The building at Cathays Park was evacuated, with all staff and students told to leave, after a blaze broke out in a fume cupboard on Tuesday.
The main building will remain shut on Tuesday with access to most areas reopened on Wednesday.
But two chemistry labs on the first floor and the restaurant will be shut until further notice.

The Lib Dem site was called "May & Co" - the same name as a Chelsea estate agent established in 1920.
The firm's boss, John Yianni, said he started getting abusive and "weird" phone calls when the campaign appeared on Twitter.
The Lib Dems said it was "an innocent mistake" and the site had been changed.
A party spokesman blamed an "over-enthusiastic volunteer" who had released an early version of the campaign which had not been checked.
He said the party had apologised to Mr Yianni.
The Lib Dem site is now called "Theresa May and Co" - but Mr Yianni said it was still close to the name of his company and he was concerned it was affecting his business.
"The damage has been done and it's probably going to get worse. You don't want to be politically biased in your business.
"People were saying: 'What are you doing getting into bed with them?'"
He said he had contacted Sir Vince Cable and the Lib Dem headquarters to ask for the campaign to be dropped.
"I was a Vince Cable supporter. I told him that," said Mr Yianni, who said he would not now be voting for the former business secretary.
He added: "This company has been going since the 20s. They should have just checked before they unleashed it.
"They have changed it into something slightly different thinking it's going to be OK but it's still the same."
Lib Dem candidate Sir Ed Davey earlier staged a protest against Conservative social care reforms, which the party has dubbed the "dementia tax," with "Theresa May & Co" placards.
Plans to make people receiving care at home liable for the full costs if they are worth at least Â£100,000 have proved controversial since they were announced in the Conservative manifesto.
Theresa May later said proposed changes to social care funding in England would include an "absolute limit" on the money people will have to pay.
She has repeatedly stressed that people will not have to sell their homes while they are alive to fund care.
Speaking at the protest, Sir Ed said there are other ways of funding social care than "setting up May & Co estate agents to come after your home".

The actor was charged with culpable homicide in Mumbai for the September 2002 incident. Mr Khan has always denied he was at the wheel.
If convicted, he faces 10 years in prison.
Mr Khan is one of Bollywood's biggest stars and has starred in more than 80 Hindi-language films.
Several of his films, including Dabangg, Ready, Bodyguard, Ek Tha Tiger, Maine Pyar Kiya and Hum Aap Ke Hain Kaun, have been huge commercial hits.
Late on the night of 28 September 2002, his Toyota Land Cruiser hit the American Express bakery in the Bandra area of Mumbai, authorities say.
The vehicle ran over five people sleeping on the street, killing 38-year-old Noor Ullah Khan and seriously injuring three others. Another person received minor injuries.
The prosecution has alleged that Mr Khan was driving the car and that he was drunk. In March the actor told the court he was not drunk and was not driving the car.
But many witnesses have said that was not the case.
A constable attached to Mr Khan's security said in a statement to the police that the "drunk" actor had lost control of the car. The policeman died in 2007 of tuberculosis.
In April, Mr Khan's driver told the court that he had crashed the car after a tyre burst.
At the scene, Simon Atkinson, BBC News, Mumbai
It's not uncommon most days to find people camped outside the house of Salman Khan hoping for a glimpse of the star.
But on Wednesday morning dozens of journalists and fans gathered as they waited for the actor to leave on the way to court.
Even Mr Khan's father was ambushed as he went on a morning stroll along the seafront encircled by security guards - proof this is a case that has gripped Bollywood and wider India.
Parts of Cardiff University's main building will remain closed on Wednesday after a fire.
[NEXT_CONCEPT]
A London estate agent has complained to the Liberal Democrats after they used the name of his company in a spoof website attacking Theresa May.
[NEXT_CONCEPT]
An Indian court is due to rule on whether Bollywood star Salman Khan is guilty of running over five men sleeping on a pavement, killing one.