The present invention refers to a method for using electric steel plant slag, ground and/or in particles of less than 6 mm, in self-reducing agglomerates, in the form of pellets or briquettes, as a fluxing material and as a secondary source of iron, in sufficient amounts to adjust the binary basicity of the final slag (ration of CaO/SiO2) of the reducing/melting process to values equal or greater than 1.1. This application enables a reject that is difficult to discard to be transformed into a secondary surce of iron and a final slag having adequate characteristics to be used in the concrete and asphalt paving industries.