## 3 Test Results
### Hardened Properties of RPC
#### 3.2.2 Effect of WB and Curing Temperature on Compressive and Flexural Strength  
Figure 7 illustrates the effect of the WB and curing temperature on the compressive strength for mixtures containing 20% SF at all ages. As the WB increased from 0.18 to 0.22, the compressive strength of the RPC mixture increased, but a further increase of WB from 0.22 to 0.26 led to a significant drop in the compressive strength, except for the 56-day specimen. The reduction of strength over increasing WB can be explained as follows: in general, the porosity of concrete increases with the decrease in the degree of hydration when the WB increases. This means that a higher water content leads to increasing the intensity of pore formation, and eventually decreases the compressive strength of concrete [37]. However, RPC is a special type of concrete containing a relatively higher SF content that increases the water demand of the mixture due to the higher specific surface area of SF. Thus, it seems that decreasing the WB below the certain optimum point results in a decrease of the compressive strength due to the insufficient hydration of the binder inside the concrete.  
Figure 6: Compressive strength of RPC mixtures.  
Figure 7: \(Cont.\)  
On the other hand, Figure 7 also shows the effect of curing temperature on the compressive strength of the RPC mixture. During the early stages of curing, the strength of RPC cured at the warm temperature (40 \(\pm\) 2 \({}^{\circ}\)C) is greater than that of the concrete cured at a normal temperature (23 \(\pm\) 2 \({}^{\circ}\)C). However, after the age of 7 days, the trend of the compressive strength development of RPC is reversed. This is the so-called 'crossover effect', which indicates that a higher initial temperature results in a more than proportional increase in the initial rate of hydration, but leads to lower strength at a later age. [PERSON] and [PERSON] [38] explained that low permeable hydration products built up around the cement particles do not have time to become uniformly distributed within the pores of the hardening paste with rapid hydration. This nonuniform distribution of low-permeable hydration products not only leads to larger pores that reduce the strength, it also hinders the further hydration of the unhydrated cement particles at later ages. Therefore, higher early-age temperature curing results in higher early strength and lower long-term strength.  
As shown in Figure 8, the 28-day flexural strength of RPC shows the same trends as for the compressive strength. For normal temperature curing, when the WB increased from 0.18 to 0.22, the flexural strength of the RPC mixture increased, but a further increase of the WB from 0.22 to 0.26 led to a strength drop. For warm temperature curing, the flexural strength of RPC mixtures generally decreased as the WB increased. Therefore, higher early-age temperature curing leads to higher early flexural strength and lower long-term flexural strength.  
Figure 8: Effect of WB and curing temperature on flexural strength.  
Figure 7: Effect of WB and curing temperature on compressive strength: (**a**) 23 \(\pm\) 2 \({}^{\circ}\)C and (**b**) 40 \(\pm\) 2 \({}^{\circ}\)C.