# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 2 Materials and Methods  
The cement was CEM I 52.5R meeting the PN-B-19707:2013-10 [46] and PN-EN 197-1:2012 [47] standards. Tables 1 and 2 list the physical characteristics and costs, as well as chemical arrangement of condensed silica fume (CSF) and cement, respectively. It can be seen that the replacement of Portland cement CEM I 52.5R by CSF was economically and environmentally justified. [PERSON] [1] found that although condensed silica fume is difficult to handle due to extreme fineness, its advantages, i.e., improving the strength and durability of high performance concrete, are so obvious that it is available not only as a by-product, but also produced specifically for the concrete industry. Quartz sand with a grain size of 0.125/0.5 mm was used as fine aggregate (FA), as well as granodiorite with 0.5/8 mm grain size was applied as coarse aggregate (CA). Size gradations of fine and coarse aggregates (quartz sand and granodiorite) were determined with the PN-EN 933-1:2012 [48] standard agreement (see Figure 1). In HPC, a light yellow color superplasticizer CX ISOFLEX 793 (CEMEX Admixtures GmbH, Salzkotten, Germany) was used, with a density of 1.07 \(\pm\) 0.02 g/cm\({}^{3}\) and PH of 6 \(\pm\) 1 at 20 \({}^{\circ}\)C, with alkali and chloride content below 1.5% and 0.1%, respectively, and based on polycarbonate ethers. It was added as a percentage of the binder content.  
The water to binder ratio was determined based on the total amount of binder components in the mixes. The percentage of CSF was defined as the mass ratio of silica fume to cement used in the control mix.  
Based on the knowledge obtained in preliminary tests, it was found that with the superplasticizer addition, it is possible to produce HPC with good workability with a maximum CSF content of 25% and W/B = 0.25. The amounts used in the reference mixture were as follows: cement--745 kg/m\({}^{3}\), coarse aggregate (CA)--1000 kg/m\({}^{3}\), fine aggregate (FA)--500 kg/m\({}^{3}\), superplasticizer--20 kg/m\({}^{3}\), and water--186 kg/m\({}^{3}\). The CSF mixtures were designed based on a control HPC with different cement substitutions with CSF. Table 3 displays the HPC mixtures details used in this investigation.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Material Properties** & **Cement** & **CSF** \\ \hline Specific surface area (m\({}^{2}\)/kg) & 484 & 17,000 \\ Water demand (\%) & 30 & - \\ Start of setting (min) & 160 & - \\ End of setting (min) & 210 & - \\ Volume stability according to Le Chatelier (mm) & 0 & - \\ Compressive strength at 2 days (MPa) & 40.3 & - \\ Tensile strength at 2 days (MPa) & 6.46 & - \\ Costs (EUR/ton) & 255 & 117 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties and costs of cement and CSF.  
Figure 1: Particle size distribution of aggregates.  
Each HPC mixture was prepared in about 8 min. Mixing was finished when a homogenous blend was obtained. Cubic samples with dimensions of 100 mm \(\times\) 100 mm \(\times\) 100 mm were made for compressive and tensile strength examinations. Cylinders with a diameter 150 mm \(\times\) height 300 mm were produced for the elastic modulus experiment, and 80 mm \(\times\) 150 mm \(\times\) 700 mm prismatic specimens with a mid-span notch were made for the three-point bending test. Cylinders and cubic samples were formed in plastic molds. In turn, prisms were prepared in wooden forms. Molds filled with the HPC blend were compacted on a vibrating table. The specimens were then removed from the molds after 48 h and matrutated for 24 days in water at 20 \({}^{\circ}\)C, and after removal from the water tank in air conditions for 2 days for testing at 28 day. Three specimens were made for each blend per the respective test. Dimensions and specimens' number are specified in Table 4.