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  • RHA is an agricultural by product


    RHA is an agricultural by-product obtained from the burning process of rice hull [7], [8]. Rice hull is the outer layer of paddy grain. About 20% of rice hull converts into RHA when it is burnt during the 5-Methyl-CTP mg generation processes [8], [9]. If RHA produced at less than 700 °C, it chemically possesses over 80% amorphous or non-crystalline silica [9]. The presence of this silica makes RHA a potential pozzolanic material. Since chemical and physical properties of RHA are similar to the Ordinary Portland Cement (OPC), RHA has the possibility to be used as SCM in concrete as a partial replacement of OPC. Arkansas supplies about 48% of the annual rice production in the U.S. Riceland Foods Inc., the world’s largest rice milling industry in Arkansas, produces about 125 millions of bushels of paddy (rice before threshing) on yearly basis [10]. This large amount of paddy yields a significant quantity of RHA. Generally, RHA is treated as waste material and is stored in nearby premises of the plant. It cannot be naturally degraded due to having silica content. As a result, RHA has emerged as a burden to the local farmers (owner of Riceland Foods Inc.) as they often run out of temporary storage spaces, which create landfill problems. The incorporation of RHA as a replacement of cement can solve the local landfilling problem and can reduce the CO2 footprint associated with the cement production [11], [12], [13]. Previous studies, mostly conducted in overseas, reported that the incorporation of RHA made durable concrete [8], [9]. RHA-modified concrete exhibited improved strength properties of concrete [6], [8], [14], [15]. Finer RHA showed better concrete strength than the coarser RHA [6], [16]. Further, ground RHA-modified concrete showed improved properties than the unground RHA-modified concrete [5]. However, RHA produced by Riceland Foods Inc. is distinct in nature, and it was never investigated for concrete production in the past other than the current study. The generation of cement gel is mainly responsible for strength development in concrete. This cement gel produced from the combination of calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H). Typically, OPC consists of tricalcium silicate (C3S) and dicalcium silicate (C2S). When these C3S and C2S come in contact with water, a C-S-H gel along with calcium hydroxide (CH) is formed as shown in Eqs. (1), (2). In addition, C-A-H is formed from the reaction between alumina (Al2O3) and water, as illustrated in Eq. (3). If an additional pozzolanic material (SiO2) such as RHA is used with cement, it will react with the available CH produced in C-S-H in the presence of water and produce a more C-S-H gel, as shown in Eq. (4) [8]. The available CH is also found after strength development process of cement paste [17]. Alkali-silica reaction (ASR) is one the common causes of concrete distress. ASR is basically a chemical reaction between alkali hydroxide and silica occurred in the presence of moisture. An ASR gel is formed and causes volumetric expansion of concrete resulting in cracking and spalling of concrete structures. Illustrations of ASR are shown in Eqs. (5), (6). During the pozzolanic reaction of SCMs, the anion of C-S-H is formed with a low Ca/SiO2. This C-S-H absorbs the alkali cations (Na+ and K+) and reduces the abundance of alkali compounds in concrete. In this way, the incorporation of SCMs reduces the ASR expansion in concrete [16]. In Arkansas, premature concrete distresses were also found in pavement and barriers due to ASR [18]. Literature showed that 12–15% partial replacement of cement by RHA can be able to mitigate ASR expansion in concrete [8].
    Materials and methodology
    Results and discussions
    Conflict of interest statement
    Acknowledgements The authors acknowledge the Transportation Consortium of South-Central States (Tran-SET) and the Office of the Provost of Arkansas State University for their financial support for this study. The authors are also thankful to Agrilectric, Riceland Foods, Inc., and Nettleton Concrete for providing the test materials for this research.