The present state of the use of palm oil fuel ash (POFA) in concrete

Construction and Building Materials 175 (2018) 26–40

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

The present state of the use of palm oil fuel ash (POFA) in concrete Hussein M. Hamada a, Gul Ahmed Jokhio a,⇑, Fadzil Mat Yahaya a, Ali M. Humada b, Yasmeen Gul c a

Faculty of Civil Engineering & Earth Resources, University Malaysia Pahang, 26300 Gambang, Pahang, Malaysia Electricity Production Directorate of Salahaldeen, Ministry of Electricity, 34007 Baiji, Iraq c School of Civil and Environmental Engineering, National University of Science and Technology, Pakistan b

h i g h l i g h t s
Palm oil fuel ash (POFA), a waste by-product, can be used to partially replace cement in concrete production.
POFA is rich in SiO2, therefore, is a good pozzolanic material.
Ultrafine and Nano POFA increase the compressive strength of concrete.
POFA reduces drying shrinkage as well as workability of concrete.
The use of POFA in concrete is favourable to the environment.

a r t i c l e

i n f o

Article history: Received 26 September 2017 Received in revised form 20 March 2018 Accepted 23 March 2018

Keywords: Palm oil fuel ash Chemical and physical properties of concrete Compressive strength and durability CO2 emissions Environment friendly materials

a b s t r a c t Concrete industry consumes considerably large quantities of natural resources in addition to generating toxic gases, such as CO2, in the atmosphere. In order to achieve more sustainability in the concrete sector, research should focus on using alternative renewable resources such as palm oil waste for concrete production purpose. Palm oil fuel ash (POFA) is a by-product obtained during the burning of waste materials such as palm kernel shell, palm oil fiber, and palm oil husk; it can be utilized to partially replace cement in a concrete mix. This paper presents a review of the applications and effects of POFA on concrete properties as reported by previous studies that have been conducted to find out POFA properties and its effects under various conditions. Chemical and physical properties of the resulting concrete have been illustrated depending on the POFA characteristics in several sources. Many studies have shown that concrete containing POFA has better compressive strength, durability and other properties than concrete containing Ordinary Portland Cement (OPC) only. Other researchers have shown more advantages of POFA replacement in concrete in specific proportions, especially minimizing CO2 gas emissions and thus improving environmental conditions. Ó 2018 Elsevier Ltd. All rights reserved.

Contents 1.

2. 3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Preparation of POFA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Environmental benefits of POFA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Nano POFA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4. Nano silica with POFA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Utilization of POFA in concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Utilization of POFA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. POFA as SCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. POFA in self-compacting concrete (SCC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . POFA properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Chemical composition of POFA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (G.A. Jokhio). https://doi.org/10.1016/j.conbuildmat.2018.03.227 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

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4.2.1. Specific gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Color . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Size and shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4. Fineness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5. Absorption of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Effects of POFA on the concrete properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Effects of POFA on fresh concrete properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Workability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Heat of hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Effect of POFA on hardened concrete properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Drying shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflict of interest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Concrete industry presents a challenge to the global environment as it consumes significantly large quantities of natural resources in addition to generating toxic gases, such as CO2. In order to achieve more sustainability in the building construction sector, researchers in this field need to focus on using alternative renewable resources, such as palm oil waste. Malaysian Palm Oil Board (MPOB) in 2012 reported that the plantation area of palm oil covers about 5.07 million hectares in Malaysia [1]. The United States Department of Agriculture reported that the production of palm oil in years 2016 and 2017 was estimated to be 64.5 million metric tons [2]. Southeast Asian countries are the main palm oil producers. Palm oil fuel ash (POFA) is one of the significant materials produced as a byproduct of the palm oil industry [3,4], which is obtained by burning the waste materials such as palm oil fiber, kernels, empty fruit bunches, and shells in the power plants to generate energy [4]. POFA can be utilized to partially replace cement in concrete production [5]. The quantity of POFA being produced is increasing with time due to the increase in the production of palm oil. Leaving this waste material without any further utilization is in itself an environmental challenge. Malaysia is one of the largest exporters and producers of palm oil all over the world [6]. Production of POFA in Malaysia alone is approximately 10 Million tons/ year [7,8]. Whereas, just 104 tons/year of POFA are being produced in Thailand, which continue to increase with time [4]. Recently, there has been an increasing interest in the use of industrial and agricultural waste materials in the construction industry, especially during the concrete preparation [9]. There is an urgent need for disposal of harmful residual agricultural and industrial products which has become a threat for human life. In recent years, many studies have emerged that indicate to use the agricultural residues in the concrete industry [5,10–12]. From environmental perspective, agricultural waste materials have been investigated by many researchers and have been shown to have better properties in concrete than the cement materials, whereas the latter also generates a high amount of CO2, which is harmful for environment [9]. Due to the fact that POFA is a geopolymer, it is environmentally friendly and consumes less amount of energy than traditional materials during production [13,14]. In Malaysia, more than 1000 tons of POFA have been dumped into lagoons and landfills without exploiting the use of this material in other industries [15]. In terms of cost saving, using POFA as partial cement replacement will reduce the cost of cement production as well as transportation of the same from cement plants to the stores. Moreover, this will improve the environment by mitigating and reducing waste materials in landfills.

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POFA is also one of the ash family of materials resulting from the burning of waste materials such as palm kernel shell and palm oil husk [7]. POFA is usually disposed in landfills, which results in the increased amount of ash deposits every year and now has become a burden [16]. Therefore, it is needed to devise new ways to benefit from these waste materials and avoid the potential risks. In the 1990s, Tay [17] started studying the properties of palm oil fuel ash as a concrete material. The study was conducted by replacing Portland cement with POFA ranging between 10 and 50%. It was noted that the compressive strength of the specimens decreased when between 20 and 50% of cement was replaced by POFA. Since then, many studies have been conducted to enhance the concrete properties, for example, Awal and Hussin [15] discovered that POFA has a significant impact to prevent and reduce the sulfate attack. In 2011, Kroehong et al. [18] conducted a study to find out the effects of POFA fineness on pozzolanic reaction of cement paste. The Ground Palm Oil Fuel Ash (GPOFA) and Ground River Sand (GRS) were used to replace the Portland cement by GPOFA or GRS at 10%, 20%, 30% and 40% by weight of cementitious materials, whereas the water to binder ratio (W/B) was 0.35 for the mixes of cement pastes. It was concluded that the effects of POFA on the cement paste and concrete mixtures increase when POFA is of higher fineness. In 2015, Rajak et al. [19] conducted research to determine the morphological characteristics of hardened cement pastes which contain Nano-POFA with particle sizes ranging between 20 nm and 90 nm. It was discovered that Nano-POFA particles have a significant effect on pozzolanic reactions in the pastes because of the filling effect. In Thailand, P. Chindaprasirt et al. [18] used the POFA and Rice-husk Bark Ash (RBA) to determine the water permeability and strength of concrete replacing the cement in the concrete by various percentages ranging between 20%, 40%, and 55% by weight. It was concluded that replacing 20% of ordinary Portland cement by POFA and RBA leads to increasing the compressive strength and workability, while the value of compressive strength decreases when the replacement quantity increases up to 40% due to the increased requirement of water [16]. The above discussion leads to the objectives of the present paper to review the state of the use of POFA in the production of concrete. In this regard, the process of the preparation of ground and nano POFA has been reviewed first. This has been followed by a review of the chemical and physical properties of POFA itself. The effects of the use of POFA in concrete on the properties of fresh concrete and hardened concrete have been discussed in the latter half of the paper. Finally, at the end, a section of discussion and conclusions has been provided that includes a few directions for the future research in this field.

