Silica fume particles are small and have a large specific area, with physical and chemical characteristics such as high purity of SiO2 and high volcanic ash activity. Adding silica fume as a mineral additive to concrete can have various good effects on its performance. The beneficial effects of amorphous and ultrafine silica fume on high-performance concrete are manifested in both physical and chemical aspects: acting as ultrafine fillers; Plays a nucleation role in the early hydration process and has high volcanic ash activity.
(1) Accelerate the hydration of cementitious material systems. After replacing equal weight of cement with silica fume, the system's 3D and RD hydration heat release significantly increased. Concrete projects that require control of early hydration heat release should pay special attention to this when selecting materials.
(2) Improve the strength of concrete. When silica fume and water reducing agent are used in combination, the water cement ratio of concrete can be reduced to a very low level, and the cement particles are filled tightly with silica fume, resulting in an increase in the compressive strength of concrete. Studies have shown that excessive silica fume content can lead to a decrease in the later strength of concrete. Silica fume concrete has good early strength and is commonly used in special projects such as emergency repairs, high-rise buildings, large-span structures, wear resistance, and corrosion resistance. Using silica fume high-strength concrete in hydraulic engineering to improve the impact and abrasion resistance strength
(3) The high-density silica fume particles are very small and can be filled in the gaps between cement particles. Dense accumulation of particles can reduce bleeding, decrease the average pore size of capillary pores, and decrease water demand. When the dosage of silica fume is between 5% and 10%, a good addition effect can be obtained.
(4) Improve the segregation and bleeding performance of concrete. After pouring concrete, water often separates from the concrete, forming a water film on the surface, also known as floating slurry, which causes uneven distribution of the upper layer of concrete and affects the quality of the building. The more silica fume is added, the more difficult it is for concrete materials to separate and bleed. When the substitution rate reaches 15%, even if the concrete slump reaches 15-20 cm, there is almost no segregation or bleeding; When the substitution rate reaches 20%~30%, it is not advisable to cause segregation when the concrete is directly placed in tap water. Due to the improvement of concrete segregation and bleeding performance by silica fume, silica fume mixed concrete can be used for underwater projects such as seaports and tunnels.
(5) Improve the impermeability and chemical corrosion resistance of concrete. Due to the addition of silica fume, the compactness of concrete is improved, greatly reducing concrete voids, effectively preventing the invasion and corrosion of acid ions, enhancing the impermeability and chemical corrosion resistance of silica fume concrete, and improving the corrosion resistance of steel bars.
(6) The effect of silica fume on the frost resistance of concrete. Regarding the influence of silica fume on the frost resistance of concrete, a large number of studies both domestically and internationally have shown that, with equal substitution, silica fume concrete with a dosage of less than 15% has similar frost resistance, and sometimes even improves (such as when the dosage is between 5% and 10%), but a dosage exceeding 20% will significantly reduce the frost resistance of silica fume concrete.
(7) Domestic and foreign practices have shown that silica fume is beneficial for suppressing alkali aggregate reactions in concrete. When calculating the total alkali content of concrete, the effective alkali carried in by silica fume is calculated based on 50% of its total alkali content.
