Coal ash is a material produced during the combustion process at coal-fired power stations. It can be used in a range of construction products with little need for further refinement, such as fillers, cement, grouts, blocks and so on.
In the UK, the types of available coal ash are as follows:
Different coal ash-based products will have varying moisture content, degree of compaction, shear strength and so on. They are categorised according to a number of industry standards setting out the levels of coal ash contained in particular products and what they can safely be applied to. For example, BS EN450-1 provides the specifications and categories for PFA use in concrete and BS EN450-2 defines quality control and statistical compliance.
Pulverised fuel ash (PFA), also known as fly ash, is a very fine ash that results from burning pulverised coal in coal-fired power stations. The term ‘pulverised’ comes from the fact that the coal used is a fine powder which can burn efficiently and quickly at a temperature of between 1,250-1,600ºC.
PFA is removed from the flue gas given off from burning coal by electrostatic precipitators. It is collected in silos where it is either sold dry for use in concrete, or ‘moistened’ for use in fills, grouts, and so on.
Alternatively, some power stations mix the ash with large amounts of water and pump the resulting slurry to lagoons. This lagoon ash is left to drain and then sold for fill and grouting applications.
The chemical composition of PFA differs from Portland cements, in that it won’t hydrate with water directly but requires lime and water. PFA is used in combination with Portland cement. Depending on the application, the ratio is typically in the range of 80-60% Portland cement to 20-40% PFA. These mixes often have slower hydration times to similar Portland cement-only mixes, but they may be more durable.
A drainage blanket or capillary break, 300-450 mm, should be placed beneath PFA-based fill materials to prevent the filler from leaching ground water as fine-grained materials have a propensity to do via capillary suction. This can depend on the thickness of the filler layer, and usually if a PFA-based filler layer is greater than 2m, a stable fill is achieved and there is no need for the precaution of a blanket or break.
There are some environmental benefits to using PFA as a building material. PFA usually replaces virgin aggregates in fill material, thereby reducing the resource-intensive process of extracting and processing such aggregates. Its cementitious properties also reduce the quantity of Portland cement required to create concrete.
In concrete, the low solubility of the PFA sulfate (gypsum) does not cause problems such as excessive retardation or expansion. However, when used as a fill material it has the potential to cause sulfate attack on some metals, such as mild steel. As such, the Specification for Highways states that PFA should not be placed within 500 mm of metallic elements, such as directly over culverts.
Furnace bottom ash (FBA) is formed from the molten material that falls to the bottom of the furnace during the combustion process at coal-fired power stations. The granular product is extracted, graded and sold as lightweight aggregate for use in concrete blocks.
Cenospheres are a coal ash by-product; fine, rigid hollow spheres that are often filled with air or inert gas. They are recovered from ash ponds or lagoons that are often situated at the same site as a coal-fired power station. These can be use as low-density fillers and in the production of lightweight concrete.
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