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Palm Kernel Shell

Empty Palm Fruit Bunches

Palm oil fiber

Fig. 1. Waste materials from palm oil tree to produce POFA [22].

1.1. Preparation of POFA Production of POFA is due to heating of significant amounts of palm oil fiber, shell, and empty palm fruit bunches as shown in Fig. 1; these wastes are employed at the palm oil mills as a main source of energy. The byproduct which constitutes about 5% of total waste weight is called POFA [20]. In the palm oil mills, the waste materials are burnt at high temperature, which reaches up to 1000 °C [19], and thus used as fuel to generate electricity [21]. In Malaysia, more than 3 million tons of POFA is produced every year [20], while in Thailand, production of POFA is more than 100,000 tons annually [5]. Instead of using electricity from familiar energy sources that causes many environmental issues and requires high cost, waste materials produced from palm oil mills can be used to generate electricity by heating up the boilers in the palm oil factories [4]. POFA constitutes about 5% of the total waste materials after burning shells and fibers to generate electricity in palm oil mills [3,17]. Out of 4 kg of raw palm oil, only 1 kg is palm oil and the rest is production residues which represent dry biomass [4,23–25]. Palm oil fronds and palm oil trunks are about 75% of the total waste; this waste is left to be recycled and used as plant fertilizer for future plant strengthening, while the remaining proportion, which is 25% and includes empty fruit bunches, mesocarp fiber, and palm kernel shells can be used to generate electricity in palm oil mills by combustion of these wastes under high temperature ranging between 800 and 1000 °C [26,27]. Preparation of POFA can be achieved in different ways depending upon the burning process and raw materials used. Noorvand et al. [20] studied initial POFA preparation by putting dried samples in an oven at the temperature of 105 ± 5 °C for 24 h. While another study by Tangchirapat et al. [5] obtained POFA through combustion of waste materials at the temperatures of up to 1000 °C, followed by using sieve (1.18 mm opening) in order to remove the large foreign particles. In a new study by Zeyad et al. [2] to prepare ultrafine POFA, three steps were adopted as shown in Fig. 2. The first step was to dry POFA to

Step 1 Wet Palm Oil Fuel Ash (W-POFA)

• Oven Dry at 105oC • Pass through number 300 sieve

Step 2

Step 3

• Grind in Ball Mill to obtain Ground POFA (G-POFA) • Treat in Gas Furnace at 500oC to obtain heat treated POFA (TPOFA)

• Grind in Ball Mill to obtain ultrafine POFA (U-POFA)

Fig. 2. Production process for ultrafine POFA [2].

remove moisture in the oven at 105 ± 5 °C, then passing dry POFA through number 300 sieve in order to remove foreign and coarse particles and also to dispose of kernels and fibers which failed to burn. The second step was grinding POFA particles to get a sufficient fineness followed by combustion in gas furnace at high temperature of up to 500 ± 50 °C to remove unburned carbon and obtain POFA with high fineness. The third step was conducting further grinding of POFA similar to the previous step to get UPOFA.

1.2. Environmental benefits of POFA In a study, Ali et al. [30] claimed that the cement industry alone consumes almost 12–15% of the energy allocated for industrial purposes [28]. Cement manufacturing process requires burning of fossil fuels, coal, petroleum coke, and fuel oils in order to maintain the temperature at 1450 °C in the oven [29]. Therefore, it generates approximately 0.97 ton of CO2 for each ton of clinker produced [30]. Around 7% of CO2 gas emissions are caused by cement production processes in factories, which increases greenhouse gases and causes environmental problems [30,31]. It was predicted in 2008 that the cement manufacturing may increase by 100% in 2020 [32]. Another study predicted the increase in the cement production due to increase in demand to reach 200% by 2050 in relation to the production rate in 2010 [33]. Besides, the energy consumed to produce 1 ton of cement is a significant amount; more than 1700 MJ/ton clinker [34,35], and about 1.5 ton of raw materials are required. Moreover, it has also been reported that the cement industry alone consumes about 5% of total energy [36]. Production of cement has been considered as the second biggest contributor of CO2 gas emissions worldwide [37]. In recent years, there has been an increased concern regarding the environmental issues and their effects on various aspects of life. The global warming is partially due to widespread solid waste materials resulting from agricultural and industrial products, which has become the most significant environmental issue worldwide [38]. According a study by Tay and Show [39], the transportation and disposal of POFA into the landfills and open fields without treatment may cause various diseases and be uncomfortable for human life. Also, the production of huge quantities of concrete due to the increased demand for the construction of new buildings will result in significant damages to the atmosphere through the generation of CO2 emissions [40]. Many researchers emphasize the need to mitigate the risk to the environment, especially resulting from the agriculture wastes, and to develop methods to use these materials in cement mortar [20,41,42]. In addition, disposal of POFA in the landfills resulting into significant environmental risks cannot be ignored. Recently, studies have been conducted replacing Portland cement by POFA rich silica as cementitious material aimed at creating sustainable construction materials [13]. In Malaysia alone, around 3 million tons of POFA was produced in 2007 as waste material [38]. These materials can cause

