Incinerator Bottom Ash (IBA) is produced in incineration facilities. It is discharged from the moving grate of municipal solid waste incinerators and widely used as a bulk aggregate fill.

Following combustion, IBA typically has small amounts of ferrous and non-ferrous metals contained within it. This ash can be processed to standardise the material and remove contaminants and is then termed ‘processed IBA’. IBA is commonly used for:

• Bulk fill for embankments
• Road sub bases
• Cement bound materials

The use of IBA to produce ‘foam concrete’ has been cited as the most probable cause of a recent explosion. It would appear the high concentration of aluminium in the IBA, was supplemented by additional quantities of aluminium to cause foaming which is a commonly undertaken process – hence the name. The gas which causes the foaming is hydrogen, produced as a result of the reaction between the aluminium, the water and the lime in the cement.

If such an activity is carried out in an area with little ventilation, this gas collects and could explode if ignited. On more than one occasion, the atmosphere has been ignited by a construction crew in the area, resulting in injuries.

IBA should not be confused with Pulverised Fuel Ash (PFA) which is the typical fill used in geotechnical applications. Having spoken with a number of suppliers of PFA and a number of chemists, we can confirm there should be no problems with the use of alternative aggregate fill materials such as PFA, as long as they come from regulated sources and contain little or no aluminium. Aluminium is generally only present as a trace element in PFA although it does contain aluminium oxide which is inert, and will not react with the lime.

With the increasing use of IBA and similar reclaimed aggregates as sustainable and economic aggregate fill materials, the source and contents of any proposed fill should be carefully considered. Where there is a risk of producing potentially explosive atmospheres, workplaces may need managing under the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) and possibly treated as confined spaces.