Applications of rice husk ash as green and sustainable biomass

Journal of Cleaner Production 237 (2019) 117851

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Review

Applications of rice husk ash as green and sustainable biomass Hossein Moayedi a, b, *, Babak Aghel c, Mu'azu Mohammed Abdullahi d, Hoang Nguyen e, Ahmad Safuan A Rashid f a

Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Viet Nam Faculty of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Viet Nam c Department of Chemical Engineering, Faculty of Energy, Kermanshah University of Technology, Kermanshah, Iran d Civil Engineering Department, University of Hafr Al-Batin, Al-Jamiah, 39524, Hafr Al-Batin, Eastern Province, Saudi Arabia e Institute of Research and Development, Duy Tan University, Da Nang 550000, Viet Nam f Centre of Tropical Geoengineering (Geotropik), School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310, Johor Bahru, Johor, Malaysia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 March 2019 Received in revised form 28 July 2019 Accepted 30 July 2019 Available online 31 July 2019

Research on agricultural wastes management, as a natural resource material, is state of the art and desirable subject in many engineering subcategories. The benefits include ease of access and implementation, affordability and environmental friendliness. Optimal use of agricultural wastes has always been a concern for humans, and the utilization of them for various purposes is an efficient way of environmental management. Among bio-waste ashes, some of them such as rice husk ash has a high pozzolanic content that stems from their abundant silica concentration. The significantly low reaction time and the high utilization of these materials, when compared to traditional mechanical methods, has generated interest from researchers. The present paper surveys the experimental studies of biomass waste ash as a pozzolanic additive for engineering applications. This paper initially provides some essential background information includes agricultural waste ash preparation procedures and its composition, then reviews the various physical and chemical pretreatment methods. Finally, the paper explores the potential application of rice husk ash as green and sustainable material in various industries. © 2019 Elsevier Ltd. All rights reserved.

^ as de Handling Editor: Cecilia Maria Villas Bo Almeida Keywords: Agricultural wastes Rice husk ash Biodiesel Soil Concrete

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Background of environmentally friendly applications of biomass agro waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Agro waste ash production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2. RHA applications in construction engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. RHA applications in renewable energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1. Introduction Newly, the use of agricultural wastes has become widespread.

* Corresponding author. Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Viet Nam. E-mail address: [email protected] (H. Moayedi). https://doi.org/10.1016/j.jclepro.2019.117851 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

About 9% of global energy production is produced by biomass used as fuel or transformed into solid fuel (OECD, 2011). In fact, it is very difficult to estimate the amount of agricultural waste, and because of the low price of these wastes, their economic values are less than the cost of gathering, transportation, and processing for advantageous use (Gil-Carrera et al., 2019). Almost one thousand million tons of these waste is produced each year and this number is increasing (Mymrin et al., 2018). 80% of the total solid waste on any

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Nomenclature AC BRHA CaO CSH CBR FAO MDD OMC PA RHA SiO2 SiC UCS USCS VOCs WRHA

Activated carbon Black rice husk ash Calcium oxide Calcium silicate hydrate California bearing ratio Food and agricultural organization of the united nations Maximum dry density Optimum moisture content Pond ash Rice husk ash Silica Silicon carbide Unconfined compressive strength Unified Soil Classification System Volatile organic compounds White rice husk ash

farm is composed of organic wastes. Agricultural wastes are comprised of toxic agricultural waste (herbicides, insecticides, and pesticides, etc.), drops and culls from fruits and vegetables, crop wastes, food processing waste, animal waste and hazardous (Obi et al., 2016). Although, these wastes are directly used as fertilizer, however, the use of agricultural waste as ash has a long history in construction. For example, “Sarouj” is a kind of binder was used in building construction many years ago. This binder has created in the Middle East and has been used to protect ice pits, earth buildings, bridge piers. It is a durable binding material made of lime and clayey soil and other additives like ash to make a hard mix, then kneaded for two days (Khaloo et al., 2019; Mousavi Haji, 2017). Many Ancient Roman structures were built with high amounts of volcanic ash in the binder mixture. These cases indicate that the optimal use of waste has always been a concern for humans, and the utilization of bio-waste materials for various purposes is an efficient way of environmental management. Although this material has been used throughout history, in many cases it has been used only on the basis of experience and not on the basis of science. However, with the advent of technology, scientists have realized that many factors affect the quality and characteristics of wastes (Calleja-Cervantes et al., 2015). This paper reviews the preparation of rice husk ash used in binder materials and the utilization of them for different applications. One of the main applications of agro-waste ashes is as an additive in concrete or soil stabilizations as pozzolan material. For example, poor soils possess a low bearing capacity and experience excess settlement, which causes many problems for designing and implementing civil engineering projects. Therefore, the mechanical properties of poor soils must be improved to a desirable level using suitable techniques (Kumar and Gupta, 2016). In recent years, other applications of ashes are the utilization of agricultural waste for renewable energy generation (Manique et al., 2012; Pode, 2016). 2. Background of environmentally friendly applications of biomass agro waste So far, much research has been done on the use of biomass ash in engineering practice, and some of these studies are used in different industries (Dias et al., 2018; Loy et al., 2018). Organic matter, such as plants, absorb minerals and silica from the soil,