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damages to the environment if not utilized by other industries such as construction industry [16,43]. Therefore, Tangchirapat et al. [3] utilized POFA in their study as a pozzolanic material in concrete in order to reduce the environmental problems by consuming POFA resulting from the palm oil industry in concrete preparation, thus reducing the burden on landfills and decreasing the environmental risk by lowering CO2 emissions. Due to the increasing demand for energy resources, researchers have started searching for new renewable resources instead of the non-renewable resources such as natural gases and petrochemicals, which contribute to global warming and cause significant harm [44]. The concrete made from agricultural wastes such as POFA has been termed as Green concrete and is considered environmental friendly. In this case, the concrete production consumes less energy than the traditional concrete made of cement; it also reduces the carbon dioxide emissions in the air. Besides, it helps control the 5–8% of CO2 emissions produced by the cement industry which leads to potential environmental risks in the future [45,46]. The waste resulting from palm oil industry constitutes about 80% of materials such as palm oil shells and fruit bunches, which can cause serious environmental issues if not treated [47]. In studies by Tay, and Tay and Show [17,39], it was said that with increased demand of the palm oil products, the waste materials produced also increase dramatically. These waste materials can potentially cause dangerous environmental problems, financial losses, and health hazards if left without any utilization in various manufacturing processes. Incorporation of POFA in concrete as partial replacement of cement is increasing with time due to enhancement of concrete in terms of physical and chemical properties. It also decreases the cost and improves the environmental situation [1]. The concrete ingredients resulting from biomass products can be more beneficial than traditional materials especially if those do not contribute to the environmental damages. In order to manage the solid waste materials resulting from the production of palm oil, many researchers have exploited this waste as partial replacement of cement in concrete manufacturing [48]. Incorporating waste materials such as POFA in concrete manufacturing will result in minimizing the impact of CO2 emissions due to decreased quantity of wastes landfilled and thus improving the environment [49]. In the long term, with increasing pressure to minimize CO2 emissions resulting from cement production, the need to use green concrete by incorporating palm oil fuel ash as pozzolanic material and partial replacement of cement will increase significantly [38]. In other words, using the waste materials in concrete production assists to reduce the volume of the solid waste materials in landfills and thus reduces harmful gases in the atmosphere [50]. The successful use of POFA and other agricultural waste materials will contribute by reduction in the dumping of these wastes in landfills, and thus reducing CO2 gas emissions. This will also result in improving concrete properties such as compressive strength, durability, and resistance to attack of chloride and sulfate. Generally, concrete consists of cement, water, fine and coarse aggregate. Consumption of these traditional materials can generate harmful gas emissions. Therefore, most of the researchers in this field who are also interested in the environmental aspects have begun to use new materials that have less impact on the environment and, at the same time, do not negatively impact the concrete properties. Although many studies have been conducted aimed at reducing the air pollution by using POFA to reduce CO2 emissions, to date no studies have mentioned the effects of POFA on the water and soil pollution. Therefore, there is an urgent need to encourage researchers to make additional efforts towards minimizing water and soil pollution in this regard.

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1.3. Nano POFA Nanotechnology has been applied in many scientific disciplines. It is also relevant here due to the unique chemical and physical characteristics of concrete composites [19]. Nano-particles are the particles that have sizes less than 100 nm (100  109 m) [51]. The integration of cement with Nano-additive replacements such as nano-SiO2, silica fume, Nano-clay, Nano-fly ash and carbon nanotubes in cement pastes has significantly improved the cement paste properties, especially, increasing the compressive strength and durability of concrete mixtures [52–55]. Many studies have been conducted to find out the potentials of Nano POFA to improve properties of concrete and cement paste, such as [18,56]. They concluded that addition of Nano POFA can enhance the concrete properties by decreasing the amount of calcium hydroxide. Rajak et al. [19] used Nano POFA as pozzolanic material in concrete mixtures. The purpose of using Nano-POFA was to understand the effects of Nano- POFA on cement hydration, especially on the microstructure properties of cement paste. POFA is a pozzolanic material due to the fact that it has a significant quantity of SiO2 in its particles [57]. Abutaha et al. [58] conducted a study to find out the impact of adding high volume Nano POFA on the cement mortar; they concluded that incorporating high volume of Nano POFA can produce concrete with the qualities of being more sustainable and with higher compressive strength. Many studies such as [3,17,39] have shown that the original size of POFA is not strong in the microstructure because of its porosity with large particle size in the natural condition. On the other hand, [59–61] suggest to grind POFA to get finer particles and thus improve the reactions with other particles in the concrete mixtures. Tangchirapat et al. [3] showed that the fineness of POFA has significant effects on the compressive strength. Due to the large particles of POFA in its original size, its microstructure composition is weak and highly porous [3,17,39]. Therefore, reducing its particle sizes to micro or Nano by grinding process to improve its reactivity with other materials and enhance its properties has been recommended [59–61]. Many studies have incorporated different fineness of POFA as cement replacement, for example, Tangchirapat et al. [3] concluded that POFA with the particle size of 7.4 mm and the replacement quantity of cement up to 10% has the same values of compressive strength as the control samples. Sobolev and Gutierrez [62] concluded that incorporation of nanoparticles with cementitious composites has the ability to improve the chemical and physical properties for composites. Kroehong et al. [18] noted that Portland cement Type I can be replaced by high fineness particles of POFA to use it as pozzolanic material with proportion up 30% of binder by weight. In a recent study by Lim et al. [63], it was claimed that using high volume of Nano POFA with particle size less than 1 mm and with the percentage up to 80% as replacement of cement can achieve high compressive strength with high quality of concrete, and thus mitigate carbon dioxide gasses emission from cement production process. In a study by Johari et al. [64], it was shown that ultrafine POFA leads to increasing the concrete workability and delay in the setting time especially when higher content of POFA instead of cement is used. Ultrafine POFA (UPOFA) contains more silica than OPC and Ground POFA (GPOFA), and thus, it can produce extra calcium-silicate-hydroxide (C-S-H) gels to make cement mortar much denser and durable. Therefore, UPOFA can be classified as F pozzolan class, while GPOFA can be classified as C pozzolan class [65]. However, to date there have been few studies conducted to combine Nano POFA and other waste materials in nano size such as nano Fly ash and nano lime. Therefore, there is a research gap in this area where it is needed to study mixing of Nano POFA with

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any other Nano composite or geopolymer to enhance the concrete properties. 1.4. Nano silica with POFA Nano silica consists of small particles sized (1–50 nm); it has spherical particles and can enhance the concrete properties in fresh and hardened states [66]. Nano silica has been added as cement replacement with POFA in order to enhance the concrete properties, such as dry shrinkage and compressive strength. Farzadnia et al. [66] conducted a study by adding Nano silica to POFA replacing 30% of cement. It was noted by the authors that the compressive strength increased by 15% compared to the control sample, the shrinkage decreased while lowering free water, and the hydration volume in cement matrix increased. POFA can be utilized as a pozzolanic material in concrete mix because it has a significant content of SiO2 [66]. Incorporation of Nano silica with unground POFA was studied by Noorvand at al. [20] in order to improve mechanical properties of concrete mix. The reduction of water absorption and the increase in the compressive strength of concrete resulted from adding Nano silica to the POFA-cement mortars. Tobon et al. [67] found that after adding Nano silica to the cement pastes, especially in the first days of curing process, the compressive strength increases due to the good distribution of C-S-H and thus causes a larger quantity of hydration compared to plain cement paste. Sanchez and Sobolev [68] reported that Nano silica is the most common among nanomaterials, which are used to enhance and improve the chemical and mechanical properties of cementitious materials and has been widely examined. Adding Nano silica to the cementitious materials ranges from small to large quantities [69–77]. After adding small amount ranging from 1% to 3% of Nano silica, it was noted that the compressive strength, durability, and microstructure of cementitious composites are modified and improved. From chemical aspect, Nano silica accelerates the C3S dissolution because of its high surface area [78]. In fact Nano silica also affects the physical properties through the consumption of calcium hydroxide (CH) and formation of C–S–H clusters to improve pozzolanic properties in cementitious materials [70]. Many studies have reported that POFA has high silica content which ranges between 50% and 70% of total weight and makes it a good pozzolanic material in the concrete mixtures, therefore, there is no need to add more silica to cement containing POFA [1]. Previous studies such as [69,76,79–83] have focused on the enhancement of properties of mortar, paste, and concrete mixtures to fast CH consumption, particularly at early ages of concrete samples to discover the high reactivity of Nano particles that includes SiO2. In a recent study by Farzadnia et al. [66], it was reported that adding Nano silica to POFA advantageously affected the microstructure and the shrinkage of concrete. However, there is still need to conduct further experimental works by adding Nano silica under different conditions to cement containing POFA. 2. Utilization of POFA in concrete POFA has many benefits to be utilized in concrete manufacturing such as enhancing the concrete properties to resist chloride attack [84], assisting to increase the drying shrinkage of concrete [85], decreasing heat development, resisting concrete sulfate attack [3,86], and reducing the effect of acidic environment on concrete [17]. In 2011, Awal and Hussin [7] studied the effects of POFA on the heat control of concrete during chemical reactions. They found that the hydration heat depends mainly on the type of chemical interactions of the materials. In Thailand, Tangchirapat et al. [3] used POFA to find out its effect on the concrete properties such as compressive strength, initial and final setting time, and

