Question

I research about "ENHANCEMENT OF CONCRETE PROPERTIES COMPRISING SILICA FUME AND FLY ASH'' .

Can you write discussion research about that title please?

Answer 1

Silica fume

Highly dense concrete can be achieved using silica fume. The size of silica fume varies from 0.005 to 0.5 microns compared to cement particles which vary from 0.5 to 100. Silica fume is generally added from 5 to 25 %. Silica fume has specific surface area of about 20,000 m2/kg, compared to 230 to 300 m2/kg of cement.

Effect of silica fume on rheological properties of concrete.

Flow of concrete decreases with the addition of silica fume demanding for increased water content in the concrete. It is because; the mix becomes more cohesive and sticky in nature which makes it hard to work with. Addition of silica fume in concrete largely reduces the bleeding and segregation in concrete due to its dense and cohesive nature.

Effect of silica fume on Hardened Concrete

Addition of silica in the concrete enhances the strength of concrete largely. A compressive strength of 270 MPa has been achieved with silica fume substituted paste. Because of the high reactive nature, silica fume produces very dense C-S-H formed when reacts with Ca(OH)2. It is observed that, silica fume also increases durability of concrete.

Fly ash

Effect of Fly ash on rheological properties of concrete.

The water demand in the concrete reduces with the addition of right quality of fly ash in the concrete. This reduced water content in the concrete helps in controlling bleeding and shrinkage in the concrete. Since fly ash is not highly reactive, the heat of hydration can be reduced through replacement of part of the cement with fly ash.

Effect of Fly ash on Hardened Concrete

Because of the pozzolonic activity of fly ash, the strength of concrete increases but fly ash not highly reactive hence it gives low early strength but later strength of the concrete increases. It also increases the dense nature of the concrete thereby reducing the permeability.

Effect of silica fume and fly ash on concrete when used together.

After understanding the behaviour of both the admixtures in the concrete, several investigations are done on the combination of fly ash and silica fume in the concrete along with cement. The results have shown that, with the increase in silica fume, slump of concrete decrease but the combination of cement, fly ash and silica fume gives better workability. It was also observed that the presence of silica fume in the concrete upto 10% increases the early strength of the concrete and presence of fly ash helps in improving the later strength of concrete.

From the result it can be seen that drawback of one material is balanced by other in the concrete. Low initial strength of fly ash is compensated by high reactive silica fume. Decrease in workability due to silica fume is compensated by the fly ash. High heat of hydration due to high reactive silica fume is balaced by low reactive fly ash. Also, the chemical composition of silica fume and fly ash shows high percentage of silica which can be used up effectively by combining with Ca(OH)2 to produce more and dense C-S-H gel, which is mainly responsible for the strength of concrete.