## 3 Test Results
### Hardened Properties of RPC
#### 3.2.3 Effect of SF and Curing Temperature on Compressive and Flexural Strength  
The effect of SF content on the compressive strength is presented in Figure 9. A similar trend has been observed for all of the curing periods, regardless of the curing temperature: increasing the SF content from 15% to 20% results in an increase in the compressive strength of RPC. However, further increasing the SF content from 20% to 25% drops the compressive strength down. This implies that the addition of SF improves the compressive strength of RPC up to a certain optimum point, but a further increase in the SF content does not result in additional strength gain. It is possible to state that the optimum amount of SF induces a strong bonding between the cement paste and the aggregate due to the filler packing effect and the pozzolanic reaction of SF. This enhanced adhesion results in a much stronger transition zone, and thus leads to a more even distribution of stresses within the concrete volume, and ultimately to larger compressive strength. However, the addition of an excessive amount of SF to the mixture does not provide continuous strength development because of the limited pozzolanic reaction. When the WB is kept constant, an increase in the SF content results in a situation where the silicates are in excess, and the CH produced by cement hydration is deficient. Hence, there is no more pozzolanic reaction that produces calcium silicate hydrate (C-S-H) lattice forms related to strength development. The other possible reason to bring the decrease of compressive strength is that the addition of an excessive amount of SF to the mixture may cause an incompatibility issue (interactions between acceptable materials that result in unexpected or unacceptable performance). Therefore, further investigation is required in order to identify this result.  
Figure 9: Effect of the WB and curing temperature on compressive strength at WB = 0.22: (**a**) 23 \(\pm\) 2 \({}^{\circ}\)C and (**b**) 40 \(\pm\) 2 \({}^{\circ}\)C.  
Furthermore, the compressive strength of RPC increases with age, but the increase in compressive strength up to 7 days is more significant when compared to that at 56 days. Hence, the addition of SF beyond a certain amount to the RPC mixture has no significant effect on strength development.  
Figure 10 shows the 28-day flexural strength of RPC. Similar to the compressive strength results, when the SF content increased from 15% to 20%, the flexural strength of the RPC mixture increased, but a further increase of SF content up to 25% led to strength drop. Interestingly, no typical trend was observed for warm temperature curing, but this can be explained by the porosity to density ratio (P/D): the lower the P/D, the higher the flexural strength. As presented in Table 4, the mixture with low flexural strength has a higher porosity and lower density. For example, while the porosity and density of the mixture containing 15% SF are 8.16% and 2.24 g/cm\({}^{3}\), those properties of the mixture containing 20% SF are 11.35% and 2.20 g/cm\({}^{3}\), respectively. In other words, the P/D of the former is 3.65, and the P/D of the latter is 5.15.