## 2 Materials and Methods  
### Materials  
Fine (0-2 mm) and coarse (2-4 mm) siliceous sands were used in the production of the mortars. These sands have dry densities of 2637 kg/m\({}^{3}\) and 2617 kg/m\({}^{3}\), respectively, according to the EN 1097-6 standard [20]. Furthermore, the fine and coarse sands have, respectively, water absorptions of 0.4% and 0.5%. In addition to these natural aggregates, the reference mortar (RM) was produced with ordinary Portland cement, OPC, type 142.5 R (CEM I 42.5 R). The remaining mixes incorporate fly ash, FA, and recycled concrete powder (RC). Class F coal FA was provided by EDP--Gestao da Producao de Energia, S.A. at the Sines Power Plant, in Portugal. RC was obtained by crushing and grinding concrete specimens previously produced in the laboratory. Its composition was designed using the Faurymethod [21] with 300 kg/m\({}^{3}\) of CEM II/AL 42.5 R cement, an effective water/cement ratio (w/c\({}_{\rm eff}\)) of 0.55, for an environmental exposure class XC3 and a target workability S3 according to the EN 206-1 standard [22]. Four types of natural aggregates were used in its manufacture: (i) coarse crushed limestone (12/22 mm) and (ii) medium-size crushed limestone (6/12 mm), as well as (iii) coarse (2/4 mm) and (iv) fine (0/2 mm) siliceous sand. The source concrete samples were manufactured and cast into 150 mm cubic and 150 \(\times\) 300 mm cylindrical specimens. After 24 h, the specimens were demoulded and cured in the laboratory for 28 days in a saturated chamber (RH \(>\) 95%). After 28 days of curing, the original concrete had an average compressive strength of 39.8 MPa, measured in cubic specimens. The chemical composition of the binders and aggregates used in the production of the mortars was analysed through X-ray fluorescence (XRF) using ZSX PRIMUS IV (Rigaku) equipment with a power of 4 kW. The results obtained can be observed in Table 1. Due to the silicous nature of the natural aggregates, SiO\({}_{2}\) is the oxide present in higher quantity in this material. In turn, it should be noted that the oxides present in higher quantities on the three binders studied (OPC, RC, and FA) are SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and CaO. However, in OPC, the high amount of CaO (67.1%) stands out, in the RC, the high amounts of SiO\({}_{2}\) (47.5%) and CaO (41.2%) are more evident, and in the FA, the high amounts of SiO\({}_{2}\) (57.5%) and Al\({}_{2}\)O\({}_{3}\) (24.4%) are the most obvious. Table 2 shows the physical properties of the different binders (OPC, RC, and FA), both in terms of their size and their density. The particle size distributions of the binders were obtained in a Mastersizer S. Analyser (Malvern Instruments) using ethanol as a dispersant. D90 (90 th percentile value for particle diameter) in RC was 147 \(\upmu\)m, while in OPC and FA it was 46 \(\upmu\)m and 43.5 \(\upmu\)m, respectively. This difference is associated with the natural aggregates present in the source concrete. The RC and FA used were less dense (2.54 g/cm\({}^{3}\) and 2.43 g/cm\({}^{3}\), respectively) than OPC (3.11 g/cm\({}^{3}\)). The morphology of the binders was obtained by scanning electron microscopy (SEM), using a JEOL JSM 7800 F. In Figures 1 and 2, SEM micrographs of FA and RC can be observed, respectively. FA has a mainly spherical structure. It can also be seen that most of the spheres have a diameter of approximately 10 \(\upmu\)m, which is in agreement with the findings for the particle size distribution analysis performed (results presented in Table 2).