1]. Given the fact that aggregates comprise around 70% by volume of concrete, alarming depletion rates of non-renewable natural resources such as river sand and limestone are expected [2]. Already, there are more than 30 billion tons of natural aggregates consumed annually for producing concrete globally. The average increase in aggregates use is estimated at 5% per year globally, which means that the total could be around 66.3 billion tonnes by 2022 [3]. According to study [4], recycled aggregate (RA) still represents only three percent of the total aggregates demand. In Europe, construction and demolition waste (CDW) occupies one third of the waste produced, but significant differences can be found in the level of recycling among different countries [2]. Similarly, the construction industry is the second largest in generating waste, either during construction or demolition [5]. Four billion tonnes of CDW are generated in China yearly [6] and around 900 million tonnes in Europe [7]. Surprisingly, only 5% of the currently generated CDW is being recycled in concrete in any form [4]. Accordingly, special attention is devoted to the potential of recycling CDW as aggregates in concrete to simultaneously reduce the threat of depleting natural resources and avoid the landfilling of CDW [2,8,9].  
In line with the European Union Directive [10] that encourages the reuse and recycling of waste materials, researchers examined the feasibility of creating sustainable concrete by replacing naturally sourced aggregates (NA) in concrete with recycled aggregates from CDW in terms of mechanical properties, durability, environmental impact (EI) and cost [11, 12, 13, 14, 15, 16]. Generally, it was agreed among researchers that in addition to the potential of being a cheaper alternative, producing recycled aggregates concrete (RAC) could reduce the EI of concrete specifically in terms of \"landfill use\". However, it was also concluded that the technical performance would be adversely affected. Using RA, instead of NA, leads to a reduction of the mechanical properties and durability of a RAC compared to conventional concrete [17, 18, 19, 20, 21].  
Regardless of the numerous studies found, there was no consensus found on the replacement ratio of fine NA by fine RA (RFA) from CDW that would achieve the optimum sustainability-potential of the resulting concrete mix. The sustainable development potential of a material depends on three main parameters: its functionality, its EI and economic viability [22]. Therefore, this paper will attempt to assess the optimum sustainability potential, based on a multi criteria decision analysis framework, which combines these three sources. However, the scope of the paper will only focus on RFA from CDW (RFA) being incorporated, with different percentages, in concrete replacing naturally sourced fine aggregates. Recently, there was a shift in the direction of the research concerning recycled aggregates focusing more on RFA made with glass, lightweight materials, etc. The reason is that these studies [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39] recommended not to include CDW as RFA anymore just on the basis of its functional properties. For that reason, in this study, the applicability of RFA in concrete in terms of all parameters (quality, environmental impact, energy consumption, toxicity and cost) was considered. Regression models were developed based on data obtained from the literature, and then, a multi criteria decision making analysis framework was used to combine the results for the mentioned parameters and to find the optimum incorporation level of RFA in concrete. However, it is important to highlight that the statistical assumption that the raw data used in the regression models follow a normal distribution was not verified. The reason is that the sample sizes were too small to eliminate any of the data as will be shown later.