# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 3 Results and Discussion
### Slump and Density  
In general, the concretes were produced with S3 consistency, according to EN 206 [31], for slumps between 100-150 mm (Table 4). However, as discussed in the literature [4; 8; 9; 19; 40], recycled cement has a higher water demand than OPC, which leads to a significant slump reduction for the same w/b. This can be attributed to the high surface area (Table 2), high porosity and subsequent water absorption [4; 8; 9; 19; 40], particle agglomeration [16] and free CaO content [9; 13]. Therefore, for incorporation percentages of RCP over 30%, high amounts of SP had to be added in order to comply with the S3 consistency class (Table 4). As mentioned, to achieve the same slump, the w/b had to be increased from 0.55 to 0.62 (13%), when OPC was replaced with only 15% RCP. The incorporation of 15% NT or 15% CF resulted in concretes with higher slumps than that with 15% RCP, but lower than that with only OPC of equal w/b, which can be explained by the intermediate porosity, nearly inert nature and absence of free CaO of well hydrated NT and CF particles. The partial replacement of OPC with RCC led to higher slumps than those obtained for the same substitution percentage of OPC with RCP. In fact, as mentioned, RCC was contaminated with about 26% non-porous aggregates, which corresponds to a higher water/cementitious fraction ratio (w/cf) for the same w/b. This led to a significant reduction of the water required for the normal paste consistency (about 32%) (Table 2). In other words, the porosity, surface area and CaO content are lower in RCC than in RCP. Figure 4 displays the workability over time of 15P65. Contrary to what is usually observed in OPC mixtures, a quick workability reduction occurred during the first 15 min, due to the exothermic hydration of free CaO, as well as the RCP absorption [9; 14; 22]. This loss of workability is expected to increase with the incorporation percentage of RCP. Then, up to 45 min, the slump was less altered. The fresh concrete faces an induction period, as shown by isothermal calorimetry analysis conducted by the authors [9; 10] and also documented in the literature [8; 19; 41]. After 45 min, the workability gradually decreased, due to the progressive hydration and setting of the binder. The SP was effective in improving the workability of concretes with RCP, having been able to compensate the significantly higher water requirement of RCP than of OPC. However, compared to OPC concretes, greater SP dosages (as high as 3.8%) were necessary for a given percentage water reduction, i.e., the SP saturation point was increased. This was attributed to the porous nature [9] and significantly higher surface area of RCP particles, which was about 8.6 times higher than that of OPC particles (Table 2), increasing the surface adsorption of SP molecules. Figure 5 shows the fresh density of the produced concretes as a function of the incorporation percentage of additions. As expected, the fresh density was more affected by the w/b than by the incorporation of RCP, which had only slightly lower density than OPC  
Figure 4: Slump of 15P65 over time. (Table 2). This and the correspondent increase in paste volume, for the same w/b (Table 3), contributed to the decrease in the fresh density, up to 2.6%, with the incorporation of RCP. Moreover, as mentioned, due to their higher water demand, RCP concretes tended to present lower workability than OPC concretes of the same w/b, which tended to promote lower compactness and higher air entrainment (Figure 6). This may explain the sharp density reduction of concretes produced with up to 15% RCP, without SP. Knowing that RCC, NT and CF have lower particle density than OPC, the fresh density of concrete decreased with the replacement percentage of OPC (Figure 5). The incorporation of 15% of RCP, RCC, NT and CF resulted in a 1.7, 1.7, 2.6 and 1.3% reduction of the fresh density, respectively. In general, the reduction of dry density as a function of the incorporation of RCP followed the same trend of fresh density.