A method of separating an iron containing phase 110 from a sodium silicate and/or calcium silicate hydraulic phase 120 in steel making slag by exposing the slag to an environment which comprises water and carbon dioxide whereby the hydraulic phase 120 is hydrated. The slag is ground either before or during mixing with water. The hydration reaction is performed at a raised pressure (e.g. 10 bar or more) at room temperature, for example in an autoclave. The method produces particles 200 comprising a core 110 of the iron containing phase surrounded by an inner layer 125 of hydrated silica and an outer layer 140 of precipitated calcium carbonate. The layers 125 & 140 can be removed from the core 110 by grinding, with the core 110 then being separated for recycling into a steel making process. The method can be used to recycle the iron in BOF and LD slags which have high phosphorous content in the calcium silicate as the phosphorous 130 is accumulated in the layers 125 140