A contractor had filled a pit to a depth of some 6m with foamed concrete. Whilst this was setting, workers started to remove steelwork using angle grinders. There was then an explosion beneath a steel walkway on which two contractors were standing, and this blew the steel plates and the workers up into the roof of the building.
The cause of the explosion was ignition of the flammable gas hydrogen evolved from foamed concrete. The liquid concrete had been aerated, or foamed, by mixing air into it by agitation in the presence of a foaming agent (surfactant). There is evidence that hydrogen was produced from the concrete as it was setting. The particular location of the pour appears to have allowed a flammable/explosive atmosphere to accumulate within a relatively confined space beneath the walkway.
The HSE has published an update on 7 July 2010 to information published by HSE on 3 December 2009 following an explosion in August 2009 involving foamed concrete in which two persons were injured.