expansion due to magnesium sulfate attack. They discovered that concrete containing POFA leads to delay in setting time depending upon the proportions of POFA replacing cement and its degree of fineness. In terms of compressive strength, the concrete samples made from cement and the POFA with original size have less compressive strength than conventional concrete samples, but the concrete samples made from cement and POFA with fine particles have compressive strength higher than traditional concrete samples. To produce lightweight foamed concrete (LFC) that has lower density than normal concrete, Lim et al. [87] used POFA as replacement of cement to get low concrete density 1300 ± 50 kg/m3 with improved properties such as compressive strength, thermal conductivity, and flexural strengths. To find out the mechanical properties of concrete containing fiber, Islam et al. [88] also used POFA with ground granulated blast-furnace slag (GGBS) as binder and oil palm shell as coarse aggregate in preparing geopolymer concrete. In order to exploit palm oil waste in the construction industry, POFA has been adopted as cement replacement up to 25% and oil palm shell as a coarse aggregate. A study concluded that POFA within 10–15% of cement replacement gives better results in terms of compressive strength; in contrast, increased POFA proportions lead to decrease in the splitting and flexural tensile strengths [89]. Liu et al. [90] utilized POFA and fly ash as concrete binder to prepare lightweight-foamed concrete containing oil palm shell (OPS) as lightweight coarse aggregate. The main purpose of the study was to evaluate the mineralogy and the morphology of oil palm shell foamed geopolymer concrete. Chindaprasirt et al. [16] examined the amount of water permeability and concrete strength containing POFA. They found that the compressive strengths of concretes containing 20% of GPOFA were higher than compressive strength of OPC concrete. But, they also found that the permeability of water was still lower than that in OPC state. It was concluded that POFA can be used as a pozzolanic material in concrete and thus achieve an acceptable permeability and strength for concrete samples. In terms of using other wastes with POFA as cement replacement, Ranjbar et al. [13] replaced the Fly Ash (FA) by POFA as a pozzolanic material and measured the volume change, the density, microstructural characteristics, and compressive strength of concrete samples. They replaced the FA contents by 0%, 25%, 50%, 75% and 100% of POFA to prepare the geopolymer mortar. All the samples were exposed to a variety of temperatures starting at room temperature as a reference to 300 °C, 500 °C, 800 °C and 1000 °C. The researchers found that, after exposing the concrete sample with POFA and FA to higher temperatures, the compressive strength starts decreasing when the temperature becomes more than 500 °C. In another study, POFA was utilized as a pozzolanic material in concrete. The POFA particle sizes in the study were between 7.4 and 15.9 mm, the Portland cement was replaced by POFA at 10%, 20%, 30%, and 40% by weight of binder [3]. Some researchers [64,91] have used high volume of ultrafine POFA as replacement of cement reaching up to 60% of total cement mass to produce concrete. POFA has a significant content of silica and thus it has the ability to replace cement in the concrete mixtures [57]. There are significant concerns related to the durability of concrete buildings as a result of erosion and reduction of the age of concrete structures as well as the generation of harmful gases into the surrounding environment. Therefore, a significant number of researchers have begun to exploit new materials that have high durability and resistance to corrosion, such as FA and POFA [92]. Some researchers have reported that the thermal properties of concrete improve if POFA is used as partial cement replacement in concrete [38]. The production of C-S-H gels results from the interaction between Al2O3 and SiO2 with Ca (OH)2 in a cement paste to produce pozzolanic material [93], and increasing fineness of pozzolanic materials will result in increasing the resistance to sulfate in concrete and reduction in the content of

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Ca(OH)2. In addition to that, concrete with high pozzolanic reaction is better in terms of enhanced concrete compressive strength along with being more homogeneous and dense. POFA satisfies the ASTM C618 chemical requirements in terms of using a binder pozzolanic material having (LOI) value less than 10%, which makes it suitable to be used in the concrete production. Many studies have been conducted to benefit from Palm oil waste applications in concrete manufacturing and it has been reported that these wastes have beneficial value [59,94,95]. In 2011, Jaturapitakkul et al. [38] claimed that pozzolanic reaction of POFA in early age of concrete sample is low and increases with time. A study on the compressive strength of concrete for 10–40% of cement replacement was reported where the researchers found the compressive strength of concrete ranging from 0.1 MPa to 4.5 MPa at 7 days and 2.5 MPa to 22.5 MPa at 90 days [60]. POFA can be mixed with other waste materials such as fly ash FA in certain proportions [1] to give high concrete quality in terms of improved workability and lowering the quantity of super plasticizer (SP). Due to the considerably large advantages acquired from the utilization of POFA and FA in concrete as partial replacement of cement, it has started gaining popularity in concrete manufacturing, especially towards minimizing cost and ecological issues. In addition, to produce foamed concrete, good quality is obtained by partially replacing cement with POFA. Even when the compressive strength value is lower than the control samples, the concrete with POFA can be used in non-structural building elements such as producing concrete blocks that do no need to be of high compressive strength [96]. Also, POFA can be combined with waste plastic resulting from plastic factories and daily use by the people to produce concrete with new properties, such as incorporating POFA with thermosetting plastic, whereas thermosetting plastic has been used by Panyakap and Panyakapo [97] with fly ash to produce lightweight concrete, which has acceptable compressive strength for nonstructural elements in buildings. In addition, POFA can be used along with egg shells, because the latter is rich in calcium oxide and enhances hydration heat during reaction of materials. Egg shells were used in concrete by [97,98]. One of the disadvantages of POFA is the high content of nonburning carbon which is considered to be higher than FA and GGBS [21]. Therefore addition of high quantity of POFA will result in the decrease of workability when the replacement reaches up to 30%, thus necessitating the addition of a superplasticizer [99]. Accumulation of palm oil waste in huge amounts results in the generation of biomass such as mesocarp fiber, empty fruit bunches, palm oil leaves, palm oil trunks, palm oil kernel shell, and palm oil mill effluent, which cause an environmental risk if not treated directly [44]. In a study by Hassan et al. [100], it was reported that biomass generation over the world was 80 million tons and this quantity is expected to reach up to 110 million tons in 2020. Therefore, academicians and researchers should play a significant role to encourage the government and non-government agencies to use these waste by demonstrating the benefits resulting from utilization in concrete production and other industries.

