Just days before a new school year was supposed to get underway, several schools in Suffolk were told of the issues relating to reinforced autoclaved aerated concrete (RAAC).
This has left schools across the county facing the prospect of either closing or partially closing as some of their buildings were constructed with the material.
Here is everything we know so far about the closures:
Why are schools closing, and which ones?
So far, we know of four schools in Suffolk who have been told to partially or fully close buildings as children prepared to return to classes after the summer holidays.
This is because of fears over concrete which could suddenly collapse.
Hadleigh High School, East Bergholt High School and Claydon High School were revealed on Friday as being among the schools affected.
The three schools are under Penrose Learning Trust, which said the schools will need to consider delaying reopening or partially closing until the issue has been resolved.
Over the weekend, it was also announced that Farlingaye High School in Woodbridge will be partially closed over the issue.
The Department for Education (DfE) said a minority of the state facilities may have to move completely and some children may be forced back into pandemic-style remote learning.
But the government has refused to publicly reveal the 104 education facilities in the country which have been told to shut buildings.
The only schools in Suffolk known to be affected are the ones that have publicly spoken about the issue.
Education Secretary Gillian Keegan told broadcasters after the issue was revealed: “Most parents should not be worried about this at all.”
What is RAAC?
RAAC is a lightweight material that was used mostly in flat roofing, but also in floors and walls, between the 1950s and 1990s.
It has a lifespan of about 30 years and its structural behaviour differs significantly from traditional reinforced concrete.
Moreover, it is susceptible to structural failure when exposed to moisture. The bubbles can allow water to enter the material.
If that happens, any rebar reinforcing RAAC can also decay, rust and weaken.
What is Suffolk County Council saying on the matter?
A spokesman for Suffolk County Council said: "We have been actively seeking further details from the DfE.
"The key is to get a clearer picture of the impact on any schools affected ahead of the start of the new academic year, so that mitigations can be put in place urgently if all or parts of buildings cannot be used."
The county council has not yet revealed a full list of all the schools affected.
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