Polymers 2022, 14, 4330. https://doi.org/10.3390/polym14204330 www.mdpi.com/journal/polymers Article Characterization and Modification of Red Mud and Ferrosilicomanganese Fines and Their Application in the Synthesis of Hybrid Hydrogels Arnaldo Ramírez 1, *, Leonir Gómez 2 , Alejandro J. Müller 3,4,5 and Blanca Rojas de Gáscue 1 1 Laboratorio de Polímeros, Instituto de Investigaciones en Biomedicina y Ciencias Aplicadas, IIBCAUDO “Dra. Susan Tai”, Universidad de Oriente, Cumaná 6101, Sucre, Venezuela 2 Centro de Investigación de Materiales (CIMAT), Universidad Nacional de Guayana, Ciudad Guayana 8001, Bolívar, Venezuela 3 Grupo de Polímeros USB, Departamento de Ciencia de los Materiales, Universidad Simón Bolívar, Apartado 89000, Caracas 1080, Miranda, Venezuela 4 POLYMAT and Department of Advanced Polymers and Materials: Physics, Chemistry and Technology, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, 20018 Donostia‐San Sebastián, Spain 5 IKERBASQUE, Basque Foundation for Science, Plaza Euskadi 5, 48009 Bilbao, Spain * Correspondence: arnaldoluisramirez@gmail.com Abstract: In this work, hybrid hydrogels were synthesized with the inclusion of two types of clay materials that are considered industrial waste: red mud (RM) and ferrosilicomanganese fines (FeS‐ iMn). These solid waste materials were characterized by studying their particle size and chemical composition, which are two key variables for their application in the synthesis of hybrid hydrogels. The morphology imaged by transmission electron microscopy (TEM) and scanning electron micros‐ copy (SEM), showed, in the case of RM, heterogeneous size and shape particles, with 73% of the particles having lengths of less than 5 μm. On the other hand, FeSiMn had particles with a circular morphology of nanometric sizes. Regarding the synthesis of the hybrid hydrogels, it was deter‐ mined that the incorporation of small percentages (0.1%) of the inorganic phases improved the ca‐ pacity of the materials to absorb water (swelling indices of 1678% and 1597% for the RM and FeSiMn hydrogels, respectively) compared to the conventional polyacrylamide hydrogel (1119%). An im‐ provement in Vickers microhardness and storage modulus (G’) was also observed: the hybrid with 10% RM presented a G’, 50 times higher than conventional hydrogel. The results show the merit of RM and FeSiMn in improving the properties of hydrogels. Keywords: red mud; modification; hybrid hydrogel 1. Introduction In synthesizing hybrid hydrogel‐type composites and nanocomposites, choosing an inorganic phase suitable to obtain the desired properties in the final material is a key step. In this regard, two variables to take into account are: (1) the particle size, which should facilitate good dispersion in the organic matrix and guarantee the homogeneity of the material; (2) the chemical composition, which is responsible for the physical and chemical interactions between both phases that occur during the formation of the hydrogel and determine its final properties. In the following investigation, two inorganic phases were evaluated in the synthesis of hybrid hydrogels, and both considered industrial wastes not studied previously in this type of application: red mud (RM) and ferrosilicomanganese fines (FeSiMn). RM is a solid residue that is generated from the alumina production process under the BAYER scheme. It is an environmental liability, mainly due to its high alkalinity (pH Citation: Ramírez, A.; Gómez, L.; Müller, A.J.; Rojas de Gáscue, B. Characterization and Modification of Red Mud and Ferrosilicomanganese Fines and Their Application in the Synthesis of Hybrid Hydrogels. Polymers 2022, 14, 4330. https:// doi.org/10.3390/polym14204330 Academic Editor: Antonio Capezza Received: 20 August 2022 Accepted: 29 September 2022 Published: 14 October 2022 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and institu‐ tional affiliations. Copyright: © 2022 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (https://cre‐ ativecommons.org/licenses/by/4.0/).