London, Sep 1 : Schools in England have been told to immediately close buildings made with a certain type of concrete that is prone to collapse, the UK government has said. There are 156 schools in England that have reinforced autoclaved aerated concrete (RAAC) in their buildings, Xinhua news agency reported, citing the Department for Education (DfE) data. The RAAC is a lightweight concrete that was used in roofs, floors and walls between the 1950s and 1990s, the DfE said. It is considered less durable than traditional concrete and engineers have warned that the material is at risk of cracking, spalling and crumbling. British Secretary of State for Education Gillian Keegan said that "the vast majority of schools will be unaffected" and support was in place to "ensure that any disruption to education will be kept to a minimum". The National Association of Head Teachers said the government was right to put the safety of pupils and staff first, but "there is no escaping the fact that the timing of this couldn't be worse," as children are due to return from summer holidays next week. /IANS