# Mechanical Characterisation and Shrinkage of Thermoactivated Recycled Cement Concrete
## 3 Results and Discussion
### Compressive Strength
This demonstrates the effective contribution of SP in reducing the negative effect of the high water requirement of RCP. As mentioned, the RCP tended to be more efficient in concretes with lower w/b. The incorporation of 15% RCP resulted in a 6.7%, 0.5% and 0.8% increase in the compressive strength at 28 days compared to OPC concrete, for concrete with w/b of 0.35, 0.55 and 0.65, respectively. This trend may be attributed to two main reasons: the similar entrapped air found in reference OPC and 15%RCP concrete with a w/b of 0.35 (Table 4); the addition of SP in concrete with a w/b of 0.35, promoting the de-agglomeration and more effective contribution of RCP particles. In spite of the aggregate contamination of RCC and the consequent reduction of the cementitious fraction, the compressive strength was not significantly different from that of RCP concrete of similar composition, especially up to 15% replacement (Figure 9). Actually, taking into account that the cementitious fraction is 74% by weight of RCC (Section 2.1), the respective w/c is about 0.57 and 0.60 for concrete with 15% and 30% RCC, respectively. Therefore, up to 15% replacement, the filler and nucleation effect of RCC should have compensated the slight increase in w/cf. However, for 30% replacement, the higher w/cf assumed more relevance in the compressive strength reduction. The incorporation of NT and CF resulted in a significant reduction of the compressive strength at 28 days, having been up to 37% lower than that of reference OPC concrete with the same w/b (Figure 9). This confirms the low reactivity of NT and CF additions, with an overall reduction of the hydration products. In fact, considering the compressive strength at 28 days as a function of the water/OPC ratio (w/c), the addition of NT and CF had little contribution to the compressive strength (Figure 10). The slightly higher compressive strength, for a given w/c, of concrete with 15% CF/NT may be attributed to an eventual filler and nucleation effect. Overall, the compressive strength increased over time (up to 90 days), independently of the type of binder and w/b (Figures 11-13). However, contrary to the general trend, a tendency for stabilization of the compressive strength between 28 and 90 days was observed for concretes with 30%RCP (w/b of 0.55, Figure 11) and 100%RCP (w/b of 0.65, Figure 12). Figure 10: Compressive strength at 28 days, \(f_{cm,28d}\), of the produced concretes as a function of the w/c. Figure 9: Compressive strength at 28 days, \(f_{cm,28d}\), of concrete with a w/b of 0.55 and different incorporation percentages of additions. Figure 11: Compressive strength,\(f_{cm}\), over time of concrete with w/b of 0.55 and different percentages of RCP. Figure 12: Compressive strength,\(f_{cm}\), over time of concrete with a w/b of 0.65 and different incorporation percentages of RCP. Figure 13: Compressive strength,\(f_{cm}\), over time of concrete with a w/b of 0.55, incorporating 15-30% of different additions. strength of recycled cement of higher surface area than OPC has been recognised [4; 9; 19]. As mentioned, since part of the mixing water is retained in the porous RCP particles, the w/b of bulk paste between anhydrous particles is reduced, which accelerates their proximity and cohesion, contributing to the early strength. Moreover, the nucleation and filler effect are more relevant at early age. In fact, the relation between the compressive strength of concrete with RCP and OPC tended to be higher at early ages (Figure 13, Table 4), especially for high incorporation percentages of RCP. Actually, concrete with RCP consistently displayed higher compressive strength at 3 days than that with OPC of equal w/b (Table 4). Comparing the reference OPC concrete with a w/b of 0.55 (R55) to that with 15% RCP of the same workability (w/b of 0.62), but without SP, the required increase in w/b led to a respective reduction of the compressive strength, of about 14.5% (Table 4).