3. Utilization of POFA 3.1. POFA as SCM There has developed an increasing interest in the recent years towards adopting new supplementary cementitious materials (SCM) that result in better chemical and physicals characteristics than traditional concrete [56,101]. Many studies have used POFA as SCM, for example, Paris et al. [49] conducted a study to find out the effects of POFA on the physical and mechanical properties as well as the durability of cement paste. Almost all of the research

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related to using POFA in the concrete mixtures has been applied in very warm climates. Due to the increasing demand of cement materials all over the world, and because of the decrease of coal fly ash as a supplementary cementitious material, there is an urgent need to use new supplementary cementitious materials such as POFA to replace PC fully or partially even in colder climates [49]. In Thailand, Tangchirapat et al. [5] used the POFA as supplementary cementitious materials to produce new concrete with high strength. In 2017, Thomas et al. [44] reviewed many studies related to using POFA as a supplementary cementitious material and the chemical and physical properties of resulting concrete in the fresh and hardened states. The authors reported that POFA gives high strength to concrete in sulfate and acidic environments, high resistance to chloride and carbonation, in addition to lowering of permeability and shrinkage. Due to the POFA advantages as a supplementary cementitious material in the long-term, it has been more extensively used in concrete preparation, whereas, many studies have focused on the determination of fresh and hardened state properties. Palm oil industry is one of the most important agro industries in South-East Asian region [102]. POFA is the newest addition to the cement paste as a SCM, which can be obtained as a by-product from the palm oil mills and is considered a significant pozzolanic material [7]. Many studies have been conducted to benefit from the agro waste materials such as POFA, FA, and rice husk ash (RHA) as SCM or as pozzolanic materials [103]. Minimizing the permeability in concrete mixes is one of the advantages resulting from using SCMs such as POFA. This alters the pore structure and thus increases resistance to the sulfate and acid attacks. In terms of concrete durability properties, POFA and other SCMs enhance concrete durability and reduce environmental damages, which leads to the reduction of concrete cost. However, adding an excessive amount of SCM in cement paste may result in lowering compressive strength because of reduced portlandite concentration in these materials [104,105]. Tangchirapat et al. [5] examined water permeability, drying shrinkage, and sulfate resistance in concrete containing POFA, and noted compressive strength up to 70 MPa at 90 days. The researchers reported that concrete thus produced has lower drying shrinkage than concrete produced from OPC, and that the concrete water permeability decreases when GPOFA proportion is increased. Concrete with POFA replacing cement shows resistance to sulfate better than conventional concrete. Due to the limited availability of concrete materials in most of the countries and the high cost of cement, POFA can be used as SCM in concrete manufacturing in order to be more economical and environment friendly. The differences in POFA particles because of incomplete burning in the boilers for some quantities can cause higher carbon content and make it darker potentially reducing its utilization as SCMs [106]. 3.2. POFA in self-compacting concrete (SCC) There are many advantages due to the utilization of waste materials in concrete, one of these advantages is self-compacting concrete (SCC) when it is required to be poured into a narrow area [107], especially when the concrete reinforcement is congested and it is difficult to conduct compaction process to the concrete during casting. Self-compacting concrete requires reduction in the aggregate materials and increase in the bonding materials, which results in additional cost; in order to overcome this problem, the alternative solution is using a SCM to replace cement partially [108]. The SCM improves concrete properties such as workability, decreases cost due to decrease in the cement amount, and enhances SCC characteristics thus achieving energy saving, which can be considered a sustainable construction method [109]. POFA is one of these SCMs that can be used as a replacement of cement

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because it contains a large quantity of silica that reaches up to 66%, which enables it to produce a variety of concrete types [110].

between 43% and 71%, and makes POFA particles with good pozzolanic properties to produce high quality concrete.

4. POFA properties

4.2. Physical properties

4.1. Chemical composition of POFA

The physical properties of POFA are affected by the burning temperature and other conditions [50,115], for instance, POFA color is grey in normal state but it becomes darker if it contains high amount of unburned carbon; and after burning further, the carbon is removed and the color changes back to grey [119]. The physical properties of POFA have been shown in Table 2. Specific gravity of POFA ranges between 2.6 and 1.89; these values are less than the specific gravity of cement.

The chemical composition of POFA has been examined by many researchers as shown in Table 1. A large variety has been reported in the experimental works, the notable variations are due to different conditions such as burning temperature, burning quantity of palm oil parts to produce POFA from different factories and other factors. These factors were identified in detail by [64,91], who focused on the higher quantity of silica due to burning some parts of palm oil tree. The chemical properties of constituent materials affect the heat of hydration of concrete [102], its compressive strength and workability. Using waste materials as a replacement of cement has seen a tremendous advancement due to their chemical composition being suitable to produce high quality concrete in line with increasingly stringent environmental legislation [111]. The main composition of POFA is silicon dioxide which ranges

4.2.1. Specific gravity A study by Tay in 1990 [17] showed that the specific gravity of unground POFA is approximately 40% lower than the specific gravity of OPC, which ranges from 1.78 to 1.97. Sata et al. [59] reported that the specific gravity increases when the POFA particle size is finer, to reach up to 2.78 because the reduction in particle sizes decreases porosity. There are many factors that affect the POFA

Table 1 Chemical compositions of POFA as reported in literature. References

Silicon dioxide (SiO2)

Aluminum oxide (Al2O3)

Iron oxide (Fe2O3)

Calcium oxide (CaO)

Magnesium oxide (MgO)

Sodium oxide (Na2O)

Potassium oxide (K2O)

Sulfur trioxide (SO3)

Loss on ignition (LOI)

[18] [112] [56] [113] [15] [102] [5] [59] [99] [90] [66] [20] [85] [16] [43] [19] [14] [63] [114] [7] [1] [115] [50] [116] [117] [118] [119] [30] [120] [3] [84] [121] [64] [86] [111] [122] [123] [89] [16] [61] [124] [125] [126] [127]

