Research on the use of slag and other steel waste, as well as the inclusion of natural fibersNatural fiber as an alternative material to the conventional use of fiberglass in polymer matrixPolymer matrix compositeComposite materials, has gained relevance in recent years. In Peru, chambira fibers (CF) and maguey fibersMaguey fiber (MF), which are extracted from the Astrocaryum Chambira palmAstrocaryum Chambira palm fiber and the maguey agave sisalana, respectively, have aroused interest as reinforcements of compositeComposite materials of a matrix composed of epoxy resin filled with foundry slagFoundry slag–FS and steel scaleSteel scale–SS (modified epoxy resin). This work proposes to explore the physicochemical properties of fibers and filling materials through techniques such as FT-IR, chemical, structural XRD, proximate carbon, and thermogravimetric analyses. Preliminary results of FT-IR spectra of natural fibersNatural fiber (MF and CF) evidence five steps: vibration OH, asymmetric vibration of CH, stretching of CO, vibration of the aromatic skeleton of lignin, vibration of hemicellulose and stretching of CO in the range of 3300 to 1000 cm−1, confirms that as MF as CF contain cellulose—forming skeletal structures, lignin—filling micro fibrils and walls, and hemicellulose—quasi indetectable, making possible their potential use as fiber-reinforced polymer (FRP) compositesFiber reinforced polymer (FRP) composites. The characterizationCharacterization of steel waste (FS and SS) prioritizes the use of FS because it contains a moderate percentage of iron and does not contain iron oxides compared to SS. Finally, the iron oxides contained in the steel scalesSteel scale (SS) don’t permit these wastes can be utilized as filling material of polymer-based compositeComposite materials without a previous treatment of stabilization.