while non-organic materials, especially silica, are found in many kinds of plants like rice, wheat, and sunflower. For example, rice husk changes from 5% to 30% silica by weight (de Sensale and Viacava, 2018; Medina et al., 2018). As mentioned before, this husk is thrown away either by disposing or burning in a boiler for processing paddy. This kind of waste material, because its ash composition mainly includes high pozzolanic content like silica (SiO2) can be used as a replacement for traditional stabilizers like cement and quicklime. Moreover, because the total weight of the ashes is lower than the soil particles, the total weight of the blend decreases, and a lightweight combination is formed, which is preferable in some civil engineering applications such as the construction of retaining wall. The use of local soils stabilized by available waste materials for civil engineering applications can make the local construction industry more sustainable. In the past, the use of cement, bitumen, limestone and other additives were commonly used for soil improvement (Baghini et al., 2013). Cement and lime-based binders are the most popular for soil stabilization via chemical reactions. Lime-based binders need water and a pozzolanic material, while, cement-based binders only need water for the reaction. However, the results are similar, based on calcium and siliceous compositions (Wong et al., 2008). In recent years, however, civil engineers have used various methods to stabilize poor soils, which include the replacement of weak soils, the improvement of mechanical properties through preloading, compaction and other static and dynamic techniques and/or stabilization with chemicals such as lime, cement, bitumen, rice husk ash (RHA), fly ash and fibres to enhance the mechanical properties (such as shear strength, bearing capacity, etc.) of soils. Although the mechanical properties of soils have been improved using various chemicals, such as oil and polymeric materials however the costs and environmental issues have limited their usage. Therefore, the use of natural materials, such as ashes from agricultural waste, is attractive for researchers due to their natural abundance and their reduced environmental impact (Bahrami et al., 2016; Carneiro and Gomes, 2019). Among biomass ash types, some such as rice husk ash has a high pozzolanic content that stems from an abundant silica concentration. The significantly low reaction time and high utilization of these materials, when compared to traditional mechanical methods, has generated interest from researchers. In the following, the preparation of ash was explained, and then the applications of biomass were reviewed. 2.1. Agro waste ash production Agricultural wastes (i.e., known as Agrowaste) or any other byproduct with low inorganics and high carbon content can be used as a precursor for producing biomass ash. Many agricultural wastes such as olive stones, bagasse, straw, cotton stalks, grape seeds (Mo et al., 2016; Vassilev et al., 2017; Yahya et al., 2015), almond, nut, hazelnut, pecan and sunflower shells (Ahmedna et al., 2000; Aygün et al., 2003; Haykiri-Acma et al., 2006; Marcilla et al., 2000), corn, oat and rice hulls (Ahmedna et al., 2000; Fan et al., 2004; Tsai et al., 1997), apricot, peach and cherry stones (Savova et al., 2001; Tsai et al., 1997) and many more, are used to produce useful materials. Also, every agricultural waste has its own composition, and according to that, many applications. Table 1 shows the composition of the most commonly used agro waste ashes investigated by researchers so far. It should be noted that the final product would vary according to the type of burning. In other words, Pyrolysis is a thermo-chemical process in which some carbonaceous substances (such as renewable organic matter like trees, plants, animal waste, industrial waste, recycled paper, etc., that are called biomass) are transformed into gas, char and oil in the presence of a gasifying agent, typically steam, carbon dioxide,

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Table 1 The composition of the most commonly used agro-waste ashes. Compositions

SiO2 (%)

Al2O3 (%)

Fe2O3 (%)

CaO (%)

MgO(%)

K2O (%)

P2O5 (%)

Na2O

Ash content (%)

Ref

Almond shell Coconut shell Rice husk Hazelnut shell Olive husk Groundnut shell Rice Hulls Sunflower seed Husk Black rice husk ash (BRHA) White rice husk ash (WRHA) Sunflower husk Walnut shell Walnut blows Cow cattle manure Sheep manure Chicken manure Bagasse Sugarcane Bagasse Oil palm empty fruit bunch Paper Apple pulp Wheat straw Wheat straw (Danish) Wheat straw wood, pyrenean oak residues pellets Eucalyptus wood Manzanita wood Olive tree prunings Wilow wood Coconut trunk Salix wood

10.7 69.3 89.39 0.98 29.4 27.7 91.42 3.65 96.2 54.1 17.8 9.9 5.18 53.5 29.3 15.92 72.29 45.88 49.10 28.1 21 36.6 38 52 2.5 17.83 5.97 57 8.08 42.66 22.7

2.7 8.8 0.22 10.42 8.4 8.3 0.78 0.62 trace trace 14.5 2.4 1.82 7.8 3.08 1.45 7.99 20.55 0.46 52.56 8.5 0.8 0.4 0.6 11.14 7.87 NA 1.4 1.39 13.94 0.6

2.8 6.4 0.4 1.34 6.3 10.3 0.14 15.7 0.05 0.03 6.4 1.5 0.85 1.7 1.95 3.34 6.16 15.45 1.28 0.81 2.2 0.6 0.2 1.1 1.25 NA 2.86 1.4 0.84 8.28 0.4

