I think if you can get a method to remove water, it will become anhydrous.

The usual method is heating to make the anhydrous salt, which is often used as a mild desiccant.  The salt is thermally stable and heating in a flow of air can be used to remove unwanted trace organic contaminants as well as the water of crystallisation.   

For organic chemical analysis it is usual to bake sodium sulphate in a muffle furnace overnight and then to keep in a sealed jar.

Interesting Saima. sodium sulphate is chemically  very stable , being unreactive towards any oxidizing or reducing agents at normal temperatures . at high temperatures, it can be converted into sodium sulphide . sodium sulphate has ten molecules of water of hydration . A simple controlled heating will lead to formation of anhydrous sodium sulphate after loosing water of hydration .  

please take it to an oven for 10 hours and 105 degree Celsius.then cold it in a desicator for 1-2 hours.then you can use it.