Citation: Barbhuiya, S.; Pang, E. Strength and Microstructure of Geopolymer Based on Fly Ash and Metakaolin. Materials 2022, 15, 3732. https://doi.org/10.3390/ma15103732 Academic Editors: F. Pacheco Torgal and Cesare Oliviero Rossi Received: 11 April 2022 Accepted: 20 May 2022 Published: 23 May 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/). materials Article Strength and Microstructure of Geopolymer Based on Fly Ash and Metakaolin Salim Barbhuiya 1, * and Edmund Pang 2 1 Department of Engineering and Construction, University of East London, London E16 2RD, UK 2 School of Civil and Mechanical Engineering, Curtin University Australia, Perth 6845, Australia; edmund.k.pang@student.curtin.edu.au * Correspondence: s.barbhuiya@uel.ac.uk Abstract: The production of Portland cement is widely regarded as a major source of greenhouse gas emissions. This contributes to 6–7% of total CO 2 emissions, according to the International Energy Agency. As a result, several efforts have been made in recent decades to limit or eliminate the usage of Portland cement in concrete. Geopolymer has garnered a lot of attention among the numerous alternatives due to its early compressive strength, low permeability, high chemical resistance, and great fire-resistant behaviour. This study looks at the strength and microstructure of geopolymer based on fly ash and a combination of metakaolin and fly ash. Compressive strengths were measured at 7, 14, and 28 days, and microstructure was examined using SEM and XRD. Keywords: geopolymer; fly ash; metakaolin; microstructure; compressive strength 1. Introduction Researchers have been examining the impact of greenhouse gases on global warming for the past three decades. Rising sea levels, changes in ocean water that threaten marine life, shifting weather patterns, and ecological degradation are all results of global warming [1]. In the next 100 years, the global temperature is expected to rise by 3 ◦ C, with a possible increase of 4.6 ◦ C. As a result of these temperature increases, the sea level is anticipated to rise by up to 28 cm [2,3]. The International Panel on Climate Change (IPCC) believes that human activity is the most likely cause of observed warming since the mid-twentieth century. Given that people are a major cause of global warming, businesses such as coal power and the cement industry must reduce emissions. A number of programmes and policies have been established in an attempt to minimise global carbon emissions and, as a result, global warming. The Kyoto Protocol [4] is an international agreement established by the United Nations Framework Convention on Climate Change (UNFCCC) in 1997 that commits signatories to emission reduction goals. Emissions trading is a concept for bolstering the Kyoto Protocol by giving economic incentives to companies, notably the concrete industry, to reduce carbon emissions [5]. The cost of carbon is expected to be around US $15 per tonne [6]. Portland cement production is widely acknowledged as a major source of greenhouse gas emissions [7–12]. This accounts for 6–7% of total CO 2 emissions, according to the International Energy Agency (IEA) [13]. As a result, various initiatives have been made in recent decades to reduce or eliminate the use of Portland cement in concrete. Geopolymer has gotten a lot of attention among the many alternatives because of its early compressive strength, low permeability, high chemical resistance, and outstanding fire resistance behaviour [14–19]. The interaction of aluminosilicate material with alkaline solutions produces geopoly- mers. As a result, the two main components of geopolymers are source materials and alkaline liquids. Natural minerals such as kaolinite, clays, micas, andalousite, spinel, and by-products such as fly ash, silica fume, slag, rice husk ash, red mud, and so on might be used as source materials [20,21]. Fly ash and slag, in particular, have become popular Materials 2022, 15, 3732. https://doi.org/10.3390/ma15103732 https://www.mdpi.com/journal/materials