# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 1 Introduction  
High performance concrete (HPC) is a concrete based on a cement matrix with natural aggregate and added additives and admixtures, which is distinguished by, among others, good workability for at least 1 h, compressive strength over 60 MPa at 28 days, and water/binder ratio less than 0.4. In order to obtain a tighter structure of the mixture, micro-fillers and nanoparticles are used, e.g., \(y\), \(\mathrm{\SIUnitSymbolMicro}\) ash, silica fume, ground granulated blast furnace slag, metakaolin, and nano-silica [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. Compared to normal strength concrete, HPC has a more homogeneous and dense structure with a smaller number of pores and shrinkage cracks occurring. For example, HPC with compressive strength of not more than 90 MPa can be easily manufactured by using moderate amounts of ordinary aggregate, silica fume, and a superplasticizer [11]. HPC is applicable to many civil engineering structures, including skyscrapers and bridges. Its use may result in the lowering of the construction costs, causing a visible reduction in the cross-sections of concrete structures [12]. This may be advantageous in columns in terms of the benefits of long span requests and reducing the consumption of steel reinforcement [13]. Although HPC is stiffer and more durable than normal strength concrete, it also has greater brittleness, which may make it more susceptible to cracking and brittle failure [14]. The high ductility of HPC may be reached by proper selection of the kind and grain size of the aggregate [15; 16] or the use of fibers [17; 18; 19; 20; 21]. Interest in the application of silica fume began with strict control of air pollution. In numerous countries, industrial plants have been forced to stop releasing silica fume into the atmosphere by using special devices for its accumulation. After the tests, it was stated that the collected silica fume can be used as an addition of a reactive binder in concrete [22]. Silica fume is an industrial by-product mainly produced during the manufacturing of ferro-silicon. It is formed by the decomposition of high purity quartz with coal or else coke and wood shavings in an electric arc furnace throughout the fall of metallic silicon or alloy of ferrosilicon. The percentage of silicon dioxide (SiO\({}_{2}\)) in silica fume largely depends on the type of product manufactured. The higher the production of silicon alloy, the smaller the amount of silica fume. Most of the silica fumes used are at least 75% ferro-silicon alloys [22]. Silica fume is a very fine amorphous substance and is recycled in concrete as a mineral additive with significant pozzolanic activity. To a large extent, the properties of silica fume depend on the type of product produced and the process of its production. The silica fume generally contains more than 90% SiO\({}_{2}\). Nevertheless, the SiO\({}_{2}\) content and the amorphousness degree can differ greatly depending on the production technique. Silica fume originates in three forms: powder, slurry, and condensed silica fume (CSF). The use of silica fume powder is quite problematic, as the loose bulk density ranges from 130 to 430 kg/m\({}^{3}\) compared to 500-700 kg/m\({}^{3}\) for CSF. Due to the very fine particles of the silica fume powder and its low loose bulk density, handling problems can also occur. Other problems associated with the use of this type of silica fume are its stickiness, clogging during transport, and bridging in the storage silo. The solution to the above-mentioned problems is the use of slurried silica fume or condensed silica fume (CSF). In this study, it was decided to use the latter. One method of producing CSF is to blow compressed air from the silos bottom filled with silica powder. As a result of agglomeration, the heavier particles fall to the bottom of the silo and are from time to time removed. Another method of producing CSF is the mechanical pressing of silica fume powder. The earliest example of the use of silica fumes is a 15% cement substitution in the tunnel construction in Norway [22].