**1. Introduction**

Concrete is a heterogeneous material formulated from natural and synthetic components, combined with water, and forms a versatile, durable, and essential building material. Worldwide, concrete production has significantly increased due to urbanization and urban development, reaching 25 billion tons per year [1]. Subsequently, the yearly production

**Citation:** Khan, K.; Ishfaq, M.; Amin, M.N.; Shahzada, K.; Wahab, N.; Faraz, M.I. Evaluation of Mechanical and Microstructural Properties and Global Warming Potential of Green Concrete with Wheat Straw Ash and Silica Fume. *Materials* **2022**, *15*, 3177. https:// doi.org/10.3390/ma15093177

Academic Editor: Dumitru Doru Burduhos Nergis

Received: 4 March 2022 Accepted: 18 April 2022 Published: 27 April 2022

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of cement has also soared to 4.1 billion tons globally [2]. Besides its usefulness, cement and concrete production has resulted in massive carbon emissions as well. Apart from the carbon emissions, the usage of raw materials in cement production is also responsible for the depletion of natural resources and its associated carbon footprint [3]. Hence, the annual manufactured carbon emissions from the cement industry are more than 5% of the total global anthropogenic emissions [4]. This vast amount of CO2 generation is also a leading factor in creating the issues related to global warming and climate change.

These issues can be controlled by following the sustainability approach in the construction sector. One such approach is to replace cement with supplementary cementitious materials (SCMs) at the construction site or during cement manufacturing. Another approach is to conserve natural resources by replacing aggregates with alternate ecofriendly materials. Hence, the search for supplementary cementitious materials has resulted in the usage of industrial byproducts such as volcanic ash [5,6], silica fume (SF) [7,8], fly ash [9,10], electric arc furnace slag [11,12], tire ash [13,14], copper slag [15,16], etc., and agricultural byproducts such as sugarcane bagasse ash [17,18], rice husk ash [19,20], woodwaste ash [21,22], wheat straw ash (WSA) [23,24], and others. The utilization of these agroindustrial wastes has been proved to be promising both as supplementary cementitious material to replace cement and/or fine aggregates [24]. Furthermore, the usage of these byproducts offers many advantages such as improved mechanical and durability properties, reduction in waste generation, reduction in cost by replacing cement/aggregates, and most importantly, the decrease in carbon emissions [25,26]. Therefore, Luhar et al. [4] reviewed the usage of agriculture waste in concrete. They concluded that agrowaste could be used as an effective supplementary cementitious material in concrete, resulting in the development of green concrete. Rattanachu et al. [27] observed that 20% replacement of OPC with finely ground rice husk ash can significantly improve the compressive strength of concrete. In addition, the utilization of SCMs also improves the durability of concrete [2]. In another study, Rashad [3] replaced fine aggregate with metakaolin at 10%, 20%, 30%, 40%, and 50% by weight. The experimental study revealed increased splitting tensile strength, compressive strength, and abrasion resistance of metakaolin mixed concrete at 40% replacement.

In developing countries, agrowaste is one of the significant issues which arise from different food crops such as sugarcane, rice, and wheat. Wheat is one of the most important cereal crops and a major food source for around 2.5 billion people globally [28]. The worldwide wheat production was estimated to be 750 million tons from 2016 to 2017 [29]. Pakistan ranked high in the wheat-producing countries around the globe. In the year 2017–2018, Pakistan produced about 26.6 million tons of wheat. In the Gulf region, the Kingdom of Saudi Arabia is known as a major wheat-producing country. The annual wheat production of the Kingdom of Saudi Arabia is estimated at 700,000 tons [30]. Pan and Sano [31] stated that one kg of wheat grain yields around 1.3 kg wheat straw. Primarily, the wheat straw was utilized to feed cattle. However, open field burning is also practiced in some cases, which causes air pollution (smog) and health issues such as respiratory diseases in that region.

Burned wheat straw produces ash, which is pozzolanic in nature. However, the pozzolanic efficiency of the WSA mainly depends on its source; therefore, its composition varies with regions primarily due to soil properties and climatic conditions [32]. In addition, the burning temperature, exposure time, and particle sizes also play a vital role in its pozzolanic behavior. Biricik et al. [33] reported that wheat straw burned for 5 h at 570 and 670 ◦C resulted in high-quality WSA. However, WSA produced at a heating temperature of 670 ◦C showed superior pozzolanic properties. Similarly, an increase in silica content with an increase in burning temperature was also recorded by Amin et al. [29]. Memon et al. [34] investigated the effects of various burning temperatures (500, 600, 700, and 800 ◦C) on the pozzolanic efficacy of WSA. They observed the transition of amorphous silica into crystalline form with an increasing temperature beyond 600 ◦C. Therefore, several

researchers observed favorable temperatures, which ranged between 570 and 670 ◦C for an improved WSA production.

