Background

Fly ash is the finely divided mineral residue resulting from the combustion of powdered coal in electric generating plants. (Also known as a coal combustion product [CCP]) Fly ash consists of inorganic, incombustible matter present in the coal that has been fused during combustion into a glassy, amorphus 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.) This 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 .5μm-100μm. They consist mostly of Silicon Dioxide (SiO2), Aluminum 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.

Uses

Owing to its pozzolanic properties, fly ash is currently used as a replacement filler in Portland cement. It can replace between 10-30% by mass of Portland cement, which can add to the final strength of the cement and increase chemical resistance. Ash used as a cement replacement must meet strict construction standards. 75% of the ash must have a fineness of 45μm or less, and have a carbon content, called the loss on ignition (LOI), of less than 4%.

It also used as a component in the production of flow able fill (also called controlled low strength material, or CLSM), which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill. Flow able fill includes mixtures of Portland cement and filler material and can contain mineral admixtures, such as fly ash. Filler material usually consists of fine aggregate (in most cases, sand), but some flow able fill mixes may contain approximately equal portions of coarse and fine aggregates.

Fly ash has also been used as filler material in lower strength applications .In higher strength applications, the strength of flowable fill mixes can range from 1380 to 8270 kPa (200 to 1,200 lb/in2), depending on the design requirements of the project in question.

More recently fly ash has also been used as a component in geopolymer mixtures.

Problems

In the past fly ash produced from coal combustion was simply taken up by flue gases and dispersed into the atmosphere. This created significant environmental concerns and health risks. These days most power plants are required by law to reduce their fly ash emisisons to less than 1% of ash produced. The remainder is collected using electrostatic precipitators or filter bags. This collected ash is either sold for use in the cement/construction industry or disposed of in ash ponds or land fill. In recent times the more and more fly ash is used beneficially - still though, more than 65% of fly ash produced from coal power stations is still disposed of. This amounts to approximately 7 million tonnes (Mt) disposed of annually in Australia, 40Mt in the United States and 100s of millions in India and China. As a result the diposal of fly ash is a growing concern for many countries world wide.

FLY-ASH MIX

PERFORMANCE RECORD, fly ash

MATERIAL PROCESSING REQUIREMENTS, fly ash

ENGINEERING PROPERTIES, fly ash