A fire crew remains at Plasgran in Manea Road, Wimblington, more than 16 hours after the incident began on Wednesday afternoon.
Road closures are expected to stay in place until midday, the fire service said.
About 75 firefighters worked into the night to put out the fire.
They also prevented its spread to neighbouring properties.
The incident was scaled down at 2300 GMT, when the fire was brought under control.

To the strains of the late reggae star's One Love, Mr Obama was shown around the sprawling house in Kingston that was Mr Marley's final home.
The US president said he was a fan of the musician, telling his museum guide: "I still have all the albums."
Mr Obama is in Jamaica to meet the 15-member Caribbean Community bloc.
But his first stop after landing in Kingston was the old Victorian-style house in central Kingston that Mr Marley called home between 1975 and his death in 1981.
Mr Obama spent 20 minutes in the museum, which now houses artefacts from the singer's life including gold records and his Grammy Lifetime Achievement award.
His arrival aboard Air Force One on Wednesday made him the first sitting US president to set foot in the country since Ronald Reagan visited 33 years ago.
He was greeted by Jamaican Prime Minister Portia Simpson-Miller and the US Ambassador to Jamaica, Luis Moreno.
Mr Obama will meet representatives of the 15 countries in the Caribbean Community group on the three-day trip to Jamaica and Panama, and is expected to discuss energy and security issues.

More than 1,000 schools in America's second-biggest city were searched on Tuesday after classes were cancelled.
Officials in New York, the biggest city, said they had received a similar threat but had ignored it as a hoax.
Los Angeles police chief Charlie Beck said it was "very easy in hindsight to criticise the decision".
"We have suffered too many school shootings in America to ignore these kinds of threats," he added.
In New York, police commissioner William Bratton and federal officials who asked not to be identified described the decision in Los Angeles as an "overreaction", Reuters news agency reports.
New York Mayor Bill de Blasio had concluded the threat contained "nothing credible".
An investigation has begun into the cause of a fire which has severely damaged a plastics factory in Cambridgeshire.
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Barack Obama has taken a tour of the Bob Marley museum in Jamaica after becoming the first US president to visit the country since 1982.
[NEXT_CONCEPT]
Schools in Los Angeles are preparing to reopen after being shut down over an email threat, in a move queried by police officials in New York.