Final shifts at the Redcar steel plant have emptied the coke ovens before they are irretrievably shut down.
The official receiver said there was "no realistic prospect" of finding a buyer for the site after its Thai owner SSI went into liquidation.
The cooled ovens may start to collapse over the weekend, making them impossible to restart.
Jimmy Donnelly, who has worked at the site for 30 years, said "steelmaking is finished on Teesside".
After the last mothballing in 2010 the ovens were kept lit and the coke was sold on the open market, he said.
"It was classed as a soft mothballing, something you can pretty easily bring back on line," he said.
"This is a hard mothballing where it's going to cost millions of pounds to even try, hundreds of millions of pounds if the coke ovens are lost, which they will be."
The last load of coal was put into the ovens to be turned into coke at midnight on Tuesday.
From that point ovens have been emptied, or "pushed", but not refilled, or "charged", and the heat will be turned off.
Community Union chairman Paul Warren said, once empty and cooled, the coke ovens would become unstable and "start to collapse".
"You won't see it from the outside, but the structural damage to the inside of the ovens will be catastrophic."
The coke, which would have been used to fire the blast furnace, has been stockpiled since steel production was halted.
It could be sold or used as part payment for suppliers, Mr Warren said.
Mr Donnelly said he was "frightened for the future".
"That is the end," he said.
"If they [the coke ovens] go cold, they will need to be rebuilt.
"In my opinion, steelmaking is finished on Teesside."