54.0 64.2 65.01 53.5 43.6 59.62 65.3 65.2 71.67 63.4 47.37 48.9 65.3 57.8 60.42 54.80 64.17 69.3 51.55 43.60 52.63 64.20 62.6 47.37 47.22 63.41 62.60 59.17 65.30 57.71 63.6 66.91 65.01 57.7 65.01 67.72 62.6 63.4 57.8 55.7 63.41 64.17 53.3 60.42

0.9 3.7 4.68 1.9 11.4 2.54 2.5 2.6 0.94 5.5 3.53 2.71 2.5 4.6 4.26 7.40 3.73 5.30 4.64 11.40 8.991 4.25 4.65 3.53 2.24 5.55 4.65 3.73 2.60 4.56 1.60 6.44 5.72 4.5 5.72 3.71 4.65 5.51 4.6 0.9 5.55 3.73 1.9 4.26

2.0 6.3 3.2 1.1 4.7 5.02 1.9 2.0 2.77 4.2 6.19 6.54 1.9 3.3 3.34 4.47 6.3 5.1 8.64 4.70 1.059 3.13 8.12 6.19 2.65 4.19 8.12 6.33 2.00 3.30 1.40 5.72 4.41 3.3 4.41 4.71 8.12 4.2 3.3 2.0 4.19 6.33 1.9 3.34

12.9 5.8 8.19 8.3 8.4 4.92 6.4 6.4 5.61 4.3 11.83 13.89 6.4 6.6 11 14.0 5.8 9.15 5.91 8.40 3.211 10.20 5.7 11.83 6.48 4.34 5.70 5.80 6.40 6.55 7.60 5.56 8.19 6.5 8.19 5.57 5.7 4.35 6.6 12.5 4.34 5.80 9.2 11.00

4.9 4.8 4.58 4.1 4.8 4.52 3.0 3.1 4.91 3.7 4.19 2.74 3.0 4.2 5.31 4.14 4.87 4.1 2.44 4.80 1.459 5.90 3.52 4.19 5.86 3.74 3.52 4.87 3.10 4.23 3.90 3.13 4.58 4.2 4.58 4.04 3.52 3.78 4.2 5.1 3.74 4.87 4.1 5.31

1.0 0.18 0.07 1.3 0.39 0.76 0.3 0.3 0.12 – – 0.05 0.3 0.5 0.18 – 0.18 – 0.07 0.39 0.564 0.10 – – 1.22 0.16 – 0.18 0.30 0.50 0.10 0.19 0.07 0.5 0.07 0.16 – 0.15 0.5 1.0 0.16 0.18 1.3 0.18

13.5 5.18 6.48 6.5 3.5 7.52 5.7 5.7 7.89 6.3 – 7.13 5.7 8.3 5.03 – 8.25 11.1 5.50 3.50 3.133 8.64 9.05 – 11.86 6.33 9.05 8.25 5.70 8.27 6.90 5.20 6.48 8.2 6.48 7.67 9.05 6.35 8.3 11.9 6.33 8.25 6.1 5.03

4.0 0.72 0.33 – 2.8 1.28 0.4 0.5 1.05 0.9 1.22 1.54 0.4 0.3 0.45 0.71 0.72 1.59 0.61 2.80 – 0.09 1.16 – 9.19 0.91 1.16 0.72 0.50 0.25 0.20 0.33 0.33 0.2 0.33 1.07 1.16 0.93 0.3 2.9 0.91 0.72

3.7 16.3 2.53 18.0 18.0 8.25 10.0 10.1 – 6.0 1.84 11.3 10.0 10.1 2.55 9.3 16.3 1.3 5.00 18.00 27.7 1.73 6.25 1.84 5.42 6.20 6.25 16.1 10.10 10.52 9.60 2.30 2.53 10.5 2.53 6.20 6.25 6.19 10.1 4.7 6.20

0.45

2.55

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properties [130] such as treatment processes [64] and the burning temperature [15]. Therefore, it can be said that the specific gravity of POFA varies depending on the factors mentioned above but does not exceeded 3.0. Other researchers [5,120] have also reported that the specific gravity of POFA increases after the grinding process due to the minimization of porosity. The specific gravity of POFA ranges between 2 and 2.5 and thus is less than the specific gravity of ordinary Portland cement. The specific gravity of POFA particles was reported as 2.16 by a study conducted to find out properties of lightweight concrete made from POFA, lime treated sewage sludge and sodium silicate [131]. While in a study by Mohammadhusseini et al. [129] the specific gravity of POFA was reported as 2.42; Islam et al. [99] reported it to be 2.15. 4.2.2. Color The color of ground POFA is dark gray, while the color is light gray for unground POFA because of the exposure to low burning temperature of the unburnt carbon content. The high burning tem-

perature for POFA leads to decrease in the unburnt carbon content proportion, and thus leads to change in its color [8]. In general, the color of POFA powder is gray and becomes dark when quantity of unburned carbon increases [102]. The decarbonation of POFA is because of the decomposition of calcium carbonate at temperatures ranging between 400 and 600 °C, therefore, the color of POFA changes from dark black to grey [63]. Zeyad et al. [111] concluded that the color of POFA particles changes due to the heat treatment which causes reduction in carbon content responsible for black color in POFA particles. 4.2.3. Size and shape Generally, the particle size of unground POFA is larger than ground POFA. In terms of the geometry, particle shape of unground POFA is spherical and porous, whereas the particle shape of ground POFA is an irregular and angular shape because it consists of crushed particles [16]. The particle size of ground POFA is smaller than cement particle size which ranges between 7.2 and 10.1 mm,

Table 2 Physical properties of POFA as reported in literature. References

Specific gravity

[117] [102] [63] [20] [85] [43] [19] [14] [5] [18] [128] [60] [7] [50] [117] [118] [119] [86] [3] [129] [99] [111] [122] [123] [88] [16] [61] [124] [125] [127]

2.31 2.42 2.56 1.97 2.17 2.6 1.89 2.33 2.36 2.42 2.39 2.22 2.42 2.16 2.2 2.42 2.43 2.36 2.42 2.15 2.59 2.2 2.42 2.14 2.43 2.48 2.14 1.81 2.6

Surface area (m2/g)

Blaine fineness, m2/g 4582 4930 7205

Median particle size, d50 (mm)

2340 493 1228

1.10 82 19.9 1.07 20–90 nm 22.78 10.1 15.6 14.58 12.30

4930 1977

57.13

13.40 145.35 12.92

Retained on sieve No. 325 (%) 10.5 0.13 75 17.1

1.5 4.98 4.30

520

0.172

88.4 33 1.0 19.5 33

4930 7.4 15.9 4930 7.670 1.72

1.136

17.10 2.06

4930 1.720 1.49 1.72 13.40

UPOFA [20]

8.0 2.1

1.0

10 1.069

96

GPOFA [142]

Fig. 3. Particle shape and size of POFA [20,142].