10.5 2.5 1.3 26.61 14.5 24.8 3.21 21 0.13 0.06 14.6 16.6 22.32 13.9 12.8 27.26 4.16 4.31 6.53 7.49 11 10.2 14 9.2 15.76 26.52 24.49 12 45.62 11.74 38.5

5.2 1.6 0.57 6.22 4.2 5.4 0.01 13.36 0.36 0.16 8.5 13.4 11.58 3.7 5.74 9.11 2.34 3.22 NA 2.36 4.1 2.5 2.6 1.8 11.96 7.25 4.94 1.1 1.16 5.37 3.6

48.7 8.8 5.04 40.34 4.3 8.5 3.71 41.21 1.62 1.11 21.1 32.9 28 6.4 23.4 23.48 4.49 1.67 12.8 0.16 24 36.7 22 21.9 14.46 7.2 10.96 2.7 13.2 10.41 15.2

4.5 1.6 0.87 5.14 2.5 3.7 0.43 NA Trace Trace 9.4 6.2 8.3 3 9.21 12.09 0.93 0.89 1.12 0.2 11 NA 4 3.2 NA 29.11 8.2 1.1 10.04 3.55 9.3

1.6 4.8 0.35 0 26.2 0.8 0.21 0.96 0.09 0.06 0.1 1 0.74 2 4.64 0.66 0.95 0.96 1.25 0.53 1.1 2.3 0.3 0.3 0.65 4.98 2.85 0.07 2.47 2.05 0.8

3.3 3.1 20.6 3.8 3.3 3.1 20.26 3.83 NA NA 1.9 2.8 2.36 15.87 20.9 17.55 7.7 12.38 7.54 8.33 2.8 5.2 3.3 7.79 3.71 0.53 NA 13.3 0.95 11.5 1.9

Demirbas¸ (2002) Werther et al. (2000) Haykiri-Acma et al. (2010) Haykiri-Acma et al. (2010) Demirbas¸ (2002) Werther et al. (2000) Miles et al. (1995) (Radovanovic et al., 2000) Fuad et al. (1993) Fuad et al. (1993) Werther et al. (2000) Demirbas¸ (2002) Miles et al. (1995) Sweeten et al. (1986) Miller and Miller (2007) (Anton et al., 2000) Gabra et al. (2001) Turn et al. (2006) Omar et al. (2011) Miles et al. (1995) rez-Garcıa et al. (2002) Sua Storm et al. (2000) Van der Drift et al. (1999) Jiang et al. (2004) Miranda et al. (2010) Stuart (1998) Osman and Goss (1983) rez-Garcıa et al. (2002) Sua Miles et al. (1995) Miles et al. (1995) Hallgren et al. (1997)

oxygen, nitrogen, air or a combination of these (Thao, 2003). However, ashes with pozzolanic properties have different production processes. When RHA burns in an open fire, the top layer of the RHA mound is exposed to open burning in atmosphere and produces black carbonized ash (called Black Rice Husk Ash (BRHA)), and the inner layer of the mound is exposed to a higher temperature leads to the oxidation of the carbonized ash to produce white ash that composes of silica (called White Rice Husk Ash (WRHA)) (Fuad et al., 1993). Fig. 1 shows the various utilizations of the rice husk ash. Ashes, biochar, and active carbons that are made using pyrolysis or similar processes are influenced by the temperature, heating rate, and particle size. It is obvious that the process omits moisture content and the volatile matter of the biomass, and the produced material presents different features than the precursor materials. These changes are mostly in the physicochemical properties such as the composition, ash content, elemental analysis, surface area, porosity, and pore structure (macropores, mesoporous, and micropores) (Tsai et al., 2001). Although these properties can change the reactivity and applicability of the ashes, they can be used as Active Carbons, that are useful as a catalyst, as pozzolanic materials for soil stabilization, as an adsorbent for wastewater treatment (ElHendawy et al., 2001), as an adsorbent for air pollution, concrete production, and many other applications. High temperatures often produce smaller particles, and they aim to crystallize inorganic ingredients, therefore reducing particle reactivity (Chandrasekhar et al., 2006; Cordeiro et al., 2009). In fact, the activation process increases the particle surface area, and physical activation of ashes may have an advantageous effect on reactivity (Cordeiro et al., 2008; Kumar and Kumar, 2011). Some researchers have pointed out that as the temperature increases, the yield of solids decreases and the yield of gases increases (Pütün et al., 2005). Since the

Fig. 1. Overview of the rice husk ash applications.

carbon percentage is fixed increasing temperature increase ash quantity and decreases the volatile matter, therefore higher temperature yields charcoals of better quality. The decreasing char yield with higher temperature is because of the secondary decomposition of the char residue or the greater primary decomposition of biomass at higher temperatures. However, some researchers have studied the temperature effects on the preparation

