Pulverised fuel ash

Pulverised fuel ash (PFA), is a by-product of pulverised fuel (typically coal) fired power stations. 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 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 as high as 11 is known, and >9 is normal. It leaches a solution dominated by sodium and sulfate, with enough boron (>15 mg 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 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 before turning to birch/willow scrub woodland. A notable feature are the Dactylorhiza orchids which often form spectacular colonies 10–20 years post dumping, only to fade away again as the woodland thickens. Hydroseeding is often used to establish vegetation onto PFA due to the inhospitable conditions of most sites.

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