# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 4 Conclusions  
High performance concrete was studied for compressive strength, tensile splitting strength, elastic modulus, flexural strength, fracture properties, and microstructure. Six mixes of HPC containing condensed silica fume in the range of 0-25% as well as a water/binder ratio of 0.25 were tested. The specimens were maturated in water at 20 \({}^{\circ}\)C and examined after 28 days. On the basis of the results of these examinations, the following conclusions can be drawn:  
* The compressive strength increases in the range of 6.5-14% with increasing CSF content compared to reference specimens without the implementation of CSF.
* The tensile splitting strength of HPC improves significantly with the addition of CSF. Tensile splitting strength increases in the range of 7-26% in comparison with the reference specimens. In the tested HPC, the values of the tensile splitting strength are in the range of 5.5-6.5% of the compressive strength, while for normal strength concrete this value is about 10%.
* The elastic modulus improves gradually as the CSF content increases to 10%, and then decreases to 25% CSF. It increases slightly in the range of 3-4.5% with the 5-20% addition of CSF, compared to the reference HPC. On the contrary, the modulus of elasticity is decreased by 0.5% for 25% CSF.
* Replacing the cement with 5-20% CSF in HPC significantly increases the flexural strength in the range of 7-16% compared to reference HPC. In contrast, for 25% CSF content, the flexural strength decreases by 2%.
* The 5-20% use of CSF significantly improves fracture energy in the range of 11-30.5% compared to reference HPC. However, the fracture energy decreases by 2% for 25% CSF.
* The characteristic length decreases in the range of 3-18% with the 5-25% addition of CSF, compared to the reference mix. HPC with 15% CSF substitution is the most brittle, which is due to the shortest characteristic length.
* The reduction of ITZ around the aggregate grains and the formation of higher-strength hydration products results in smaller microcrack widths in all CSF-added HPC.
* The microcrack width gradually decreases in the range of 11-2% with increasing CSF content from 5% to 20%, in comparison with the reference HPC. However, the microcrack width increases by 2% for 25% CSF.
* HPC containing CSF is characterized by a significant increase in compressive strength (up to 14% with 20% and 25% CSF) and flexural strength (16% with 5% and 10% CSF). However, the most significant improvements can be seen in splitting tensile strength (up to 26% with 10% CSF) and fracture energy (30.5% with 5% CSF). Significant de  
Figure 15: (**a**) Variation of microcrack width with SF content; (**b**) correlation between microcrack width and fracture energy after 28 days.  
creases in the characteristic length (up to 18% with 15% CSF) and the width of the micro-cracks (13% with 10% CSF) are related to the greater brittleness and better durability of HPC concrete with the addition of CSF, respectively. In turn, the increase in modulus of elasticity is negligible (up to 4.5% with 10% CSF). Therefore, condensed silica fume can be used to produce high performance, sustainable concrete, but replacement should not exceed 20%.  
This research received no external funding.  
Not applicable.  
The data that support the findings of this study are available from the corresponding author, P.S., upon reasonable request.  
The author would like to thank the CEMEX Company for donating the materials for this study, as well as every person/department who helped throughout the research work. The careful review and constructive suggestions by the anonymous reviewers are graciously acknowledged.  
The author declares no conflict of interest.