## 3 Results and Discussion
### Compressive Strength  
Figure 4 presents average compressive strength values of HPC after 2, 7, 14, 28, 56 and 90 days of curing (tested on 100 mm cube specimens), as well as 2 years (730 days) after specimen preparation (tested on 150 mm/300 mm cylinders). To unify the analysis process, the tested compressive strength values were converted into those corresponding to 150 mm cubic specimens using conversion factors for HPC: 0.99 and 0.86 for 100 mm cubic and cylindrical specimens, respectively [23]. Due to the possible lower accuracy of the cylinder-cube strength value conversion and dispersion of the results obtained on cylindrical specimens, it can be concluded that the compressive strength of low-clinker HPC mixtures increased continuously over time. Additionally, it can be stated that within the period of 90 days to 730 days after specimen preparation, a less significant increase in concrete strength was observed (0.8-1.4%) than within the 28-90 days curing period (9-16%). This could be caused by a change in the  
Figure 4: Average compressive strength of low-clinker HPC; *—compressive strength values tested on two specimens, converted using conversion factor of 0.86 for cylindrical specimens. storage conditions of cylindrical specimens. Comparing compressive strength values of C100_SFA15_SF3 mixture and a corresponding mixture C555_SFA15_SF3 with a 45% replacement of cement by GGBFS, it can be seen that the compressive strength of HPC decreased with an increase of GGBFS content. The biggest differences between these two mixtures were found in the case of early strength, with 50% and 39% lower strength values at 2 and 7 days of curing being obtained for the C55_SFA15_SF3 mixture. After 28 days of curing, the compressive strength of the designed low-clinker HPC varied between 76.1 and 91.7 MPa. It should be highlighted that for the C100_SFA30_SF3 mixture, the increase in the FA content in comparison to C100_SFA15_SF3 mixture did not lead to an increase in the compressive strength. All concretes were characterized by relatively low early strength (7.2-16.3 MPa after 2 days of curing). According to the EN 206 standard, the rate of compressive strength development of the designed HPC at 20 \({}^{\circ}\)C was assessed by calculating the ratio of the average compressive strength after 2 days of curing (\(f_{cm,2}\)) to the average compressive strength after 28 days of curing (\(f_{cm,28}\)). The calculation results are presented in Table 4.  
Based on the calculations, the rate of compressive strength development for HPC mixtures containing 100% CEM III/B as the main binder was described as slow, whereas the C55_SFA15_SF3 mixture with a reduced amount of Portland cement clinker was characterized by a very slow increase in compressive strength. The evaluation of the long-term compressive strength development rate was done by comparing the values of compressive strength on the 28 th and 90 th days of concrete curing. The following values of the \(f_{cm,28}/f_{cm,90}\) ratio were obtained: 0.87 for the C55_SFA15_SF3 mixture, 0.89 for the C100_SFA15_SF3 mixture and 0.92 for the C100_SFA30_SF3 mixture. It can be stated that the compressive strength development rates were similar, unlike in the case of the \(f_{cm,2}/f_{cm,28}\) ratio. The average increase in the compressive strength between the 28 th and 90 th days of curing was equal to 12%, while the highest value of 16% was obtained for the C55_SFA15_SF3 mixture. The obtained compressive strength values have been compared with time-dependent concrete strength development models proposed in:  
* ACI Committee 209 [24]: \[f_{c}(t)=\beta_{cc}(t)\cdot f_{c28},\] (1) \[\beta_{cc}(t)=\frac{t}{a+b\cdot t}\] (2)
* JSCE [25]: \[f_{c}(t)=d\cdot\beta_{cc}(t)\cdot f_{c28},\] (3) \[\beta_{cc}(t)=\frac{t}{a+b\cdot t}\] (4)
* and Eurocode 2 (EN 1992-1-1), MC 2010 [26; 27]: \[f_{c}(t)=\beta_{cc}(t)\cdot f_{c28},\] (5) \[\beta_{cc}(t)=e^{[1-\{\frac{2}{7}\}^{05}]}\] (6)