## 1 Introduction
Another is between the old cement paste and the new paste, and the third ITZ is between the NA of the recycled aggregate and the new cement paste. The two ITZs between recycled aggregate (stone and mortar) and the new paste are where the chemical reactions between both generate loose and pore interfaces [30; 31]. These ITZs are thus weaker and limit the mechanical properties of CRA [30]. Concrete with 100% CRA shows a loss in compressive strength with respect to coarse NA at 28 days, for the same effective w/c ratio and amount of cement, from 10% to 37% [9; 17; 27; 32; 33; 34], approximately proportional to the replacement level [35; 36] and depending on the relative strength of the new \begin{table}
\begin{tabular}{c c} \hline \hline
**Acronym** & **Meaning** \\ \hline RA & Recycled aggregate \\ CRA & Coarse recycled aggregate \\ RCA & Recycled concrete aggregate \\ RAC & Recycled aggregate concrete \\ NA & Natural aggregate \\ NAC & Natural aggregate concrete \\ CDW & Construction and demolition waste \\ EAFS & Electric arc furnace slags \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Acronyms used in the article. paste and CRA [37]. This decrease in compressive strength may make it necessary to use about 5% more cement to achieve the same strength as with NA, thus compromising the cost-effectiveness and sustainability of CRA [9; 12]. From 28 to 91 days, the relative increase in compressive strength with recycled aggregate concrete (RAC) is sometimes greater than with NA due to the hydration of unhydrated cement grains [37]. The splitting tensile strength is typically lower but often not by as much as compressive strength, but there are studies where the tensile strength can even be higher with CRA than with NA [9; 38], while the modulus of elasticity can decrease by 15% for 30% replacement and 45% for 100% replacement [35; 39]. This reduction is because CRA has a lower modulus of elasticity than NA and due to the increase in the effective w/c ratio to maintain workability constant. Reductions in the modulus of elasticity result in increases in the peak strain of concrete under monotonic compression--[32] reports an increase of 20% for aggregate replacement of 100%. It is also known that the use of CRA but also the new cement paste affect the fatigue behaviour of concrete, reducing the fatigue limit and fatigue life [40; 41]. Other authors have shown that the multi-recycling of the concrete that contain CRA is limited and that after three recycling cycles, CRA are mostly composed of mortar and new NA are needed in the mix design [42]. The drying shrinkage of concrete with CRA can be 50% higher [43] than with coarse NA, while chloride ions' penetration can reach up to 150% increases [24; 44] due to the high permeability of this concrete. Resistance to carbonation is linked to the porosity of concrete [15] and, therefore, to the porosity of CRA, although it is also strongly linked to the chemical composition of concrete [24]. The high porosity of CRA makes carbonation depths increase between 22%-187% for 100% replacement [24; 33; 45] in comparison to NAC. Some authors propose the use of more crushing stages to eliminate the attached mortar and thus obtain rounder and less porous aggregates [45; 46], the use of acids or heat to disaggregate the mortar of RCA [47; 48], thermo-mechanical processes [49; 50] or the use of several mechanical systems for on-site processing [51]. Another solution presented by the literature is the use of crushed bricks or steel slag [13; 52] to compensate the loss of strength and durability, due to their pozzolanicity. However, these beneficiation techniques add new steps to the aggregate production process and increase production costs and the environmental impact of CRA production. Currently, there is an increasing trend towards the use of steel slag in concrete [23].