Effects of silica fume on portland cement hydration and mechanical properties

The construction sector is growing rapidly in parallel with the population growth rate. With this growth, the need for concrete and cement, which is an important component of concrete, is increasing day by day. Portland cement production is responsible for 5-7% of global carbon dioxide emissions and significantly harms the ecological balance. In addition, approximately 5% of the energy consumed in the industry is from cement production. Therefore, research on cement continues intensively in order to reduce both serious environmental concerns and energy costs. This research focuses especially on the use of pozzolanic materials, which are considered as industrial waste, as cement replacement materials. One of these materials, silica fume, is among the most researched high-quality supplementary cement materials due to its significant increase in the general performance of cement and concrete. In this study, the hydration reactions and mechanical properties of silica fume-substituted cements will be investigated. In the first part of the study, the physical, chemical, mineralogical, molecular and microstructure characterizations of silica fume and CEM I 42.5 R type Portland cement were determined. In the second stage, 5 types of cement were prepared by substituting silica fume into Portland cement at the rates of 0%, 2.5%, 5%, 7.5% and 10% by weight. In the third stage, water requirements, setting times and volume expansions of the pastes produced with these cements were determined by standard cement tests. In the fourth stage, the hydration mechanisms of these pastes on the 28th day were investigated by advanced analytical techniques such as X-ray diffraction, fourier transform infrared spectroscopy, simultaneous thermal analysis, scanning electron microscopy and energy dispersive X-ray spectroscopy. In the last stage, the compressive strengths of the cement mortars on the 2nd, 7th, 28th and 90th days were determined. The results showed that the compressive strengths of the mortars could be improved by substituting silica fume at all hydration ages. This is due to the fact that silica fume, with its nano-sized particles and large surface area, supports the hydration of C3S and C2S and the formation of calcium silicate hydrate phase. However, silica fume and free lime reacted to form additional calcium silicate hydrate gel, thus densifying the microstructure of cementitious composites and reducing voids. These findings show that when silica fume is substituted for Portland cement, an environmentally friendly, cost-effective and sustainable solution can be provided. In addition, it has been determined as a result of standard tests that cements produced with 10% silica fume substitution meet all the requirements of the 52.5 R type cement, which is a higher strength class.