# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 3 Results and Discussion
### Microstructure  
The addition of condensed silica fume changes the chemical composition and properties of HPC. Figure 11 shows that as the content of CSF increases, the color of HPC becomes darker and the number and size of pores increases. This is due to a decrease in workability of HPC with an increase in the CSF substitution due to its high water demand and high specific surface area. [PERSON] and [PERSON] [50] observed that workability deteriorated, and higher viscosity caused the formation of air bubbles when the silica fume substitution surpassed a certain value. This can decrease the superiority of the HPC structure and the interfacial zone between its ingredients, and consequently decrease the compressive, tensile splitting, and flexural strength, as well as elastic modulus and fracture properties. After testing the tensile splitting strength, 3 specimens of each HPC mixture were prepared for microstructure analysis. Using a scanning electron microscope (SEM), HPC micropores and microcracks were assessed depending on the CSF content. Figures 12 and 13 show images of all 6 HPC mixtures in 200\(\times\) and from 4000\(\times\) to 20,000\(\times\) magnification, respectively. CSF acts similarly to a binder to alter the structure of pores and form a dense HPC microstructure with a reduced amount of interfacial transition zone (ITZ). Because of this, HPC permeability is reduced. Thus, the solutions penetration containing harmful sulphates as well as has a chance to be meaningfully reduced. The structure of the pores largely determines the durability and permeability of HPC. On the basis of the image analysis of several dozen microphotographs, it was found that the pore dimensions were in the range of 64-190 \(\upmu\)m, 47-186 \(\upmu\)m, 40-180 \(\upmu\)m, 36-160 \(\upmu\)m, 32-145 \(\upmu\)m, 25-100 \(\upmu\)m, for HPC with CSF content of 0%, 5%, 10%, 15%, 20%, and 25%, respectively. It was established that the pores number reduced with increasing CSF substitution. A reduction in macropores size was also found due to the CSF presence. The microstructure of HPC is improved due to the high pozzolanic activity of the CSF (in Figures 12 and 13). The binding activity is revealed by the CSH gel, which is formed as a result of the reaction of CSF with Ca(OH)\({}_{2}\) throughout hydration in an alkaline environment [19]. The resulting CSH gel fills holes and reduces the porosity of cementitious hydration products (see Figure 13). The CSH gel acts as a binder and provides a stronger bond between the aggregate grain and the paste. Consequently, it decreases the amount of free Ca(OH)\({}_{2}\), densifies ITZ, and finally increases HPC strength. Ca(OH)\({}_{2}\) reacts with CSF as it cannot hydrate directly with water [19]. The addition of CSF improves strength to a limited extent, but its higher amount, i.e., 25% content, and the lower amount of water in HPC may result in inhomogeneity of the mixture and low adhesion. [PERSON] et al. [57] stated that due to the high pozzolanic activity, the porosity of the concrete is decreased, but the ITZ is improved through the consumption of calcium hydroxide and a marked increase in the number of hydration products. As previously mentioned, [PERSON] and [PERSON] [50]  
Figure 11: Split cross-sections of notched HPC beam specimens. stated that the addition of 15-20% silica fume decreased porosity and improved strength due to the pozzolanic reaction and filling effect. In contrast, 25% silica fume increased yield stress and plastic viscosity, which may result in the entrapment of additional air. Figure 12: SEM microphotographs in 200\(\times\) magnification for HPC with CSF content of (**a**) 0%; (**b**) 5%; (**c**) 10%; (**d**) 15%; (**e**) 20%; (**f**) 25%. Figure 13: SEM microphotographs in magnification from 4000\(\times\) to 20,000\(\times\) for HPC with CSF content of (**a**) 0%; (**b**) 5%; (**c**) 10%; (**d**) 15%; (**e**) 20%; (**f**) 25%. No ettrinigite crystals were observed on HPC microphotographs, regardless of the addition of CSF.