Mode I Fracture Behavior of Metakaolin-Based Geopolymers Made of Mine Tailings
Authors: Asadizadeh M. , Clements C. , Hedayat A. , Tunstall L. , Gelber Bolanos Sosa H. , Tupa N. , Yanqui Morales I. 
Published: August 2024

ABSTRACT:
Converting mine tailings rich in aluminum oxide and silicon dioxide into a geopolymer using an alkali activator offers an eco-friendly solution for mitigating the environmental impact of mine tailings and reducing associated disposal costs. In the realm of construction materials, thorough investigations into the mechanical properties of geopolymers are essential to gain a better understanding of their performance when subjected to varying loading conditions. This includes a comprehensive understanding of their fracture behavior when subjected to mode I loading, a critical aspect for their effective deployment in mode I loading scenarios. In this study, a semi-circular bend (SCB) test fixture was employed to analyze mode I fracture behavior of geopolymer specimens, a research topic that has been largely unexplored. Different geopolymer compositions comprising mine tailings (50% wt.), low-reactive slag (50% wt.), and varying metakaolin percentages (10% wt. and 20% wt. of total solid) were prepared by mixing the solids with a 10 M NaOH solution with a liquid/solid ratio of 20%. Using a cylindrical mold, we cast geopolymer specimens, which were then subjected to curing at slightly elevated temperatures over an eleven-day period. The semi-circular specimens were prepared by cutting the cured samples into notched disks, and a series of three-point bending tests were conducted. We delved into the examination of mode I fracture toughness and its sensitivity to the percentage of metakaolin through the utilization of digital image correlation techniques. Our research showed that by increasing metakaolin from 10% to 20%, the crack tip opening displacement (CTOD) increased from 19.29 μm to 37.63 μm; however, the fracture process zone (FPZ) decreased from 2.40 mm to 1.93 mm..

1. INTRODUCTION
Mine tailings (MTs) refer to the finely ground remnants left over after the valuable metals have been extracted during mineral processing. This process generates a substantial volume of MTs in various forms, such as solid or slurry phases. In the year 2022, it was approximated that global mining activities generated over 14 billion metric tons of tailings annually (Cacciuttolo & Cano, 2022). For example, gold or porphyry copper mining activities can yield substantial volumes of MTs containing heightened levels of sulfides, which can result in many environmental challenges. These issues include the occupation of extensive land areas and the contamination of surface and groundwater with hazardous leachates comprising heavy metals and acids (Galhardi & Bonotto, 2016;
Werner et al., 2019). To mitigate and address these environmental impacts, it is crucial to implement effective containment, management, and remediation measures. Researchers have investigated various strategies to alleviate these consequences. Some methods include using MTs for backfill purposes (Behera et al., 2021), the production of lightweight aggregates (Mostafa Asadizadeh et al., 2023, 2024; Yliniemi et al., 2016), incorporating them as supplementary cementitious materials (SCMs) (Ince et al., 2021; Maruthupandian et al., 2021; Simonsen et al., 2020), integrating them into alkali-activated binder systems (Koohestani et al., 2021; N. Zhang, Hedayat, Bolaños Sosa, et al., 2023b; N. Zhang, Hedayat, Han, et al., 2022), and converting them into construction materials such as bricks (N. Zhang, Hedayat, Figueroa, et al., 2023; N. Zhang, Hedayat, Perera-Mercado, et al., 2022) and ceramics (Shabani et al., 2023). MTs with adequate aluminosilicate content can be effectively converted into environmentally friendly construction materials through the alkaline activation and geopolymerization process. Geopolymers containing aluminosilicates are significantly influenced by the silicon (Si) to aluminum (Al) ratio and consist of four distinct cell structures: sialate (Si:Al=1), sialate siloxo (Si:Al=2), sialate disiloxo (Si:Al=3), and sialate-multisiloxo (Si:Al&gt;3). The mechanical characteristics of geopolymers are impacted by the Si:Al ratio, and some natural materials like MTs lack sufficient reactive aluminosilicates (i.e., Si:Al &lt; 1) for efficient geopolymerization. Hence, additives such as fly ash (N. Zhang, Hedayat, Bolaños Sosa, et al., 2023a, 2023b; N. Zhang, Hedayat, Figueroa, et al., 2023; N. Zhang, Hedayat, Perera-Mercado, et al., 2022), slag (He et al., 2022; Huang et al., 2024; Khair et al., 2024; Qing et al., 2022; Tang et al., 2022; X. Zhang et al., 2023), and metakaolin (Borçato et al., 2023; Ferreira et al., 2022; He et al., 2022; Xie et al., 2023; Yang et al., 2022) are frequently incorporated to enhance the quantity of reactive aluminosilicates and alter the cell structure in alkali-activated geopolymers, facilitating successful geopolymerization. While there has been considerable research conducted on geopolymers derived from raw materials like fly ash or slag, there has been comparatively less emphasis on geopolymers derived from a combination of MTs, low-reactive slag, and metakaolin (Chindaprasirt & Rattanasak, 2018; N. Zhang, Hedayat, Perera-Mercado, et al., 2022). Geopolymers produced from mine tailings via alkali activation using alkaline solutions of varying molarities, spanning from 2M to 15M, have demonstrated uniaxial compressive strength (UCS) in the range of 2MPa to 50MPa (Singh et al., 2015). To ensure the suitability of geopolymer materials for engineering and construction purposes, it is crucial to consider the potential impact of static loading conditions. Throughout their lifespan, geopolymer concrete and bricks may encounter various static load conditions, leading to different types of fractures such as tensile opening (mode I), in-plane shearing (mode II), and out-of-plane shearing (mode III). For instance, geopolymer bricks in a masonry wall may be vulnerable to uneven settlements, resulting in in-plane opening. While some research has explored the fracture behavior of MTs-based geopolymers under mode-I loading conditions (N. Zhang et al., 2021; N. Zhang, Hedayat, Bolaños Sosa, et al., 2023a, 2023b; N. Zhang, Hedayat, Bolaños Sosa, Tupa, et al., 2021b), there is not comprehensive research on the effect of metakaolin content on the mode I toughness of MTs-based geopolymers. This present study aims to examine the effect of metakaolin content on the fracture characteristics of a geopolymer derived from mine tailings (MTs) and low-reactive slag using Semi Circular Bending (SCB) tests. Following the production and curing of the MTs-based geopolymer samples, SCB specimens were tested under three points bending loading to determine the critical stress intensity factor (fracture toughness) for mode I. Furthermore, Digital Image Correlation (DIC) was employed to capture the full strain distribution of the specimens during loading, facilitating accurate detection of crack opening and shear displacements, crack tip opening displacement (CTOD), and fracture process zone (FPZ) through DIC analysis.