**1. Introduction**

Increased greenhouse gas (GHG) discharges have caused the melting of the Antarctic and Arctic polar ice caps. This has resulted in significant environmental problems on Earth [1]. The manufacture and transportation of building materials, as well as the installation and construction of structures, require considerable energy and produce significant volumes of GHG. In the European Union's member states, buildings use around 50% of the total energy consumption and contribute to almost 50% of the CO2 emissions in the environment over their life cycle, which includes construction, operation, and destruction [2,3]. The building sector is still experiencing an increase in demand for concrete [4–9]. Ordinary Portland cement (OPC) is a critical component of concrete that contributes considerably to GHG emissions [10–13]. OPC production causes around 5–8% of worldwide CO2 emissions [14–17]. Annual cement usage is over 4000 million tons and is predicted to reach approximately 6000 million tons by 2060 [18]. These GHG emissions have been a significant contributor to climate change [19–21]. In recent years, there has been a rise in the figure of thorough studies on the many triggers of climate change (natural and man-made), their effects on living conditions, and possible adaptation and mitigation techniques [22–27]. Blended cement manufacturing demands the use of a number of different cementitious components because of the higher energy and emission issues associated with OPC production [28]. Industrial waste utilization as supplementary cementitious materials (SCMs) is one of the methods that might cause a significant reduction in the usage of OPC, while also eliminating the risks connected with the disposal of waste materials from varied sectors [29–33]. Therefore, the most efficient technique for reducing the carbon footprint of

**Citation:** Amin, M.N.; Ahmad, W.; Khan, K.; Sayed, M.M. Mapping Research Knowledge on Rice Husk Ash Application in Concrete: A Scientometric Review. *Materials* **2022**, *15*, 3431. https://doi.org/10.3390/ ma15103431

Academic Editor: Dumitru Doru Burduhos Nergis

Received: 6 April 2022 Accepted: 4 May 2022 Published: 10 May 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/).

the construction industry is to replace OPC with suitable alternative SCMs [34–37]. There are several binders that might be utilized in concrete to decrease GHG emissions from the concrete industry [38,39]. Utilizing recycled/waste materials in concrete is a viable method of mitigating the impact of environmental challenges [40]. This not only meets the increasing need for concrete, but also significantly reduces the direct danger to society [41]. Numerous researchers in the building sector have focused on the utilization of waste resources, particularly SCMs [42,43]. The production of environmentally friendly concrete has been critical to decreasing GHG emissions [44]. Agriculture wastes such as rick husk ash (RHA), sugarcane bagasse ash, olive oil ash, etc., as well as industrial wastes, are being utilized to partially replace OPC in the manufacture of sustainable concrete [45–49]. By polluting air and water systems, dumping these waste materials in the open ground creates a major environmental threat [50]. Globally, rice husk is produced by nearly 110 million tons and RHA by 22 million tons [51]. Rice husk is effectively and extensively used as a fuel in numerous nations for rice paddy milling operations and electricity production facilities [52]. This procedure results in the formation of a pozzolanic substance known as RHA, which contains more than 75% silica by weight (after incineration, 20% of the rice husk remains in the form of RHA) [23]. The ash formed by this operation is often dumped into water flows, contaminating the water and causing ecological damage [53]. Utilizing waste materials in concrete might enhance the durability and strength of the material owing to the pozzolanic effect [54]. This decreases industrial demand for OPC, lowering the expense of producing concrete and mitigating the negative impacts of CO2 discharges during the OPC production process [28]. Given RHA's advantageous characteristics as an SCM, its use is not limited to cementitious concrete, but may also include geopolymer concrete, self-compacting concrete, fiber-reinforced concrete, pavement blocks, bricks, and high-performance nanocomposites [55–61].

The key properties of SCMs are their compatibility with aggregates (similar to OPC) and their better pozzolanic nature [62,63]. The application of RHA in concrete has sparked tremendous interest in the usage of sustainable and environmentally friendly SCM [64–67]. RHA has amorphous nature, high surface area, and compatibility with OPC-concrete, which results in outstanding pozzolanic capabilities [55,68–70]. Each kilogram of rice milled yields 0.28 kg of rice husk [71]. As a result, an enormous quantity of waste is generated annually. These rice husks are utilized as a fuel source in a variety of sectors to generate heat energy, including incineration and combustion units [72–74]. After the complete burning of rice husk, around 20–25% RHA by weight is formed [56]. A very small quantity of the RHA is subsequently employed as a field fertilizer, and sadly, most of it is thrown in open landfills [73,75]. RHA includes amorphous silica and calcium oxide and so may be utilized efficiently as an SCM in concrete [76–78]. Utilizing RHA in concrete results in better durability and strength, reduced material expenditures owing to OPC savings, and ecological advantages associated with waste material disposal [48]. RHA has been employed in recent studies as a partial replacement for OPC as well as fine aggregate in concrete mixes [79–81]. The properties of RHA concrete vary by the amount of OPC or fine aggregate replaced, the RHA grain size, the chemical characteristics of RHA independent of the water-cement ratio, and aggregate size/shape in the matrix [82]. However, for optimum strength growth, it is advised that around 10–25% of OPC be replaced [55,56]. The use of RHA in concrete has a number of benefits, as depicted in Figure 1. RHA has been researched for its possible use in cement-based composites as SCM or fine aggregate replacement. Also, natural aggregate extraction uses substantial energy and leads to increased CO2 discharges [83]. As a result, issues about the manufacturing and use of OPC may be reduced, while natural resources can be conserved. Thus, including RHA into cementitious materials reduces the demand for OPC and fine aggregate and results in an ecologically beneficial building material. Furthermore, waste management issues can be alleviated by the use of RHA in construction materials.

**Figure 1.** Benefits of RHA concrete.

As research on RHA concrete develops in response to the expanding environmental concerns, scientists face information constraints that may stymie creative investigation and scholarly collaboration. As a result, it is vital to create and apply a method that enables researchers to obtain critical information from the most reliable sources feasible. A scientometric method may assist in overcoming this shortcoming via the software application. The intention of this work is to conduct a scientometric analysis of bibliographic records published on RHA concrete up to 2021. Using a proper software tool, a scientometric analysis may undertake a quantitative examination of massive bibliometric data. Conventional review studies are weak in their ability to connect diverse sections of the literature in a complete and accurate manner. Science mapping, co-occurrence, and co-citation are a few of the most demanding parts of modern-day exploration [84–86]. The scientometric analysis identifies sources with the most articles, keyword co-occurrence, the most prolific authors in terms of papers and citations, and areas actively engaged in RHA concrete research. The Scopus database was utilized to extract bibliometric data for 917 relevant articles, which were then evaluated using the VOSviewer program. As a consequence of the statistical and graphical depiction of authors and countries, this study will aid academics in forming joint ventures and exchanging novel ideas and methods.
