Seminar Reports on Slica-Fume Concrete

Silica fume also called Micro silica is an amorphous (non-crystalline) poly morph of silicon dioxide. Micro silica is very narrow in size and collected as a by-product of the silicon and ferrosilicon alloy production. Its average particle diameter is 150 nm and is spherical in shape.
Micro silica can act as pozzolanic material in High performance concrete due to its extreme fineness and high silica content. ASTM C1240 and EN 13263 are the standard specifications of silica fume used in cementitious mixtures. Both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the pozzolanic reactions between the silica fume and free calcium hydroxide in the paste are the reasons for the improvements of properties of the Portland cement. The permeability of concrete to chloride ions is also
reduced by the addition of silica fume to concrete, which protects the reinforcing steel of concrete from corrosion, especially in chloride-rich environments such as coastal regions and humid continental roadways and runways (because of the use of deicing salts) and saltwater bridges.

Silica fume works on two levels in cementitious compounds. A chemical reaction called the “pozzolanic” reaction is the first one. Calcium Silicate hydrates (CSH) and Calcium Hydroxide (CH) are produced on hydration (mixing with water) of Portland cement. The source of strength in concrete is said to be the CSH gel. Fresh concrete produces additional CSH when silica fume is added to the fresh concrete. CSH is produced by a chemical reaction of silica fume with the CH. By this reaction we can Increase the compressive strength and chemical resistance. The second function is a physical one. Silica fume can fill the voids created by free water in the matrix because it is 100 to 150 times smaller than a cement particle. In this physical function, silica fume refines the micro structure of concrete, creating a much denser pore structure and this function is called particle packing. Silica fume reduces the number and size of capillaries that would normally enable contaminants to infiltrate the concrete so that, impermeability is dramatically increased. Silica fumes are more resistant to aggressive environments hence the silica fume modified concrete is not only stronger, it lasts longer.

Improvement in the properties of fresh and hardened concrete due to the addition of silica fume include

1) Workability: The slump loss with time is directly proportional to increase in the silica fume content with the addition of silica fume. This is due to the introduction of large surface area in the concrete mix by its addition. The mix remains highly cohesive and also the slump decreases.

2) Segregation and bleeding: The free water is consumed in wetting of the large surface area of the silica fume and hence the free water left in the mix for bleeding also decreases. Water within the concrete is not allowed to come to the surface because silica fume blocks the pores in the fresh concrete. Thus silica fumes significantly reduce bleeding.