Citation: Waqas, R.M.; Zaman, S.; Alkharisi, M.K.; Butt, F.; Alsuhaibani, E. Influence of Bentonite and Polypropylene Fibers on Geopolymer Concrete. Sustainability 2024, 16, 789. https://doi.org/10.3390/su16020789 Academic Editors: Antonio Caggiano and Syed Minhaj Saleem Kazmi Received: 14 October 2023 Revised: 3 January 2024 Accepted: 9 January 2024 Published: 17 January 2024 Copyright: © 2024 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/). sustainability Article Influence of Bentonite and Polypropylene Fibers on Geopolymer Concrete Rana Muhammad Waqas 1 , Shahid Zaman 1 , Mohammed K. Alkharisi 2, *, Faheem Butt 1 and Eyad Alsuhaibani 2 1 Department of Civil Engineering, University of Engineering and Technology, Taxila 47050, Pakistan; rana.waqas@uettaxila.edu.pk (R.M.W.); engr.shahid742@gmail.com (S.Z.); faheem.butt@uettaxila.edu.pk (F.B.) 2 Department of Civil Engineering, College of Engineering, Qassim University, Buraidah 52571, Saudi Arabia; e.alsuhaibani@qu.edu.sa * Correspondence: m.alkharisi@qu.edu.sa Abstract: Bentonite is one of the SiO 2 -rich pozzolanic clay types that has been enormously employed as a cost-effective and eco-friendly supplementary cementitious material in ordinary Portland cement (OPC) concrete. However, the use of bentonite in geopolymer concrete (GPC) has not been explored very widely. Further, the research available on the effect of utilizing treated bentonite in GPC is limited. The practical application of GPC is also very limited due to its significant shrinkage and high brittleness compared to OPC concrete. There are several studies available that have highlighted the use of polypropylene fibers (PPF) in improving the mechanical properties of GPC by reducing drying shrinkage and enhancing ductility. However, the effect of PPF on the durability properties of GPC needs to be addressed. Further, the effect of the combined integration of bentonite and PPF on the mechanical and durability properties of GPC has not been reported yet. The aim of this study is, therefore, to investigate the individual and combined effect of bentonite and PPF on the workability, mechanical properties, and durability of fly ash (FA)-based GPC. Bentonite replaced 10% of FA, and PPF was added at varying proportions (0.5%, 0.75%, and 1%) for raw and treated bentonite. Slump test was used to assess workability, while compressive, tensile, and flexural tests were utilized to evaluate the mechanical properties. Water absorption, acid attack, and abrasion resistance tests were used to evaluate durability. The results showed that bentonite and PPF significantly enhance mechanical properties, especially when combined with treated bentonite, with the highest improvement observed for mixtures with 1% PPF. The compressive strength was improved by an extent of 10% and 18% for raw bentonite-GPC and treated bentonite-GPC, respectively, compared to the control mix without bentonite. The durability test results revealed that water absorption of raw and treated bentonite-GPC mixtures at the age of 90 days was decreased by 16% and 21%, respectively, compared to the control mix (without bentonite). The mass loss of raw and treated bentonite-GPC mixtures in sulphuric acid solution was 5% and 10% lower, respectively, than the control mix (without bentonite). The mass loss of raw and treated bentonite-GPC mixtures in abrasion resistance tests was 6% and 12% lower, respectively, than the control mix (without bentonite). For durability performance, mixtures with 0.5% PPF perform the best, while higher PPF contents negatively impact the GPC durability. Keywords: bentonite; durability properties; geopolymer concrete; mechanical properties; polypropylene fibers 1. Introduction Climate change is a rising global challenge that is driven by increasing pollution and CO 2 emissions. Researchers are collectively working on environmentally friendly methods to protect the environment, including sustainable living practices and the development of eco-friendly technologies. The construction sector is also seeking eco-friendly, low-CO 2 emitting materials. Each year, approximately 4 billion tons of ordinary Portland cement Sustainability 2024, 16, 789. https://doi.org/10.3390/su16020789 https://www.mdpi.com/journal/sustainability