The invention provides a method of processing a starting material in the form of a metal oxide-containing raw metallurgical slag to obtain a processed slag product. The method includes admixing the raw slag with a reductant to obtain a reaction mixture and heating the reaction mixture to cause the reductant to reduce the metal oxide in the slag, to obtain molten metal, in particular containing ferromanganese, and molten processed slag, in particular having a manganese content of at most 10% by mass. The method further includes separating the molten processed slag from the molten metal and causing or allowing the molten processed slag to solidify, thereby to obtain a solid processed slag product. This processed slag product may be, as desired, a filler for use in brick-making or for use in formulating a ready-mix concrete, an extender for extending a cement or for producing a blended cement, or an aggregate for use in the construction or building industry.