Pulverised Fuel Ash
Pulverised fuel ash is a by product of pulverised fuel (typically coal
Coal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...

) fired power station
Power station
A power station is an industrial facility for the generation of electric energy....

s. The fuel is pulverised into a fine powder, mixed with heated air and burned. Approximately 18% of the fuel forms fine glass spheres, the lighter of which (c. 75 %) are borne aloft by the combustion process. They are extracted from the flue gasses by cyclones and electrostatic precipitation.

The resultant material is used as engineering fill and as a component for concrete. It has been widely used, particularly in the UK, for concrete block production. The blocks are lightweight and have excellent thermal insulation properties. PFA can undergo a pozzolanic reaction
Pozzolanic reaction
The Pozzolanic reaction is the chemical reaction that occurs in hydraulic cement, a mixture of slaked lime with amorphous siliceous materials , forming non-water-soluble calcium silicate hydrates...

 and become brittle over time. PFA has a fine dust texture and is grey in colour.

When newly produced the dust is strongly alkaline; a pH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 as high as 11 is known, and >9 is normal. It leaches
Leaching (chemical science)
Leaching is the process of extracting minerals from a solid by dissolving them in a liquid, either in nature or through an industrial process. In the chemical processing industry, leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from...

 a solution dominated by sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 and sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

, with enough boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

 (>15mg l-1) to kill most plants, though coastal species often tolerate the salinity and boron to grow on young ash lagoons. In dry conditions these solutes rise to the surface to form a hard salt crust, impeding all plant growth, though hardy grass species such as Vulpia myuros
Vulpia myuros
Vulpia myuros, or Rat’s-tail Fescue, is an annual grass species of the genus Vulpia. It was probably originally native to Eurasia, but it can now be found nearly worldwide as a naturalized species....

can later colonise it. A "waxy" layer has been known to form in some locations which inhibits root penetration, however, mixing crushed rock into the top layer has been found to inhibit the formation of the "waxy" layer.

As the ash weathers, its salinity, boron level and pH all fall; the former two are largely removed from surface layers after 5 years outdoors, while pH declines towards 7 in a generally linear fashion at a rate of about 1 pH unit per 20 years. The floral succession approximates to that of a coastal dune system, without the wind-blown deposition, so salt-tolerant plants are replaced by an attractive sward of legumes and perennials
Perennial plant
A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...

 before turning to birch
Birch is a tree or shrub of the genus Betula , in the family Betulaceae, closely related to the beech/oak family, Fagaceae. The Betula genus contains 30–60 known taxa...

Willows, sallows, and osiers form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere...

 scrub woodland. A notable feature are the Dactylorhiza
Dactylorhiza , is a genus of terrestrial plants in the orchid family ....

orchids which often form spectacular colonies 10-20 years post dumping, only to fade away again as the woodland thickens. Hydroseeding
Hydroseeding is a planting process which utilizes a slurry of seed and mulch. The slurry is transported in a tank, either truck- or trailer-mounted and sprayed over prepared ground. Helicopters have been used to cover larger areas...

is often used to establish vegetation onto PFA due to the inhospitable conditions of most sites.

External links

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