Mechanical and Microstructural Studies of High Performance Concrete with Condensed Silica Fume

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This article is an extended version of the conference paper “Influence of silica fume on mechanical and fracture properties of high performance concrete” published in Procedia Structural Integrity as a part of the Special Issue for the 3rd International Conference on Structural Integrity (ICSI 2019). Tests were carried out to evaluate the compressive strength, tensile splitting strength, modulus of elasticity, flexural strength, and fracture properties of high performance concretes (HPC) having different levels of condensed silica fume (CSF) replacements for cement. It was found that CSF replacement for cement by up to 25% may have a favorable effect on the mechanical properties. HPC containing CSF was characterized by quite large increases in compressive strength (up to 14%) and flexural strength (16%). However, the most significant improvements in mechanical properties were obtained for splitting tensile strength (26%) and fracture energy (30.5%). There were slight reductions up to 2% in the elastic modulus, flexural strength and fracture properties at the 25% level of CSF substitution for cement. Microstructural studies showed that the narrowest microcracks and the smallest pores in the interfacial transition zone (ITZ) between the paste and grains of aggregate occurred in the HPC having 10% CSF. In addition, a reduction of ITZ around the aggregate and the formation of more high-strength hydration products was observed in all CSF-added HPCs. The outcomes reported that CSF can successfully replace cement. It is suggested that the substitution should not exceed 20%.