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of ashes with a chemical activator-like ZnCl2 and found that there is not any relation between soaking time and the char yield, whilst the char yield decreases with temperature (Tsai et al., 1997). Generally, there are two phases for the preparation and manufacturing of ashes: the first is carbonization of material at a temperature lower than 800  C in the non-oxygen condition and the second is activation of the carbonized product by physical or chemical techniques. Physical activation consists of carbonization of raw materials followed by the activation of the produced ash at a higher temperature in the presence suitable oxidizing gases like CO2, steam or mixtures of them. This is done because of the fact that the ashes produced at a low temperature do not have acceptable properties in order to be used as pozzolanic materials. The activation gas mostly is CO2 as because it is accessible, easy to handle, clean and it simplifies control of the activation process because of the low rate reaction at temperatures near 800  C (Zhang et al., 2004). It must be noted that there is not a certain temperature, duration and even oxidizing gas prescribed for producing suitable ash. In fact, in some precursors, a longer activation duration may lead to a greater adsorption capacity of the resultant ash (Zhang et al., 2004). For example, the presence of ample oxygen during pyrolysis is an important factor necessary for decreasing the carbon content of ashes (Nair et al., 2006). However, for limit and small applications, advanced techniques and ovens are not achievable and affordable to obtain these standards. Another technique for producing ash is chemical activation in which the two above-mentioned steps are done simultaneously in one step. This means that the raw material is blended with chemical activating agents, such as oxidants and dehydrating agents. Table 2 shows the processes, advantages, and disadvantages of these two techniques for the preparation of ashes, drawing from different references. In another side, it is also important to choose the type of inexpensive material. This means that the raw material should be readily available, and the most cost-effective method should be used to prepare it. Table 3 shows the most common raw materials and preparation techniques used for producing ash as reported in different references. As is clear from the above tables, recently, much research has been done on agricultural wastes. However, in the present paper, an overview of the use of RHA as a material with the greatest application in construction engineering and renewable energy production is carried out. 2.2. RHA applications in construction engineering Due to the high amorphous silica (SiO2) content, RHA is a pozzolanic material (Das et al., 2018). Schematic process diagram of rice husk in soil stabilization shown in Fig. 2. Silica is a essential ingredient for various industrial applications, including the production of concrete and its lower energy alternatives such as geopolymers, alkali-activated materials (Bernal et al., 2012; Castaldelli et al., 2013; Detphan and Chindaprasirt, 2009), as pozzolanic material used in construction such as silicates, silicon carbide (SiC), pure silica, and as refractory materials in the production of glass. Moreover, RHA may be used as a filler in polymer composites or as an adsorbent and reinforcing agent (Moraes et al., 2014; Soltani et al., 2015). However, different applications need purification and processing operations, such as particle size segregation, grinding and acid leaching (Vayghan et al., 2013). In addition to pozzolanic materials, RHA is composed of iron(III) oxide (Fe2O3), potassium oxide (K2O), phosphorus pentoxide (P2O5), calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na2O) and carbon (C) (depending on the combustion situations).

The first time RHA was used in concrete was in the year 1924 (Stroeven et al., 1999). After that, RHA has been used in many systems such as geopolymeric and alkali-activated systems (Mehta and Siddique, 2018) that aim to reduce using ordinary binders and lower environmental effect than cement or lime (Habert et al., 2011). In India, each year about one hundred million tons of paddy is produced which generate more than four million tons of RHA (Kumar and Mittal, 2019). According to the food and agricultural organization of the united nations (FAO) report in 2016, about 533 million tons of rice were produced worldwide (FAO, 2018). From every ton of rice harvested, it is estimated that 0.2 tons of shell and 18%e22% ash are obtained, although this depends upon the weather conditions and the geographical features. It is also estimated that burning one ton of RHA produces 220 kg of ash, of ^ and Fournier, which about 94 kg is constituted by silica (Bouzoubaa 2001; Prasas et al., 2000; Prusinski and Bhattacharja, 1999). The quality of RHA depends largely upon the production method and the production conditions must be carefully considered to convert the ash into active pozzolanic materials. However, due to its lack of cementitious properties, RHA cannot be used alone for construction applications (Choobbasti et al., 2019). Therefore, it is used along with binders such as lime, cement, calcium chloride and lime sludge for construction applications like the stabilization of soil (Basha et al., 2005; Brooks, 2009; Sharma et al., 2008). In most cases, the binder type should be selected based on soil characteristics. For example, in the clayey soil, chemical improvement with lime is very efficient in comparison with cement, due to the long-lasting reactions between the pozzolanic soil and the calcium compound (Lindh, 2001; Rogers et al., 1996). Therefore, because of the abundant RHA production found worldwide, it is conveniently available for use in construction applications. Additionally, the significantly low reaction time and the high utilization of these materials, when compared to traditional mechanical methods, has generated interest from researchers. When the rice husk burns, depending on the burning temperature, the ash is produced in the form of cristobalite, quartz or tridymite. Parameters that affect the surface area of ash samples are the temperature, the duration of combustion and the treatment of the rice husk before burning (Nair et al., 2006). Some researchers believe that decreasing in the surface area is because of pore opening and crystalline growth (James and Rao, 1986a). Burning the rice husk at a long high temperature and low temperature produce crystalline structure and unburnt carbon, respectively (Saraswathy and Song, 2007). Also, time has the same effect. If it is intended for use as a substitute or admixture in concrete, an amorphous state is necessary in order to make ash with high pozzolanic activity. RHA can also reduce the number of binders used for construction applications which can lower the costs of cement and lime used in civil engineering projects, making RHA of international interest. Therefore, it seems economical to use RHA as a supplement to improve the mechanical and physical properties of soil and concrete, especially in areas with high rice production capacity. This is very important for rural areas facing a cement shortage because it significantly reduces the price of cement. Pozzolanic reactions form products like that found in the hydration of Portland cement (Little et al., 1987). As an example stable element, such as calcium silicates and calcium aluminates, that act as natural adhesive elements like cement paste and increase the strength and durability of the material. Unfortunately, pozzolanic reactions are relatively slow, multi-step reactions dependent upon the reaction time, temperature and the water content, in which temperatures below (above) 13  C to 16  C slow down (increase) the reaction rate. The slow reaction rates must be increased through additives, such as those found in RHA. High silica concentrations are found in the jagged, rectangular elements found on

