Concrete, the most used cement-based material, represents a multi-phase heterogeneous product that is strongly affected by the characteristics of each constituent. The use of Recycled Aggregates (RAs) has the potential to reduce the overall environmental impact of concrete production but lower mechanical performances and reduced workability are generally reported. The impact grows going from coarse recycled aggregates (CRAs, 10-4 mm) to fine recycled aggregates (FRAs, below 4 mm). Fractions of CRAs, although with restrictions, are already accepted in concrete production whereas FRAs are generally excluded (Evangelista et al 2015). This is mainly due to the presence of residual hydrated cement paste (HCP) on RAs (Katz 2003, De Juan & Gutiérrez 2009, Bonifazi et al 2018). The HCP adheres on the surface of RAs on a thin layer known as interfacial transition zone (ITZ). In fresh Recycled Aggregates Concrete (RAC) a secondary ITZ is formed between the RAs-HCP-new cement matrix resulting in a doubled ITZ. This double layer of ITZ represents two planes of weakness responsible for a low packing density, low bulk density and high-water absorption capacity. Furthermore, being cement hydrated products directly influenced by water molecules interaction, the distribution of RAs inside the cement matrix leads to the formation of areas of local depletions of water availability that results in substantial differences in cement minerals phase distributions. The microstructure of RAC depends on the types, amounts and distribution of its constituent and cement phases (anhydrous and hydrated products) existing a fundamental link between the processes that form the material and its macroscopical properties. Combined quantitative X-ray diffraction, scanning electron microscopy (SEM) and X-ray computed microtomography (micro-CT) analyses have been performed on different RAC samples where 10-4 mm, 4-2 mm, 2-0.6 mm, 0.6-0 mm have been respectively replaced in a standard mix design distribution. Rietveld analysis technique has been applied to the bulk wt. % estimation of crystalline anhydrous products whereas SEM analysis has been performed to obtain information on hydrated amorphous phases in section. Finally, micro-CT provided evidence related to micro-porosity in the area surrounding RAs which could be related to different local water absorption. The different impact of RAs diameters in changing RAC microstructure and cement phases assemblage is described and related to the mechanical performance obtained after 7 – 28 – 90 days of hydration. 

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