Under the contract, more than 300 unionized custodians would be replaced with lower-paid non-union workers by Friday.
Critics say the contract is a step backwards for labor in the state.
"Our public body shouldn't be taking positions that undermine working men and women. They shouldn't be leading a race to the bottom. That is exactly what we're seeing at O'hare Airport with this contract. We are taking workers who earn a decent wage and replacing them or offering to pay them less," said Jorge Ramirez, president of the Chicago Federation of Labor.
The organization says the new contractor has already hired nearly 400 non-unionized custodians to start work on Friday- including 100 workers from the current contract.
The Office of the Mayor issued this statement on Tuesday:
"Mayor Emanuel always puts Middle Class families first. Whether it is cracking down on pay-day lenders that prey on families living paycheck-to-paycheck; expanding opportunities through new innovative partnerships with labor; or refusing to increase property and sales taxes, Mayor Emanuel has proven to be a passionate fighter for the hardworking middle class families of Chicago."