A method for treatment of metallurgical wastes i.e. slags, which are in liquid form by introducing water into the liquid slag thus causing it to cool at a slow rate so that the solidified slag contains a porous crystalline or utrified structure according to the rate of cooling, which structure is stable at ambient temperature. Another object of the invention concerns an apparatus for treating solid metallurgical slags, comprising a continuous conveyor formed of molds and means adapted to diffuse water into the liquid slag.