**1. Introduction**

Traditional Portland cement (OPC) is regarded as the most widely used construction material in the world for the production of mortars. Large amounts of energy derived from the combustion of fossil fuels are used in the production of OPC, resulting in the emission of greenhouse gases, such as carbon dioxide (CO2). According to earlier research, 1.5 metric tons of raw materials are required to produce one metric ton of cement, resulting in the emission of 0.8 metric tons of CO2 into the environment [1].

Many studies have been conducted in an effort to reduce the OPC contents in concrete mixtures by partially or entirely substituting the OPC with a mineral addition or industrial

**Citation:** Yazid, M.H.; Faris, M.A.; Abdullah, M.M.A.B.; Ibrahim, M.S.I.; Razak, R.A.; Burduhos Nergis, D.D.; Burduhos Nergis, D.P.; Benjeddou, O.; Nguyen, K.-S. Mechanical Properties of Fly Ash-Based Geopolymer Concrete Incorporation Nylon66 Fiber. *Materials* **2022**, *15*, 9050. https://doi.org/10.3390/ma15249050

Academic Editor: Claudio Ferone

Received: 6 October 2022 Accepted: 13 December 2022 Published: 18 December 2022

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**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/).

by-product such as fly ash, slag, or silica fume in order to reduce CO2 emissions [2]. Due to the substitution of aluminosilicate materials, geopolymers have been introduced as alternatives to OPC in the construction field. Geopolymers can be made from any raw materials that have a high silica (SiO2) and alumina (Al2O3) composition as their main constituents, can react with a concentrated alkaline solution, and have thermal energy for curing to speed up the reactions [3].

One of the commonly used aluminosilicate materials is fly ash. Fly ash is a by-product that is produced from burning anthracite or bituminous coal. Fly ash is widely available over the world, possesses pozzolanic properties, and is high in alumina and silicate, but its application has been limited so far. Despite the fact that coal-burning power plants are environmentally harmful, the amount of energy produced by them is increasing due to the huge global supply of high-quality coal and the low cost of energy produced from these sources [4–6].

Due to the material's high compressive strength and low tensile strength, geopolymer has been shown to possess mechanical properties similar to those of hardened cement (brittle). Fiber reinforced concrete (FRC), also known as conventional concrete, is made by randomly adding tiny fibers to the concrete mixture to increase its brittleness. If a crack develops in plain concrete while it is being loaded, it spreads quickly and results in a loss of load carrying capacity. In contrast, with FRC, the break is intercepted by the fibers scattered throughout the matrix, causing it to slow down and even come to a stop. This process, known as the "crack bridging effect", increases the toughness of the concrete and maintains its capability of supporting a load even after the first crack appears [2].

The type of fiber, fiber content, bonding strength between the fiber and matrix, and mechanical properties of the fiber are significant in improving the mechanical qualities of geopolymer concrete (GPC). Steel fiber (SF) and polypropylene fiber (PF) are the two most common forms of fiber [7]. Metallic fibers frequently enhance flexural strength due to their higher stiffness, whereas non-metallic fibers regulate plastic matrix shrinkage due to their larger aspect ratio and surface contact area [8]. PF is believed to improve the performance of concrete owing to its high impact resistance, greater strain to failure, fine crack-free finish, increased water permeability resistance, and subsequently improved durability.

On the other hand, fibers from a variety of materials, including metal-based fibers such as steel and stainless-steel alloys; carbon-based fibers such as PAN rayon and mesophase pitch; synthetic fibers such as polyvinyl alcohol, polypropylene, and polyethylene; natural fibers such as jute, sisal, bamboo, and coconut; and inorganic fibers such as silica and basalt, are frequently used in composite materials [9].

The high strength, high modulus fibers, such as steel, glass, asbestos, carbon, and etc., are primarily used to acquire superior strain hardening after peak load, fracture toughness, and resistance to fatigue/thermal shocks, whereas the low modulus, high elongation fibers, such as nylon, polypropylene, PET (Polyethylene terephthalate), polyester, and shredded tire wastes, are potentially used in, but not limited to, enhancement of energy absorbed [10].

The most common steel and polypropylene fibers employed in nylon fiber research were quite few. A synthetic substance is nylon. Nylon is a smooth, thermoplastic substance that may be melted and processed into a variety of "films, fibers, or forms". Nylon fiber was chosen because it has excellent hardness, resilience, and durability qualities. It is also easily available in a wide range of colors, can be dyed, is resistant to soil and filth, has high abrasion capabilities, and can be cut into various cross sections. Nylon is resistant to a range of materials, hydrophilic, heat stable, and generally inactive. After the first fracture, nylon is most effective in increasing concrete's load-bearing capacity, flexural toughness, and impact resistance [11].

The prospect of adding fibers as reinforcement to a geopolymer matrix is therefore the subject of investigation. These materials' flexural strength and fracture toughness should both be strengthened by the addition of fibers, as well as the energy that the geopolymer can take before suffering damage. Strengthening geopolymers with short fibers is particularly effective because of how easily they can be dispersed. A fracture becomes more ductile

and less brittle as fibers are added. The material's cracks are less numerous, and they are smaller in size, with a maximum crack width. This is especially true of microcracks, which are less likely to spread [12].

The main objective of this study is to investigate the strength of the fly ash geopolymer concrete reinforced fibers at two different curing times, which are 28 days and 90 days. The nylon66 fiber has been introduced in this study due to its good properties. This study also focuses on the effect of interlocking plastic bead (diamond end shape) toward the geopolymer concrete strength properties.