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while the cement particle size is 10–20 mm [59]. In order to find out shape and size of POFA particles, a scanning electron microscopy (SEM) as one of the analysis tools has been used by many researchers [19,20,43,113,132–141], most of whom concluded that GPOFA particles are smaller in size and less porous compared to UPOFA particles. SEM analysis for GPOFA and UPOFA can be seen in Fig. 3. Although POFA particles have different size and most of the POFA particles have spherical shape, these still have specific gravity less than the cement particles [25]. Regarding the shape of POFA, Lim at el. [63] conducted tests on the UPOFA particles to find out morphological structure by Field Emission Scanning Electron Microscope (FESEM). They noted that UPOFA particles have irregular, thinner, and crushed particles as shown in Fig. 4. Other researchers such as [128,135] reported that POFA has thinner, irregular, crushed particles together with spherical particles, with some air gaps present. 4.2.4. Fineness Fineness is one of the significant physical properties of POFA. The pozzolanic nature of the material and the hydration rate depend on the fineness of POFA particles. With the finer particle size, the compressive strength of concrete increases directly [20,86,126]. The UPOFA particles are larger than OPC, on the other hand, the GPOFA particles are finer than OPC as can be seen in Table 3. Finer POFA particle size can be obtained in ball mills through grinding process [3,5,59]. The porosity of POFA can also be reduced by decreasing particle size through the grinding process [143]. Besides, Paya et al. [144] showed that the grinding process of POFA particles leads to smaller particles with less porosity.

Fig. 4. UPOFA shape by Field Emission Scanning Electron Micrograph (FESEM) [63].

Fig. 5. Distribution curve of particle size for POFA and OPC [125].

The fineness of POFA can be measured through the mass percentage passed or retained on sieve No. 325. Due to the fact that particle size of GPOFA is smaller than that of OPC, the specific surface area of OPC is smaller than that of GPOFA as listed in Table 3. The physical and chemical properties of the waste materials resulting from agricultural combustion residue are affected by the material treatments inside palm oil mills [49]. Some of these treatments are calcination temperature, acidic or alkaline pretreatments, and the Blaine fineness of POFA which may differ from 300 to 1800 m2/kg depending on the treatment method [4]. The Particle size of POFA and OPC influence the pozzolanic activity in these materials directly. Khalid et al. [126] claimed that grinding process by rod bar for POFA particles is the best method to get further fineness of particles with high surface area. The distribution curve of POFA particles size in Fig. 5 shows that POFA particles have smaller size than that of OPC particles. In addition to that, there is large quantity of unburned carbon which may affect the concrete workability by absorption of a huge amount of super plasticizer. The carbon particles in the unburnt residues can be mitigated or disposed through heating these particles at a high temperature of up to 500 °C for one hour [64,134]. Awal and Shehu [102] used various particle sizes of POFA after grinding process for half an hour, where the specific surface area was 4930 cm2/g, while the ash retained on 45 mm sieve was 10.5% only. As shown in Fig. 5, POFA particle size in normal state is larger than OPC particles. While POFA particles have size 2.99 mm, UPOFA particles have size 2.06 mm, which is considered finer than OPC particles [44]. Another study divided POFA particles into 3 sections, small, medium, and large particle sizes, which have median sizes of 7.4, 15.9, and 183 mm, respectively. The POFA particle fineness and

Table 3 Effects of the particle size POFA on concrete properties. References

Particle size of POFA (mm)

Positive effect

[99] [64] [59]

45 2 11

[5]

10.1

[3]

7.4

[85] [61] [18]

10.1 2.2 2.1

[125] [127]

10 1.069

Adding 10% POFA had higher compressive strength than control concrete at 90 day Improved workability due to reduction in water demand. In addition, enhanced the compressive strength up to 104 MPa at 28 days. Use of UPOFA particles in high strength concrete is effective in minimizing the superplasticizer volume in high compressive strength concrete, less than the quantity required in silica fume state. Small particle size of POFA can be used as a cement replacement in high strength concrete to minimize the water permeability and produce high compressive strength. The small POFA particle size can replace cement in the concrete mixture due to the ability to resist sulfate and improve compressive strength at the age of 90 days, especially when 20% is added as replacement. The grinding process for POFA to produce micro particle size enhances the concrete durability and reduces water absorption. Finer particles of POFA improve compressive strength higher than POFA with coarse particle size Grinding POFA to get high fineness particles assists to use it as pozzolan material and, therefore, it can be used as cement replacement up to 30% of total cement weight. Due to small particle size of POFA, it can be used a replacement of cement by 20% in self-compacting concrete SCC. Effect of using ultrafine POFA particles was acquiring more strength especially after adding 10 ML of Na2SiO3 aq to NaOHaq

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the specific gravity increases whenever the grinding process is used to get smaller particles. Jaturapitakkul et al. [86] discovered that increasing POFA fineness leads to decrease in the compressive strength in addition to improving resistance against sulfate attack. Lim at al. [63] used UPOFA with particle size 146% less than OPC to find out impact of Nano POFA on the hydration heat and microstructure properties of cement mortar. UPOFA particles have high surface area than that of GPOFA and OPC particles. 4.2.5. Absorption of water One of the physical properties of cement mortar and concrete paste is the ability to absorb water. A few studies have been conducted to examine the water absorption of concrete containing POFA as a replacement of cement material. One of these studies in 1990 showed that absorption of water increased whenever the quantity of POFA content increased in concrete mixture [17,39]. Islam et al. showed that absorption of water in concrete containing POFA as a replacement of cement with proportion range between 10 and 70% increases because of the delayed hydration and tendency of POFA to absorb water [99]. In a recent study to determine water absorption for concrete containing POFA, tests were conducted at two ages of 28 and 90 days. Firstly, the disc specimens were dried at 105 ± 5C for 2 days and left at room temperature before being immersed in water for curing purpose [99]. Absorption of water was determined through observing the weight of the samples at 30 min and 72 h. Lau et al. [131] noted that increase of temperature of sintering leads to decrease in the water absorption of light weight concrete containing POFA and lime treated sewage sludge. Mixing waste polypropylene (PP) with POFA was done by Mohammadhusseini et al. [129], who investigated the durability properties of sustainable concrete compound, which consisted of POFA and PP (carpet fiber). They noted that mixing POFA and PP in concrete assists to decrease the slump value, the water absorption and chloride penetration. In a study by Yahaya [145], it was claimed that POFA with pulverized fuel ash PFA used as replacement of concrete by 10%, 20%, and 30% has ability to enhance concrete porosity due to minimizing voids within concrete microstructure and thus producing concrete with high density. It is known that some of the materials have good absorption of water due to the microstructure properties of their composites, and it is generally agreed upon that materials containing finer particle have improved resistance to water transportation through them. Therefore, further studies should be conducted using nanomaterials as cement replacement such as Nano POFA, Nano FA, and Nano GRBA [59]. 5. Effects of POFA on the concrete properties 5.1. Effects of POFA on fresh concrete properties 5.1.1. Workability Workability is a significant characteristic to determine the concrete quality. Decrease of concrete workability occurs due to higher amount of unburned carbon in POFA, especially when the replacement level of cement is high [21]. Slump test is generally used to identify the workability; Awal and Shehu studied the slump of concrete which contains different percentages of POFA in various concrete mixtures; they found that whenever the percentage of POFA is increased in concrete mix, the workability also increases and the slump value decreases [102]. On the other hand, Tay and Show [85] noted that workability of concrete decreases with increased amount of POFA percentages in the concrete mortar. They concluded that no segregation in concrete sample occurs when the compacting factor value is more than 0.93 [39]. In a recent study by Noorvand et al. to assess the desired workability, nanosilica