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Table 2 Processes, advantages, and disadvantages of the preparation of ashes. Method of activation

Advantages

Physical activation

 Activation temperature (600e900  C) The most common activator gas is CO2 because it is:  Easy to use  Various temperature and time needed for  Accessible carbonization and activation.  The low rate of reaction  Changing temperature and time may  Facilitates control of the activation process because of the low completely change the character of  rate of reaction at temperatures near 800 C [15]. produced ash.

Chemical activation

     

Disadvantages

Commonly used raw material

Almond shells (Savova et al., 2001) Corn cob(El-Hendawy et al., 2001) Oak(Zhang et al., 2004) Corn Stover (Fan et al., 2004) Corn hulls(Zhang et al., 2004) Rice Straw (Ahmedna et al., 2000) Rice husk (Yalçın and Sevinc, 2000) Rice hulls (Ahmedna et al., 2000) Apricot stones (Savova et al., 2001) Pecan shells (Ahmedna et al., 2000) Peanut hulls (Girgis et al., 2002) Chemical activation was used in many studies for various  Environmental concerns of using chemical Rice husks (Yalçın and Sevinc, 2000) Rice straw (Oh and Park, 2002) agents. precursors. Almond shells (Aygün et al., 2003) peanut Cheap and available chemical agents such as KOH, ZnCl2,  Some chemicals may not be available hulls (Girgis et al., 2002) H3PO4, and K2CO3. Corn cob (Tsai et al., 2001) One step. Olive seeds (Stavropoulos and Zabaniotou, Activation and Carbonization carried out at relatively lower 2005) Hazelnut shells (Aygün et al., 2003) temperatures. Cassava peel (Sudaryanto et al., 2006) Chemical agents can be easily recovered (Tsai et al., 2001; Pecan shells (Ahmedna et al., 2004) Zhang and Tang, 2010). Macadamia nutshells(Ahmadpour and Do, Better porous structure part of the added chemicals (such as 1997) phosphoric acid and zinc salts) Apricot stones (Aygün et al., 2003)

the outer surface of RHA, whereas the middle and inner portions contain lower silica concentrations (Muthadhi et al., 2007). Overall, pozzolanic reactions save energy and reduce environmental pollution during the production of lime, cement, and concrete while increasing the durability of these materials. As mentioned before, there are two different stages in RHA decomposition: carbonization and decarburization. Carbonization results from the decomposition of volatile organic compounds (VOCs) in the rice husk and occurs at temperatures around 300  C (Maeda et al., 2001). Decarburization results from carbon ignition in the rice husk at higher temperatures and in the presence of oxygen. It must be noted that up to 1972, most research utilized ash derived from uncontrolled combustion (Nair et al., 2006). However, some researchers reported that burning rice husk under controlled conditions yields ash with high silica in noncrystalline form (Wen et al., 2019). Note that the melting temperature for rice husks is estimated at about 1440  C, which is also the melting temperature for silica (Trubetskaya et al., 2016). Chopra et al. (Chopra, 1981) established that for temperature up to about 700  C, the silica was in non-crystalline form and silica crystals expanded with a time of burning. The parameters affecting the production of RHA include the geographical location, ignition conditions (temperature and ignition time) (Xiong et al., 2017), heating rate, fineness and colour of the produced ash as well as the rice type and its production year (Nair et al., 2006). Depending upon the thermal range and the husk's ignition time, different types of amorphous and crystalline silica are produced (Costa and Paranhos, 2018; Mor et al., 2016). These products have different properties, and it is very important that ash must be produced with the appropriate properties for specific uses (Nehdi et al., 2003). Yogananda and coworkers (Yogananda et al., 1983) concluded that pre-grinding of RHA before mixing with the binder is necessary to achieve higher strength mortars, and the long-term strength of RHA pozzolans in some conditions presented a reduction in strength after 28 days. Clearly, the production of RHA under uncontrolled conditions is not beneficial and applicable. Ignition conditions have the greatest effect on the quality of the produced ash. The reactivity of pozzolans has an adverse

