What is Silica fume
Silica fume is a very fine particle, but how we get it, we produce it when we heat the sand upto two thousand degrees C, we get ferrosilicon. In addition to this, we get silica fume from the separation of waste gases through a bag-house filter. Furthermore, ferrosilicon are used to deoxidize the steel, in the steel industry, and silica fume is used in the construction industry. To get back to the point, particles of silica fume are so fine that, if we take one kg of silica fume, we can spread it into the area about ten thousand square meters, that is twenty to thirty times more, when we compared it with cement. By using it in concrete, we can increase the strength durability and finishing of concrete. But until we understand its chemistry, all this cannot be achieved.
why Silica Fume used in concrete
The chemical formula of both silica fume and sand is SiO2. But both have different behavior in concrete. One is very reactive in concrete, while the other is inert. That may be due to, there type of solid structure. As the structure of sand is crystalline, so it cannot dissolve in concrete, but silica fume is a non-crystalline pozzolanic material which is very reactive. The reactivity of silica fume depends on the percentage of SiO2 in it. Usually, SiO2 percentage goes from 70 to 98 percent, but its percentage should be more than 85% for construction work.
Size comparison of Silica Fume particles with other ingredients of concrete
Because the size of the particles of silica fume is much smaller than the size of the particles of cement, so it fills the gap inside the cement pores. and makes concrete denser—consequently, concrete strength increases.
Size Ranges of ingredients of concrete
- Silica fume particles 0.5 µm
- Cement particles 45 µm
- Sand Grain 2.00 mm
- Coarse aggregate particles 20.00 mm
Effect of Micro-silica on fresh concrete
- Because of the microscopic size particle, it holds all the ingredients of the concrete together. and increases the attraction between all the ingredients of concrete. Therefore, its use improves the cohesion in concrete
- It can overcome the problem of bleeding if we use it in concrete. The reason is, due to its microscopic size particle, which requires more water for lubrication. Further, mostly silica fume concrete is designed with a low w/c ratio. Due to both of these, the problem of concrete bleeding goes away.
Effect of Micro-silica on hardened concrete
- We already know that cement reacts with water after mixing the ingredients of concrete, producing C-S-H gel and Calcium Hydroxide. Since silica fume is a pozzolanic material, it reacts with calcium hydroxide. as a result of this, compressive strength of concrete increases
- We know that if we use micro-silica in concrete, it densify the concrete therefore reduces the permeability of concrete. Due to low permeability, the aggressive chemical cannot penetrate the concrete in the future and decreases the chloride damages to reinforcement, sulfate , acid, or other chemical attacks.
- Due to all these reasons, the durability of concrete increases.
Replacement of cement with Micro silica
It depends on the amount of calcium hydroxide in concrete; the reason is that, micro-silica will react to it. Therefore we shall use between the range of 5 to 15 percent of the quantity of cement. If we use micro-silica less than 5 percent, it will not show any remarkable effects on the concrete. And if we use micro-silica more than 15 percent, more research is required
The disadvantage of Micro Silica
- As cement and silica fume is reacting with water and calcium hydroxide, respectively. These two reactions produce heat at the same time and increase the expansion and shrinkage. as a result of this, it increases the risk of the crack in concrete
- Because the size of the particles of micro-silica is much smaller, hence the water demand of concrete increases for lubrication. If we increase the quantity of water in concrete, it will increase W/C ratio and decrease concrete strength. Hence there is always a demand for water-reducing admixture.
- As silica fume reacts with calcium hydroxide, decrease the available amount of calcium hydroxide in concrete. resulting in less absorption of carbon dioxide gas to form calcium carbonate.