The cause of the explosion was ignition of the flammable gas hydrogen evolved from foamed concrete. The particular location of the pour appears to have allowed a flammable/explosive atmosphere to accumulate within a relatively confined space beneath the walkway.
HSE has investigated the mechanism by which hydrogen was generated. The particular concrete mix included incinerator bottom ash aggregate (IBAA), which has been shown to contain a significant proportion of aluminium which is known to react with cement/concrete mixtures to form hydrogen gas.
Where use of IBAA is being considered it is advisable to adopt certain precautions that can be found on the HSE website.
HSE has decided to take no action requiring the omission of IBAA from foamed or general concrete mixtures for use in civil engineering works, and has thus not 'banned' IBAA from being used.