# Effects of Fineness and Dosage of Fly Ash on the Fracture Properties and Strength of Concrete
## 4 Results and Discussion  
### Compressive Strength  
Compressive strengths were determined at five ages of 7, 14, 28, 56, and 91 days on concrete stored under moist-curing conditions. The results are given in Table 4. As can be seen in Table 3, the slump values of all concrete mixtures were in the range of 240-260 mm, and similarly, the slump-flow fell in the range of 550-640 mm. This indicated that all mixtures had quite similar workability. In addition, all mixtures were proportioned to have an equivalent target strength of 63 MPa at the age of 28 days. This was generally achieved, except for mixture F1R2. In this case, the strength equivalence was achieved later, at the age of 56 days. Figure 3 shows the strength development for each concrete mixture. At an early age (7 days), as expected, the control mixture (R0) without fly ash displayed higher strength than the other fly ash mixtures. The early strength gain of the control mixture was superior to that of the fly ash mixtures, indicating that replacing any amount of cement with fly ash of various fineness may reduce the strength gain of the concrete. In addition, the concrete incorporating more fly ash (20%) presented lower strength than that with lower fly ash content (10%). Nevertheless, the curve slope of the fly ash mixtures in Figure 3 tended to be steeper than that of the control mixture at later ages. In other words, the strength increment versus age for the fly ash mixture was larger than that of the control mixture. Consequently, the strength of each fly ash mixture exceeded that of the control mixture at later ages. On the other hand, it can be seen in Table 4 that for the same replacement ratio (20%) of fly ash, mixture F3R2, had the highest strength at an early age (7 days), followed by F2R2, and the F1R2 had the lowest. This revealed that incorporating finer fly ash in concrete may fill the micro-voids much better and produce higher packing density, resulting in a larger strength enhancement. In addition, a beneficial effect of the fineness of fly ash on the concrete strength can be seen in Figure 4; the compressive strength of fly ash concrete increased in accordance with the enhancement of the fineness level of fly ash for each age and various fly ash replacement ratios. Moreover, as seen in Table 4, there was a significant strength gain from 7 to 28 days, and again from 28 days to 91 days for the mixtures containing finer fly ash. For example, the strength increment of the mixtures F2R1 and F3R1, which contained finer fly ash, displayed an increased rate from 100% at 7 days to 125% and 126% at 28 days, respectively, and 139% and 140% at 91 days, respectively, which were obviously larger than those of the corresponding mixtures F1R1 that increased from 100% at 7 days to 119% at 28 days and 137% at 91 days. At 28 days, the strengths of the finer fly ash mixtures of F2R1, F2R2, F3R1, and F3R2 were shown to consistently exceed that of the control mixture. This was particularly significant considering that the cement replacement of finer fly ash in each case was only 10-20%. In addition, the highest strength achieved at 91 days was 76.5 MPa for the mixture F3R2 with 20% F325 fly ash replacement, while the strength obtained for the corresponding control concrete (R0) was the relatively lower value of 71.2 MPa.