The use of industrial waste in the synthesis of phosphate ceramics opens the door to new ways of production and marketing of this kind of product, as well as contributing to the reduction of the environmental impact of industrial waste. In the present study, chemically bonded phosphate ceramics (CBPC) were synthesized from steel slags (electric arc furnace slag (EAF), basic oxygen furnace slag (BOF), and ladle furnace basic slag (LF) and aqueous solutions of H3PO4. A chemical analysis was performed of the steel slags, which were selected through standard sampling before being ground and milled to obtain the appropriate granulometry. In the evaluated slags, iron oxides and calcium oxides, present both in their free state and in compounds with other elements, were identified as the elements with the greatest presence and potential to form CBPC. Design of experiments (DOE) was applied to form multiphase cements, in order to make the variables present in the obtainment of cements workable, thereby allowing the formation reactions of cement to be better understood. The cements obtained were analyzed through XRD, SEM-EDS, FTIR, XPS, and mechanical compressive strength tests. The synthesized cement exhibited mainly vitreous structure for the iron phosphate phases (Fe(H2PO4)2) or (FeHPO4), and mainly crystalline structure for the calcium phosphate phases, present as dihydrogen phosphate of calcium hydrate (Ca(H2PO4)2·xH2O) and brushite (CaHPO4·2H2O). Compression strength between 10 and 19 MPa was recorded for the different compositions evaluated, with short setting times.