Posted By Bob Youel for interest only / or when you are facilitating re risk assessment: I am passing on the info the credit is to others "There was a serious explosion last week in a sewage pumping station being decommissioned when contractors were injured as a result of a gas explosion. The dry well had been filled with a large volume of foamed concrete over the previous three days and a spark from an angle grinder being used to cut handrails is believed to have ignited an explosive gas mix that had accumulated between the surface of the poured concretre and the underside of the chequer plate flooring around the former dry well. The concrete contained a significant percentage of flyash. The investigation is on-going. However there are other anecdotal reports of small fires from hydrogen evolved form foamed concrete used to cover gas/ water mains laid in the ground. There are research papers (from Sweden)on the internet which show that there is quite a potential for hydrogen evolution from flyash , particulatly in alkaline conditions- as found in concrete. The initial advice is- keep the area well ventilated- avoid void spaces where gas can accululate- avoid hotwork unless the area has been tested as " gas free" for hydrogen"