**1. Introduction**

It is crucial to keep industrial waste under control and to reduce its impact on the environment as much as possible. Due to rapid urbanization and a large increase in the world's population [1], a large amount of solid waste has been generated and this is increasing rapidly in Turkey. According to the Turkish Statistical Institute, up to 16 million tons of FA are being created yearly, but only two or three percent can be used efficiently in the cement and concrete industry. FA, GGBFS as a waste material of steel factories, and QP with high reserve areas in Turkey are capable of having alkali-activated binder properties in terms of concrete production technology. Therefore, the goal of this research is to determine the optimal amount of waste materials to use in the production of lightweight aggregate for concrete mixes. Some other discussions have been had in the literature regarding the recycling of waste materials into new building materials, such as municipal solid waste incineration bottom ash (MSWIBA), silica fume (SF), and rice husk ash (RHA) for use as cement replacement materials. It was found that the use of FA as

**Citation:** Ibrahim, M.A.; Atmaca, N.; Abdullah, A.A.; Atmaca, A. Mechanical Properties of Concrete Produced by Light Cement-Based Aggregates. *Sustainability* **2022**, *14*, 15991. https://doi.org/10.3390/ su142315991

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 19 October 2022 Accepted: 29 November 2022 Published: 30 November 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/).

partial replacement for cement in concrete resulted in enhanced durability of the produced concrete in addition to reducing CO2 emissions [2]. The influence of FA in the cement hydration process and its pozzolanic reaction inside concrete has been investigated by the authors in [3–5]. However, several researchers have investigated using FA and GGBFS in producing artificial LWA [6–13] such as: recycling bottom ash in artificial LWA to be used directly in concrete [14] and minimizing cumulative quantities and preserving the environment. The aggregate in concrete provides a large percentage of (often between 65 and 75 percent) and contributes significantly to the material's heavy weight; therefore, it makes sense to investigate the possibility of turning waste materials into lightweight artificial concrete. The different chemical and physical properties of waste materials directly affect the mechanical and durability of the produced concrete [15]. It was observed in some studies that the replacement of 30 percent of recycled aggregate with the natural aggregate had no significant impact on the performance of concrete when compared to natural concrete [16–18]. Therefore, the goal of this research was to determine the optimal amount of waste materials for use in the production of lightweight aggregate for concrete mixes.

Different materials and procedures may be used to produce artificial aggregate, such as cold-bonding and sintering techniques [7,19]. According to BS EN 13055, aggregates are considered lightweight aggregates if their particle densities do not exceed 2000 kg/m3. Lightweight artificial aggregate was produced via the sintering method using alkaline palm oil fuel ash (POFA) combined with silt [20]. Although sintering consumes more energy than cold bonding, the benefits of achieving the sintered aggregate properties in less time outweigh waiting 28 days for cold-bonded aggregate to cure [21].

Lightweight aggregate manufacture via cold bonding was developed in the early 2000s, and FA was used as the dry powder. In this process, a million tons of waste materials was employed to generate aggregate, and this was found to be an appropriate material for producing lightweight aggregates [22–25]. The cold-bonding process has been widely used to utilize various types of waste materials and protect the environment by controlling the leaching of pollutants [26]. The technique can thus be considered a proper treatment channel for recycling waste materials [27]. Moreover, the technique is more economical than the sintering method, but the crushing strength of aggregates usually gives lower results [10,28,29]. In addition, the technique has more advantages in terms of cost, energy consumption, and gas emissions [30].

Concrete with different compressive characteristics can be made from a wide variety of structural components around the world, simplifying construction while increasing durability and versatility. Depending on the intended function, concrete may be poured in almost any form, shape, or color. Concrete may now be found in many locations, thanks to the tremendous expansion in the building industry. However, concrete has some drawbacks including its low tensile strength and heavy weight. Many research studies have looked at ways to amplify these unfavorable traits. High-strength concrete may be enhanced with recycled cement kiln dust and fibers made from recycled polyethene [31]. Artificial aggregates are also employed to reduce the concrete's weight. For industrial purposes, a variety of light aggregates have been employed successfully. For example, bottom FA, concrete waste powder, and pulverized granulated blast-furnace slag [32] may all be used when using a cold-bonding technique. Various studies examined the mechanical behaviors of concrete produced with cold-bonded lightweight FA aggregates [10,13]. Sintered and cold-bonded artificial aggregate produced by utilizing washed sludge ash (WAS) and GGBFS was used to produce concrete with mechanical properties comparable to ordinary concrete with a lower oven-dry density [33]. This form of artificial aggregate may be utilized in various concretes with varying mechanical properties. Depending on the aggregate content, the compressive strength may be readily achieved at concrete manufacture between 20 and 80 MPa. The use of 10% FA with recycled aggregate resulted in a slight increase in the axial compressive strength of the concrete [34]. Furthermore, the addition of FA to the concrete with recycled aggregate improved the workability—due to the spherical and flat shape of pellets—in addition to the mechanical and durability properties of concrete [2].

Although lightweight concrete has a lower strength than normal-weight concrete, it has some advantages such as having a reduced dead load, being eco-friendly, being low cost, and having higher seismic and fire resistant properties [35–37]. From an environmental protection point of view, using FA artificial aggregate as a fine aggregate in concrete could reduce CO2 emissions by up to 60% compared to conventional concrete [38]. Furthermore, replacing 50% of the cement with FA as a binder could reduce greenhouse emissions by 54% [39]. In addition to the environmental impact of using an FA artificial aggregate, the cost of producing concrete can be reduced by 13–15% compared to conventional concrete [40]. Sintered artificial aggregate is widely preferred in concrete production in order to provide a higher strength. Using sintered FA aggregate in lightweight concrete production has resulted in good mechanical and durability properties [41]. Similarly, the fly ash cenosphere (FAC) features include being hollow spherical, lightweight, and airfilled [42]. It is favored in different industries due to its high workability, low conductivity and bulk density, and its thermal resistance [40]. FAC is also a fine aggregate in sustainable lightweight concrete production [43]. A combination of 50% FAC and 75% SFA is suitable for producing sustainable lightweight concrete [43]. However, a high volume of FAC and SFA (up to 75%) leads to a significant reduction in lightweight concrete strength; the strength could be enhanced by adding silica fume [44,45]. Based on these previous studies, it is observed that a high volume of FAC and SFA could be utilized in lightweight concrete production with the aid of silica fume.

There is a lack of research on the use of QP in manufacturing aggregate and its influence on the mechanical performance of the bond strength effects of lightweight aggregates between concrete and reinforcing bars in particular. Hence, this study was focused on the influence of an artificial aggregate made from FA, GGBFS, and QP powder on the properties of concrete. Additionally, the effect of lightweight aggregate on the adhesion of concrete to steel was examined. The experimental findings were compared with the outcomes of conventional concrete made using typical aggregate. Due to prior research, the optimal value of the aggregates was determined based on the water absorption, bulk density, and crushing strength. These aggregates were substituted for conventional coarse aggregate with varying ratios to assess the concrete's compressive strength, modulus of elasticity, and tensile strength, and the strength of the bond between the aggregates and the cement paste.

#### **2. Materials and Methods**

During this experimental study, concrete was cast using three different types of artificial LWAs to examine their influence on concrete's mechanical and fracture properties. Different replacement ratios (20%, 40%, and 60%) were applied with natural coarse aggregate by weight.
