The main purpose of this study is to evaluate the characteristics of two by-products obtained during ferroalloys production, SiMn slag and Mn oxide filter cakes, and their possible use as complementary cementing materials for commercial Portland cement manufacturing, considering 5% and 15% additions. The results obtained have revealed that by-product characteristics make them suitable to be added to blended cement production processes since they fulfill the European regulations for the chemical, physical and mechanical requirements. Regarding the Mn oxide filter cakes use, a strength loss has been detected in blended cement mortars after a 90 days curing, although pore size distribution in this case is similar to that of a control mortar.