relationship with un-burnt carbon even though it has a high content of amorphous silica. James and Rao showed that isothermal burning at a minimum temperature of 400  C is needed for the complete elimination of organic content from rice husk and to release silica (James and Rao, 1986b). Nair et al. compared the properties of ashes from different field ovens. In this research three types of field ovens with maximum temperature from 500 to 600  C were investigated: annular oven (requiring 9 h for complete burning), brick oven (requiring 72 h for complete burning) and pit burning (requiring one week for complete burning) (Nair et al., 2006). The features of ashes support the priority of an annular enclosure over the other two types of field ovens. According to their experimental results, higher strength pozzolans produce from RHA samples with lower values of loss on ignition and higher specific surface area. Saraswathy and Song mentioned that adding RHA to a concrete mixture increases its corrosion resistance (Saraswathy and Song, 2007). In fact, the RHA makes a calcium silicate hydrate (CSH) gel that covers the cement particles which is less porous and highly dense. Consequently, it will prevent the cracking and corrosion of the concrete. The results showed that using RHA up to about 30%, decreases the chloride penetration, reduces permeability and enhances the corrosion resistance and strength properties. Kim investigated the effect of combining RH with gypsum in the production of drywall boards (Kim, 2009). He pointed out that at RH contents up to about 30%, the modulus of elasticity and modulus of rupture increased, though at contents more than 40% these modules decreased. Tang et al. showed that making an 8% cement mixture with propylene fibre in clay soils significantly increased the cohesion, internal friction angle, and unconfined compressive strength (UCS) (Tang et al., 2007). In other work, Alhassan studied the permeability and the bearing capacity of high-plasticity clays (CH clays) by mixing RHA and lime. In this study, a soil mixture was made by mixing 2%, 4%, 6% and 8% lime (by dry soil weight) with up to 8 ± 2% RHA (Alhassan, 2008a). The effects of RHA in the soil-lime mixture were investigated with respect to their UCS and their permeability coefficient for curing periods of 7, 14 and 28 days. Results showed that larger lime and RHA ratios yielded increased UCS values; with

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Table 3 Most common raw materials used for producing ash. Base material

Carbonization condition ( C =minute)

Activation condition ( C =minute)

Chemical or physical treatment

Particle size (mm)

Comment

Ref.

Almond shell

N.A

750/600

ZnCl2

1000e1250

-

N.A

NA

H3PO4

N.A

-

800/60 (15  C =hour) 800/60

steam

200e1000

One-step process pyrolysis/activation

400/60

850/60

physical

1500e2000

various samples (either with CO2 or N2)

Aygün et al. (2003) Ahmedna et al. (2004) Savova et al. (2001) Marcilla et al. (2000)

400e800

-

ZnCl2

1435

The impregnation ratio fluctuates 20e175%wt

500/30 & 500/120 soak time. 700/30 &700/120 soak time. 800/30 &800/120 soak time. 500-800/60 soak time. 500/120

-

ZnCl2

1440

-

KOH & K2CO3

1190e1680

Physical and chemical and activation

850/60

Steam

500e2000

Physical activation/two-steps

500/120

500/120

H3PO4

500e2000

Chemical activation

500/120

600-700/120

Pure steam

500e2000

Steam-pyrolysis/one-step scheme

Rice husk

400/60 -

600/60 600/180

steam ZnCl2/CO2

200-16 mesh () Malik (2003) N.A Different salt Solutions/CO2 participated in the activation method Yalçın and Sevinc (2000)

Rice Hulls

750/60

900/240 and 900/ CO2/N2 1200

10e20 mesh

900

ca.3 cm

Corn-cob

Rice straw 700/60 to 1000/60 (10  C/min)

KOH

Fig. 2. Flow diagram for the synthesis Rice husk ash in soil stabilization.

6% RHA and 8% lime content, the permeability coefficient was optimized, and a curing period of 21 days caused it to reach its maximum UCS value. The permeability coefficient for the cured specimens decreased with increased ash content with the minimum values occurring for a 6% RHA and 8% lime mixture with a curing period of 21 days. Brooks studied the stabilization of CH clay with coal ash and RHA (Brooks, 2009). In this work, the RHA had a 90% silica content

Tsai et al. (1997) Activation and carbonization are performed concurrent, the best Tsai et al. starting time and temperature, impregnation ratio 175 wt% (1998)

Tsai et al. (2001) El-Hendawy et al. (2001) El-Hendawy et al. (2001) El-Hendawy et al. (2001)

