what is high volume flyash concrete?

Answer 1

Fly Ash is is the finely divided mineral residue resulting from the combustion of powdered coal in electric generating plants. It is also called pulverized fuel ash. Fly ash consists of inorganic, incombustible matter present in the coal that has been fused during combustion into a glassy, amorphous structure. Coal can range in ash content from 2%-30%, and of this around 85% becomes fly ash. (The remaining 15% is called bottom ash and isn’t lifted up by the flue gases.)

Fly ash material is solidified while suspended in the exhaust gases and is collected by electrostatic precipitators or filter bags. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 µm to 100 µm. They consist mostly of silicon dioxide (SiO2), aluminium oxide (Al2O3) and iron oxide (Fe2O3), and are hence a suitable source of aluminum and silicon for geopolymers. They are also pozzolanic in nature and react with calcium hydroxide and alkali to form cementitious compounds. Fly ash also contains some heavy metals.

Answer 2

Fly ash is a pozzolanic material that is often used as a mineral admixture to concrete. Fly ash is used to replace a portion of the portland cement used in the mix. However, when using fly ash as a mineral admixture, there must be enough calcium hydroxide to react with the fly ash that you add. The theoretical limit is 25% by weight of cement (you can replace up to 25% of the cement with fly ash). However, durable structural concretes with greater than 50% replacement of fly ash have been made. This is because not only does fly ash react with calcium hydroxide to form calcium silicate hydrate (remember, only 25% wt. of cement will do this) but it also improves the workability of the concrete, when combined with a water-reducing admixture. So, even though there are excess fly ash particles within the concrete that have not reacted with the calcium hydroxide, their beneficial effect on workability actually improves the quality of the concrete (even though they chemically do not contribute to the strength).

Any concrete that uses more fly ash than 25% (wt cement) would be considered high volume fly ash concrete. With high volume fly ash concrete, you will see less early age strength, but the long term strength is about the same as with normal concrete.