# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 3 Results and Discussion
### Microstructure
The reason was probably too little water, which is necessary for the CSF pozzolanic reaction and the formation of ettrinigite. In HPC, regardless of the addition of CSF, there were no individual CSF gel particles, which can usually be located in normal concrete. The obtained HPC was characterized by a very dense structure, which, however, decreased slightly with greater CSF substitution. Calcium hydroxide Ca(OH)\({}_{2}\) appeared sporadically and took the form of small local crystals (see Figure 13). ITZ in concrete is a transitional phase between aggregate grains and hydrated cement slurry, which determines the final strength and durability [22]. The HPC microphotographs show that the dense structure of the paste is practically in contact with the edges of the aggregate grain. It can be concluded that CSF contributes to the disappearance of ITZ in HPC, while in ordinary concrete there is a Ca(OH)\({}_{2}\)-rich porous region with a thickness of approximately 40-50 \(\upmu\)m surrounding the aggregate grains. In this study, CSF10 and CSF15 also showed less non-hydrated cement and smaller pore areas in the boundary zones of the aggregate grain and in the dense paste structure compared to other HPCs mixes. [PERSON] and [PERSON] [22] noticed that with the increase of the water-cement ratio in concrete with the addition of CSF, the ITZ thickness and the degree of orientation of calcium hydroxide crystals increased. There are clear differences in the average microcrack width between the HPC specimens, as shown in the SEM microphotographs in Figure 14. The microcracks of CSF10 were smaller than those of the high condensed silica fume content. Besides, HPC with 25% CSF had a higher average microcrack in comparison with the control mixture. This means that in HPC with CSF substitution up to 20%, post-hydration structures of high strength were formed. It can also be noticed that there is a trend between the microcracks width and the fracture energy. Additionally, a high content of silica fume can considerably increase the possibility of microcracking caused by autogenous shrinkage, especially in the case of HPC with a high content of binder supplementary materials and a low W/B ratio, which can negatively affect the mechanical properties [58]. In general, autogenous shrinkage increases with increasing CSF substitution and can cause microcracks. In HPC, however, it may exert stresses at the boundary of the aggregate grain and cement paste, which, in turn, may lead to an increase in the bonding strength with the aggregate and reduction of cracks development. The formation of more high-strength hydration products, e.g., CSH, as well as the improvement of ITZ around the aggregate grains by minimizing or even eliminating it, resulted in narrower microcracks, which translated into greater stresses between the grains and the matrix, and improved bond strength to the aggregate. Differences in the microcracks width depending on the CSF content are shown in Figure 15a. The average microcrack width decreased with increasing CSF substitution up to 10%. It was found that the HPC microcrack width regularly increases with the addition of CSF above 10%. The use of the CSF reduced the microcrack width by approximately 11%, 13%, 5.5%, and 2%, in comparison with the control specimen when 5%, 10%, 15%, and 20% CSF content was used, respectively. Nevertheless, the microcrack width increased by 2% for 25% CSF content in comparison with the reference HPC. This proves the effective reaction of CSF with CH and the formation of CSH, which reduced the width of microcracks and improved the bond strength in ITZ, as well as increasing matrix strength. [PERSON] and [PERSON] [59] found that the poor quality of the bond between the aggregate and the cement slurry is mainly caused by a high concentration of pores and microcracks as well as the directional orientation of CH crystals and their high content. [PERSON] et al.