Ahmedna et al. (2000) Two-stage method

Oh and Park (2002)

that provided a good pozzolanic factor. He pointed out that the UCS increased from 660 to 1300 kPa and the California bearing ratio (CBR) value increased from 1.5% to 10% by adding 0e12% RHA. However, a higher RHA content decreased the UCS. The optimum value was obtained with a 12% RHA mixture. The optimum coal ash content was 25%, and larger coal ash quantities decreased the UCS. Yadu et al. showed that RHA and fly ash both decreased the dry density of soil, and the optimum values for fly ash and RHA were 12% and 9%, respectively (Yadu et al., 2011). These values increased the CBR values to 190% and 50% for fly ash and RHA, respectively, while the corresponding UCS values increased up to 200% and 80%, respectively. Sarkar et al. showed that increasing the RHA decreased the dry density of soil and increased the optimum water content. Moreover, a mixture with 10% RHA is the optimum mixture value for the maximum unconfined compression strength (Sarkar et al., 2012). Gupta and Kumar reported the findings of an experimental study on clay soil improved with cement, pond ash (PA), and RHA (Gupta and Kumar, 2017). In their study, Modified Proctor tests and CBR tests were performed. The specimens contained different amounts of admixtures, and the soil used was kaolin clay. According to the Unified Soil Classification System (USCS), the soil was CL (clay with low plasticity). The percentages of RHA used were 0%, 5%, 10%, 15%, and 20% and those of PA were 30%, 35%, 40% and 45% of the total mass of the combination with cement contents of 0%, 2% and 4%. They mentioned that the optimum percentage from the literature for RHA content is 10%e15% (Basha et al., 2005) and for PA content is 30%e40% (Kumar Bera et al., 2007). The results showed a reduction in the maximum dry density (MDD) and a concurrent rise

H. Moayedi et al. / Journal of Cleaner Production 237 (2019) 117851

in the optimum moisture content (OMC) with the addition of RHA and PA. Some other studies have also shown the same results for RHA, fly ash- and lime-stabilized clay mixtures (Jafer et al., 2018; Mir and Sridharan, 2013; Sharma et al., 2018). The higher rate of the pozzolanic reaction between soil, RHA, and PA, the higher the OMC (Anwar Hossain, 2011). The rise in the OMC in the case of stabilized soil samples is because of the higher water required for finer grains and the subsequently improved hydration. Gupta and Kumar pointed out that there is a significant decrease in the MDD and improvement in the OMC with the mixture content of RHA and PA in mixed stabilization (Gupta and Kumar, 2017). Alhassan reported that the approximately lower specific gravity rates of RHA and PA could be the cause for the decline in the MDD values (Alhassan, 2008b). Further reduction in density with increasing cement dosage is as a result of the rapid reaction of stabilizer (cement) with the soil which alters the flocculation and base exchange aggregation, and consequently, the void ratio of the mixture increases thereby decreasing the density of the whole combination (Gupta and Kumar, 2017). Recently, the application of RHA as a filler in the formulation of various polymeric materials is considered by many researchers. For example, the replacement of commercial silica by RHA has been considered (Fernandes et al., 2018). RHA acted like crystalline silica in most experiments; show that it has the ability to use as an alternative for silica with few losses of properties. These researches have emphasized the favorable performance of RHA as a substitution for other fillers (Azadi et al., 2011). However, nearly all researches have studied the applicability of RHA as thermoplastic polymer composites or a filler in rubbers, and the potential of the by-product as a reinforcing agent in thermostable epoxy polymers has not been investigated (Fernandes et al., 2018). Chaunsali et al. reported the advantageous use of RH and sugarcane bagasse-based combined biomass ashes (Chaunsali et al., 2018). In the research a cementitious binder was comprised of clay, hydrated lime and biomass ash) using 2 Mol/L NaOH solution at a liquid-to-solid mass ratio of 0.40. They concluded that between the two combined biomass ashes, the one with the higher amorphous values led to a mixture with higher strength and denser reaction product. Although this article presented many construction engineering applications for ash, there are still unknown uses that will be identified by researchers.

toxic wastewater, hard separability, difficulty in recycling after the reaction, its high corrosivity, and requiring the purification step (Kumar et al., 2018; Liu et al., 2016), the research of eco-friendly catalysts has been becoming a state-of-the-art research subject. According to these researches, many catalysts such as alkaline earth oxides or mixed oxides can be used instead of base catalysts due to recoverable, reusable, and non-corrosive (Gracia et al., 2018; Singh et al., 2017; Wang et al., 2017). Among them, calcium oxide (CaO) because of the low cost during preparation, nontoxicity, high activity, and, possibility to be extracted from waste or natural materials have attracted a lot of attention (Kouzu and Hidaka, 2012; Roschat et al., 2018). There are different biomass waste materials can be used in the production of biodiesel. Many waste shells as a catalyst or source of CaO have been used such as Meretrix shell (Nair et al., 2012), mussel shell (Hu et al., 2011), Mollusk shell (Nakatani et al., 2009; Viriya-Empikul et al., 2010), bones (Chakraborty et al., 2011; Obadiah et al., 2012), cockle shell (Boey et al., 2011), Turbonilla striatula shell (Boro et al., 2011), mud crab shell (Boey et al., 2009), eggshell (Viriya-Empikul et al., 2010), oyster shell (Jairam et al., 2012). Moreover, some researchers have considered ash-based catalyst for synthesis of biodiesel, they are cocoa pod husk ash (Ofori-Boateng and Lee, 2013), wood ash (Sharma et al., 2012), oil boiler ash (Ho et al., 2012), palm fruit ash (Yaakob et al., 2012), and RHA (Nedeljkovic et al., 2018). Biochar is made by thermal decomposition of biomass under no available oxygen at a temperature below 700  C (Liu et al., 2018). Because of its high carbon content, low cost, fine-grained, porous texture, large surface areas, the wide availability of feedstock, and good thermal stability is used as the catalyst support for the transesterification reaction (Kosti c et al., 2016; Lee et al., 2017). The pyrolysis of RHA produces biochar that has been used to catalyse transesterification and esterification reactions (Li et al., 2014, 2013). Schematic process diagram of rice husk in biodiesel production shown in Fig. 3. The first use of silica from RHA for biodiesel production was done (Chen et al., 2013). They mentioned that RHA could be used without thermal pretreatment and further drying. In order to use RHA as catalyst support for biodiesel synthesis, usually, rice husk is dried, milled and sieved. After that, it is washed with deionized water, filtered and dried at about 105  C and then calcinated at the furnace. Since many factors such as catalyst preparation and reaction conditions are involved in the activity of catalysts, typically, a series of experiments is performed by changing the effective

