# Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume
## 1 Introduction
CSF impacts the HPC in twofold, key ways. Firstly, CSF acting a part to the reaction of hydration between water and cement by reacting with the hydroxide of calcium to form a gel of calcium silicate hydrate (CSH), which is the binder between the aggregate grains and provides the mechanical strength of HPC. What is more, the ultra-fines constituent part of CSF (\(<\)1 \(\upmu\)m) occupies the holes between the particles of cement, consequently acting as a micro-filler, reducing the cement porosity [23]. This results in lower permeability and better HPC durability. However, the homogeneous dispersion of CSF in HPC can be problematic. The heterogeneity can be caused by the influence of ball bearing as well as local problems. Thus, heterogeneity in the structure of pores may cause serious durability issues [22]. From some perspectives, CSF is a very favorable mineral supplement mainly for the production of high and ultra-high performance concrete, but caution must be taken through mixing to obtain appropriate particles' dispersion. In addition, CSF can accelerate the setting time by around 60% [24]. As a consequence, the compressive strength, tensile strength, bonding strength, freeze-thaw resistance, chemical attack resistance, and abrasion resistance of HPC showed significant improvement [25; 26; 27; 28; 29]. It is worth mentioning that CSF increases the concrete performance, but its efficiency is reduced when high doses of CSF or small amounts of water are used to complete pozzolanic reactions [30]. Notably, the effect of CSF filling is more leading than its pozzolanic influence. The greatest recognized impact of CSF on HPC is the enhancement of aggregate grain--cement paste interface [31; 32; 33]. CSF increases the HPC's request of water due to its ultra-finess and direct addition. Therefore, CSF is used in a composition with a superplasticizer in order to control workability. However, investigators revealed some shortcomings associated with its use in HPC, i.e., the plasticity reduction throughout the manufacture and an increase in shrinkage during maturation [23; 33]. [PERSON] and [PERSON] [34], as well as [PERSON] and [PERSON] [35] reported that experimental relations between compressive strength and physical properties, for instance elastic modulus, flexural strength, and split tensile strength, approved for normal concrete, should not be used for HPC. [PERSON] and [PERSON] [36] stated that the crack is better localized and resembles homogenous material behavior for HPC, but the matrix is more brittle compared to normal strength concrete. [PERSON] [13] studied the mechanical assets of high performance CSF concrete maturated in water at 20 \({}^{\circ}\)C and 50 \({}^{\circ}\)C with cement replacement by CSF in the range of 12-28%. The results obtained at 20 \({}^{\circ}\)C showed that the compressive and splitting tensile strengths reduced when the CSF addition was augmented from 16% to 20%. However, the problem was not evident to samples maturated at 50 \({}^{\circ}\)C. The results also showed that there was no noteworthy effect on the relation between compressive and splitting tensile strength. Moreover, the tensile strength was in the range of 4-6% of the compressive strength value. In addition, compressive strengths above 90 MPa were related to decreases in elastic modulus, especially when the CSF replacement was greater than 16%. [PERSON] et al. [37] observed a maximum 28-day concrete strength ranging from 5 to 20% CSF replacement with cement. [PERSON] et al. [38] reported an average increase in compressive and splitting strength of about 10% in ultra-high performance cementitious mixtures with the addition of different doses of CSF and nanosilica ranging from 0.5 to 3%. [PERSON] et al. [39] highlighted that a mixture of CSF and nanosilica has a greater influence on the sulfate resistance of concrete compared to separately addition. Researchers also take issue about the optimal CSF addition [17; 40] to obtain the best performance of HPC.