Many modern chemicals can be added to the mixture to achieve special objectives. 'Superplasticizers' are chemicals which improve workability, which means the ability to shape the concrete before it has set. Pigments can change the dull grey colour. Corrosion inhibitors can reduce rusting on the steel bars. 'Air entrainment' is blowing little bubbles in the concrete before it sets. This helps the concrete survive the freezing and thawing of a cold climate. Blast furnace slag can be mixed with concrete. It turns the colour into almost pure white and makes the concrete stronger.