2.3. RHA applications in renewable energy Biodiesels are a kind of green alternative energy sources produced from natural or biological resources such as agriculture and food wastes or even some algae, whose chemical and physical properties are similar with diesel oil with no need of engine modification (de Mello et al., 2017). They have many benefits including, cheapness, biodegradability, cleanness, non-toxicity and eco-friendly (Taufiq-Yap et al., 2014; Zhao et al., 2018). Biodiesel is produced by transesterification reaction from refined vegetable or animal oils with methanol being catalysed by base (or alkalis catalysts such as KOH, and NaOH), acids, enzymes, and even supercritical phase (Dai et al., 2014; Thirumarimurugan et al., 2012). Previous researches have been shown that alkali catalytic transesterification rate because of mild conditions, homogeneous base catalysts, short reaction time, high yield, achieved at the low reaction temperature, and easy operation is better than acid catalysts, therefore, they considered as the most attractive process to date (Helwani et al., 2009). However, due to industrial pollution, inevitable production of

7

Fig. 3. Flow diagram for the synthesis biodiesel from (RHA).

8

H. Moayedi et al. / Journal of Cleaner Production 237 (2019) 117851

parameters, and an optimum preparation condition for the highest catalytic activity is recommended. Some studies have shown that the catalyst principally depended on the structure, basicity, and pretreatment temperature of the catalyst (Zhao et al., 2018). They pointed out that the catalyst 30% RHA prepared in 800  C for 4 h, had the highest catalytic activity (yield: 91.5%) so that even after eight reaction cycles, the catalyst had a high yield about 80% of biodiesel (Chen et al., 2013). However, the maximum yield could reach 93.4% under proper conditions (Zhao et al., 2018)). Using RHA as a carrier increases reusability, stability, and application compatibility, of the biocatalysts (Ulker et al., 2016). RHA also has been used as support for the immobilization of recombinant rhizopus oryzae lipase (Martin et al., 2013). Bonet-Ragel et al., showed that RHA had a similar performance of commercial ones (RelOD) (Bonet-Ragel et al., 2018). However, because of its intrinsic characteristics, it was not recovered for possible utilization in successive biotransformation. They suggested that as support of lipase in the enzymatic biodiesel production, this problem should be solved for a future application. Wang and coworkers produced biodiesel from soybean and RHA activated carbon obtained by the hydrothermal process (Wang et al., 2018). They used the polyethylene glycol and calcium oxide on the RHA activated carbon (CaO/AC) catalyst as a heterogeneous catalyst to produce biodiesel. They pointed out that, the catalyst presented very high activity for transesterification of soybean oil. A group of researchers has studied the transesterification of soybean oil with methanol and RHA catalyst that was modified by Li2CO3 at atmospheric pressure (Chen et al., 2013). The catalyst was made using 1 g RHA with 1.23 g Li2CO3 which calcinated at 900  C in air for 4 h. They concluded that the as-prepared catalyst presented remarkable catalytic activity. The mentioned studies show that RHA is an attractive catalyst for industrial application in biodiesel production. 3. Conclusions The use of agricultural wastes as a sustainable and renewable energy resource and has huge potential as low-cost precursors for the production of valuable materials has long been considered by researchers, and much research has been done. The benefit of this waste is sustainability, environmentally friendly and the diversity of agricultural wastes in the various regions of the world is very high. Therefore, in each region, there are different species of plants and animals depending on the climatic conditions; any waste can have unique chemical compounds. The review presents a review of rice husk ash and describes their characteristics, advantages and limitations and explored main industrial applications. The study revealed that although agricultural wastes have been used for many years in civil engineering operations, there are still many aspects of these wastes that must be discovered by future researchers. For example, their various applications in civil engineering and biofuel processing technology can differ with regard to the chemical compounds and preparation of agricultural wastes. According to the above review, due to the existence of various compounds in agricultural waste ash, combinations of different ashes should be considered. With regard to the purpose of producing ash, whether it is used as an absorbent in water or soil, catalyst, pozzolanic material in soil stabilization and/ or other purposes, its characteristics are different according to the burning method. It is concluded that using rice husk ash for silicon materials production are a sustainable option. However, further study on sustainable indicators and other factors like rice husk ash applications or properties and technology for production are also needed.

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