# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 3 Results and Discussion
### Compressive Strength  
Figure 8 shows the compressive strength at 28 days of concretes with different w/b as a function of the incorporation percentage of RCP. Depending on the incorporation percentage of RCP and w/b, the compressive strength varied between 33.2 and 90 MPa at 28 days. These concretes ranged strength classes from C20/25 to C70/85, according to EN 206 [31], making them suitable for the production of common structural concrete. For the same w/b, the compressive strength was not significantly influenced by the RCP content, even for high incorporation percentages, demonstrating the potential of this new binder. In fact, as also demonstrated in previous studies [9; 10], RCP could effectively rehydrate and generate similar hydration products to those produced by OPC. Therefore, RCP is an active binder that clearly distinguishes itself from other non-hydraulic additions. For the same w/b, the compressive strength tended to increase slightly for incorporation percentages of RCP up to 15%. The maximum increase of about 6.7% was found  
Figure 8: Compressive strength at 28 days,\(f_{cm,28d}\), of concrete with different incorporation percentages of RCP and w/b. Figure 7: Dry density, \(\rho_{dry}\) of concrete with different incorporation percentages of additions. in concrete with a w/b of 0.35. It is possible that the finer RCP particles promoted filler and nucleation effect, improving the hydration development and microstructure refinement, due to their high surface area [16; 20; 22]. This should be more relevant at early age strengths, as discussed later. For up to 15% replacement of OPC with recycled cement, [PERSON] and [PERSON] [20], [PERSON] et al. [21] and [PERSON] et al. [22] also reported no significant changes in compressive strength. In this study, the maximum compressive strength in concrete with w/b of 0.55 was attained for 5% RCP. In cement pastes, [PERSON] and [PERSON] [20] reported that the optimum strength was achieved for 5-10% recycled cement in pastes with a w/b of 0.3. However, the incorporation of more than 15%RCP tended to decrease the compressive strength. This may be attributed to the diluted effect of the OPC amount, as well as the reduction of the workability and consequent compactness of concrete with high incorporation percentage of RCP. As documented by [PERSON] et al. [9] in a previous study with an equal recycled cement thermoactivated at 650 \({}^{\circ}\)C, the compressive strength at 28 days of RCP pastes was about 28% lower than that of OPC pastes of equal w/b, displaying less long-term development of outer hydration products than OPC. In fact, part of the mixing water is rapidly consumed in the hydration of internal products within the porous structure of recycled cement particles [10]. On the other hand, the estimated volume of entrapped air tended to increase with the replacement of OPC with RCP (Table 4). The high water requirement of RCP reduces the fluidity of concrete and this affects its compactness and, as a consequence, the mechanical strength. The full replacement of OPC with RCP in concrete with a w/b of 0.65 led to an increase of about 12 L/m\({}^{3}\) in the volume of voids and to a 17% reduction of the compressive strength at 28 days (Table 4). In addition, the increase in air entrainment for 15% RCP incorporation also contributed for the optimal compressive strength found in 5% RCP concrete (Figure 6). The exception occurred for 40% RCP concrete, in which an inversed trend was observed. This may be explained by the addition of high amounts of SP that effectively dispersed, not only the agglomerated RCP particles, but also the OPC particles. Moreover, the estimated air entrainment was reduced (Figure 6). Therefore, a more effective and homogeneous growth of hydration products should have been attained and the above mentioned side-effects were compensated. This may also explain the lower compressive strength reduction obtained in 100% RCP concrete than in the recycled cement pastes produced by [PERSON] et al. [9] without the addition of SP.