# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 3 Results and Discussion  
### Compressive Strength  
The capacity of HPC to carry compressive loads till failure is known as compressive strength. The tests were carried out taking into account the recommendations of the PN-EN 12390-3:2019-07 [49] standard in order to determine the compressive strength after 28 days. The detailed composition of the reference mixture and the five HPC mixtures with the CSF addition changing from 5 to 25% are shown in Table 3. Three cubic samples cast per each mix. The influence of the CSF content on the development of compressive strength is presented in Figure 2.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Test** & **Shape** & **Dimensions (mm)** & **Number** \\ \hline Compression & & 100 \(\times\) 100 \(\times\) 100 & 18 \\ Splitting tensile & Cube & 18 \\ Modulus of elasticity & Cylinder & 150 \(\times\) 300 & 18 \\ Three point bending & Notched beam & 80 \(\times\) 150 \(\times\) 700 & 18 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Specim specification.  
Figure 2: Differences in compressive strength with CSF substitution.  
Figure 2 shows that CSF substitution with cement up to 25% can result in a noticeable enhancement in compressive strength. Increasing compressive strength in HPC has been noted with increasing CSF exchange. The increases were set at approximately 6.5%, 13%, 13.5%, 14%, and 14% for 5%, 10%, 15%, 20%, and 25% CSF replacement, respectively. The above outcomes may be influenced by the increase in the cement paste-aggregate interface bond strength caused by the filling effect of CSF. The hypothesis is supported by compression failure modes that are more explosive for CSF HPCs.  
[PERSON] et al. [11] examined cylinders 150 mm in diameter \(\times\) 300 mm in height during compression. They reported strength increasing to 15% CSF replacement and then decreasing to 25% of the CSF used in this study. [PERSON] [13] reported an investigation on the compressive strength of CSF HPC at W/C = 0.35. The compressive strength after 28 days was from 74 to 110 MPa with CSF content up to 28%. Samples maturated at 20 \({}^{\circ}\)C with 12% and 16% CSF replacements presented a leveling of strength gain after 28 days. On the other hand, those with more than 16% CSF continued to increase compressive strength after 28 days. Moreover, the outcomes indicated that the compressive strength up to 28 days evidently decreased when the CSF content augmented in the range of 16-20%. [PERSON] et al. [3] examined the effect of silica fume (SF) content on compressive strength of ultra-high strength concrete (UHSC). The SF content had a noteworthy influence on the compressive strength up to the 7 th day of maturation. Afterwards, the increase in compressive strength was around 11%. A noteworthy increase in compressive strength was found with an increase in SF substitution in the range of 0-20%. The compressive strength tended to decrease when SF exceeded 20%. The UHPC compressive strength without SF after 28 days was 89.8 MPa. When 10%, 15%, 20%, and 25% SF substitution was employed, the compressive strength augmented by about 18%, 16%, 28%, and 25%, respectively. In the present work, it was found that the compressive strength gradually increased with increasing CSF exchange up to 20%, and then remains constant up to 25% of the applied CSF content. A similar behavior was observed by [PERSON] and [PERSON] [50], who reported that the 15-20% amount of SF improved strength and reduced porosity as a result of the pozzolanic reaction and the filling effect. They also reported that at 25% SF, yield stress as well as plastic viscosity increased, which may affect the entrapment of additional air. On the other hand, in these investigations, it was observed that HPC with 25% CSF content developed a compact and very dense matrix, as illustrated in Figure 3.