When Aluminium Sulphate is used instead of Potassium Aluminium Sulphate (often called Potassium Alum) the hardener often contains some Sulphuric acid.

It is possible as Gerald states to use an Alum like Sodium Aluminium Sulphate (Sodium Alum) in a harening fixer.

The term Alum itself is used in different trades to signify quite different compounds so is quite unspecific.