Research studies have highlighted the efficacy of the WSA both as filling and pozzolanic material in concrete and mortar. The addition of WSA in mortar increased the compressive and flexural strength [35]. Moreover, the improvement in the mechanical properties of cement mortar was attributed to its filling ability. Furthermore, the addition of WSA in cement mortar or concrete also improves its durability. Qudoos et al. [36] noted that extensively ground WSA exhibited higher compressive strength at all ages than control specimen with 20% cement replacement. Similarly, Amin et al. [29] also observed that 15–20% cement replacement with WSA showed significant enhancement in the strength and ductility of concrete samples at 91 days. WSA as cement or sand replacement decreased the water adsorption and increased the resistance against acid and sulfate attacks. Al-Akhras [37] used WSA as cement replacement up to 15% by weight and concluded that concrete containing WSA showed better resistance to freeze and thaw damage than control specimens. In addition to its standalone performance as a pozzolanic material, the behavior of WSA was also evaluated as a binary mix with other pozzolanic materials such as metakaolin, fly ash, bentonite clay, millet husk ash, and others [3,26,32,38]. The usage of bentonite clay along with WSA was effective in consuming the free lime. Moreover, it improved the resistance of the cementitious matrix against acid attack [38]. The effectiveness of WSA and millet husk ash (MHA) combined was evaluated by Bheel et al. [39]. The results showed improvement in the flexural, tensile, and compressive strength of the specimens containing 15% MHA and 30% WSA combined.

The binary and ternary blends of various agroindustrial wastes have increased its pozzolanic activity. Among many, SF is widely used component of ternary blends to enhance mechanical as well as durability performance of concrete. Generally, SF is obtained as a byproduct during the production of silicon and ferrosilicon alloys at a very high temperature around 2000 ◦C in an electric furnace arc [40]. Oxygen is eliminated by heating highly pure quartz with coke or coal. An ultrafine powder is obtained, having high porosity and specific surface area in which the silica ranged between 85% and 95% [41]. SF is most commonly used in concrete as a dry, densified form consisting of agglomerates of size from 10 microns to several millimeters. These agglomerates may only partially disintegrate during normal concrete mixing [42]. In order to disperse SF effectively, sonification techniques [43] or the Holland method [44] of mixing SF concrete in a laboratory mixer are necessary for improving the microstructure and pore size of the materials. Murthi et al. [45] presented the effects of OPC, bagasse ash, and nanosilica on the fresh and hardened properties of high-performance concrete. The experimental investigation showed that the addition of nanosilica significantly enhanced the early age strength; however, it reduced the setting time. SF is a highly siliceous material, and its addition to the cement matrix increases the formation of calcium silicate hydrate (C-S-H) gel, resulting in a denser microstructure of the cementitious system. The cement replacement with 10% SF and 20% WSA in lightweight concrete improved its density and strength for structural applications [46]. The microstructure investigation of cementitious matrix containing a ternary blend of OPC, SF, and WSA indicated better mechanical and durability performance than the control specimen [47]. Based on previous research, it has been highlighted that the usefulness of the SCMs increases with the addition of SF. SF, being one of the most reactive siliceous materials, could accelerate the pozzolanic activity inside the cementitious system when used in combination with agro-industrial ashes. Despite several studies, it has been noted that the existing literature is limited to the use of WSA as an individual SCM with low replacement levels.

The purpose of this study is to investigate the novel use of WSA and its blend with SF as a cement substitute to produce environmentally sustainable concrete that does not compromise on its mechanical properties. Therefore, the impact of WSA and its blend with SF, as a high-volume replacement of cement, on the mechanical, durability, and microstructural characteristics of concrete were investigated. Additionally, X-ray

fluorescence (XRF) and X-ray diffraction (XRD) methods were used to measure the physical and chemical properties of cement, WSA, and SF. Concrete specimens were prepared, namely, control (100% cement), three binary with only WSA (C/WSA: 90/10, 80/20, and 70/30), and three ternary containing WSA along with SF (C/WSA/SF: 70/25/5, 60/33/7, and 50/40/10). In ternary mixes, a large amount of cement was substituted (up to 50%). Mechanical properties, such as the compressive and split tensile strengths with aging (7, 28, and 91 days), and water absorption (WA) and apparent porosity (AP) after aging for 91 days were evaluated for hardened concrete samples. Additionally, the effect of WSA and its blend with SF on the microstructure and pore structure of the cement paste matrix was studied by employing scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM–EDS), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and nitrogen (N2) adsorption isotherm analysis. At the end, the global warming potential (GWP) of all the concrete mixes was calculated in kg CO2-equivalent per unit concrete and kg CO2-equivalent per unit concrete/MPa by using the green concrete lifecycle assessment (LCA) tool.
