Citation: Ngnintedem, D.L.V.; Lampe, M.; Tchakouté, H.K.; Rüscher, C.H. Effects of Iron Minerals on the Compressive Strengths and Microstructural Properties of Metakaolin-Based Geopolymer Materials. Gels 2022, 8, 525. https:// doi.org/10.3390/gels8080525 Academic Editor: Esmaiel Jabbari Received: 4 July 2022 Accepted: 17 August 2022 Published: 22 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). gels Article Effects of Iron Minerals on the Compressive Strengths and Microstructural Properties of Metakaolin-Based Geopolymer Materials Dimace Lionel Vofo Ngnintedem 1 , Marco Lampe 2 , Hervé Kouamo Tchakouté 1,2, * and Claus Henning Rüscher 2, * 1 Laboratory of Analytical Chemistry, Department of Inorganic Chemistry, Faculty of Science, University of Yaounde I, Yaounde P.O. Box 812, Cameroon 2 Institut für Mineralogie, Leibniz Universität Hannover, Callinstrasse 3, D-30167 Hannover, Germany * Correspondence: hervetchakoute@gmail.com (H.K.T.); c.ruescher@mineralogie.uni-hannover.de (C.H.R.); Tel.: +237-677979617 or +491-7627699163 (H.K.T.); +237-695036859 (C.H.R.) Abstract: The current study aims to investigate the influence of iron minerals on the amorphous phase content, compressive strengths and the microstructural properties of the geopolymer materials. Geopolymer materials were prepared by the substitution of metakaolin by 10 and 20 wt.% of each iron mineral sample. Sodium waterglass from rice husk ash was used as a hardener, and metakaolin was used as an aluminosilicate source. The X-ray patterns show that the iron minerals denoted FR and FB are associated with hematite and magnetite, respectively. FY contains goethite together with a significant content of kaolinite and quartz. It is observed in the XRD patterns and FTIR absorption spectra that the additions of hematite, magnetite and goethite remain largely unreacted in the geopolymer binder. The compressive strengths of the related geopolymer composites show some significant variations indicating certain effects for mechanical stability obtained: 10 wt.% replacement of metakaolin by hematite increased the compressive strength from 51.1 to 55.5 MPa, while 20 wt.% hematite caused a decrease to 44.9 MPa. Furthermore, 10 and 20 wt.% replacement with FB revealed decreased values 47.0 and 40.3 MPa, respectively. It was also found that 10 and 20 wt.% of FY caused lower values of 30.9 and 39.1 MPa, respectively. The micrographs of geopolymer materials present some voids and cracks. The denser matrix is related to a superior gel formation producing a better glue between the crystalline additions. The unsubstituted geopolymer sample provides with about 50% the highest X-ray-amorphous content, whereas the substituted samples range between 35 and 45%, indicating systematically smaller gel contents without any clear trend with the compressive strength variation, however. The strength dependencies reveal more complex interaction between the gel and crystalline additions. Keywords: hematite; magnetite; goethite; metakaolin; geopolymer materials; gels 1. Introduction Iron is found mainly in the Earth’s crust in minerals of iron oxide and iron oxyhy- droxide such as hematite, magnetite, maghemite, goethite, limonite, etc. Amongst these minerals, hematite (α-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) are mostly used as an ore for making metal iron. Iron minerals exist to some extent in aluminosilicate sources such as kaolin and more significantly in laterite. Most iron minerals generally present in these aluminosilicate sources are hematite and goethite. The iron mineral content in the aluminosilicate sources depends on the geological formation. Goethite contained in the raw aluminosilicates are transformed to hematite during the calcination process, which is performed at about 700 ◦ C using these sources for the preparation of geopolymers. Several researchers have used hematite and magnetite nanoparticles as additives to increase the hydration process of the Portland cement mortar and concrete. For example, Kani et al. [1] investigated the Gels 2022, 8, 525. https://doi.org/10.3390/gels8080525 https://www.mdpi.com/journal/gels