35

was utilized with concrete containing POFA in order to reduce the quantity of super plasticizer in concrete mortar [20]. Aldahdooh et al. concluded that the workability of concrete increases when using ultra fine POFA particles instead of OPC. The workability increases because of the lower carbon content and the lower loss of ignition (LOI) of the ultra-fine POFA [56]. Concrete containing unburned carbon leads to decrease in the workability of concrete. Islam et al. studied the behavior of concrete containing POFA. They noted that the concrete workability will be reduced when replacement level of POFA is more than 30% [99]. Also, the slump is not influenced if the POFA content is less than 20%, compared with Palm oil shell to produce lightweight aggregate concrete (LWAC) without POFA [99]. Other studies, such as [4] have concluded that increasing the amount of POFA replacement will result in decreasing the concrete workability level due to the increase in water demand. Yusuf et al. [43] investigated impact of H2O/Na2O molar ratios on the UPOFA and ground blast furnace slag GBFS. They noted that using Na2O in UPOFA and GBFS has positive impact on the workability of concrete, while it has negative impact on the compressive strength of the concrete sample. Another study by Ariffin et al. [113] used sodium hydroxide and sodium silicate, NaOH and Na2So3, as alkaline solution that have been mixed together for 5 min; in addition, super plasticizer was used to achieve required workability ranging between 80 and 100 mm. They noted that the concrete paste had low workability making it not suitable to be cast in molds. Salami et al. [136] reported that to achieve the required workability, water should be added to fresh concrete containing POFA. Muthusamy et al. [146] used POFA as cement replacement in various percentages with oil palm shell OPS as a coarse aggregate in order to benefit from the waste materials generated from palm oil industry, and mitigate the hazardous impacts to the environment. They noted that using POFA with 20% to 30% achieved the best results in terms of workability and compressive strength. Slump tests were conducted by Bashar et al. [122] to investigate the concrete workability; the slump value was recorded zero due to the use of high quantity of POFA, which caused absorption of high quantity of water resulting in decreasing the concrete workability. Awal and Mohammadhusseini [123] used 20% POFA as replacement of cement and various proportions of waste carpet fiber (WCF) ranging between 0.25% and 1%. They noted that the slump value decreases from 210 mm for the mix without fiber to reach 25 mm with the addition of only 1% fiber into the concrete mix. The addition of 20% of POFA also improved the workability by reducing slump value. Islam et al. [88] conducted a study to examine the extent of concrete incorporating oil palm shell as coarse aggregate and POFA with GGBS as binder replaced in concrete. They concluded that the addition of fibers to concrete results in reducing the concrete workability due to large surface area and the ability to absorb high quantity of water. It is noted that there is a lack of studies in literature that focus on evaluating the workability of concrete made by combining POFA and other plastic materials [147] in order to fully benefit from the effects of plastic properties on the workability. 5.1.2. Heat of hydration Awal and Shehu noted an increase in the concrete temperature value containing 50%, 60% and 70% of POFA compared to 100% OPC in the beginning. However, the total temperature increase reduced in concrete containing POFA and thus the occurrence of peak temperature was delayed [102]. On the other hand, Lim at el. [63] conducted a study on the impact of high volume Nano POFA on the hydration temperature and microstructure properties of cement mortar. They concluded that a high volume of Nano POFA reduces the heat of hydration of cement mortar; it can also be used to treat thermal cracking resulting from large temperature increase in

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Table 4 Compressive strength for concrete containing the POFA as replacement of cement. References

Compressive strength in MPa at 28 days with replacement value 0%

[30] [102] [120] [5] [153] [64] [122] [89] [59] [86] [119]

67.1 46 77.5 58.5 137 91 41.8 77 31.9 44.4

10%

81.3 59.5 142

42.4 81 31.9

20%

85.9 60.9 153 98 38.4 86 31.6

30%

60%

70%

68.0 36

65.5 28

104

79 30.1

98 31.4

80%

5.2.1. Drying shrinkage Drying shrinkage is responsible for cracks occurring because of the loss of water from concrete paste, and it occurs particularly in dry and hot weather [85]. Many studies have been conducted to find out the impact of POFA as cement replacement on drying shrinkage in concrete. Farzadnia et al. [66] investigated the effects of adding various dosages of Nano silica on short-term drying shrinkage of cement mortars containing POFA as partial replacement of cement during the first 28 days. They noted that the concrete samples with 30% POFA as cement replacement showed increased compressive strength by 15% during 7 to 28 days of treatment, while drying shrinkage decreased by 7.5%. At the same time, the hydration volume of concrete mix increased. A significant result from a study by Lau et al. [131] is the shrinkage index. They used POFA, lime treated sewage sludge, and sodium silicate to produce lightweight aggregate; all these materials had been sintering under three temperatures of 1160, 1180 and 1200 °C. The researchers noted that the fire condition and adding more binder affects the shrinkage index and water absorption. The shrinkage index can be defined as the percentage for changing in pellets’ diameter before and after burning as illustrated in equation below [148].

ð1Þ

where d1 is pellets diameter before burning and d2 is pellets diameter after burning. However, adding some materials to the concrete mix is expected to change the drying shrinkage value. Other materials can be used instead of Nano silica, such as Nano clay [149,150] and Nano alumina [151] to control the drying shrinkage of concrete. 5.2.2. Compressive strength Most of the researchers have conducted their studies to improve the compressive strength of concrete containing POFA. For example, Muthusamy and Zamri [114] concluded that 20% of POFA as cement replacement is the optimum level for compressive strength of concrete at 28 days. In another study by Islam et al. [89], it was found that 10% of POFA is the optimum level to replace cement in the concrete mix. The values of the compressive strength

30

27.5 35.7

5.2. Effect of POFA on hardened concrete properties

d2  d1  100 d1

50% 69.0 41

79.8 58.8 146

mass mortar. However, there are few studies that illustrate the impact of Nano POFA along with other Nano composites of waste materials on the heat of hydration of cement. Therefore it is needed to study the use of high volume mixing Nano POFA and Nano fly ash and find out how it affects the heat of hydration. In addition to the potential utilization of POFA as cement replacement in cold climates, it will also help understand its effects on the heat of hydration of cement in climate change conditions.

Shrinkage index ¼

40%

29.5

27.0

of concrete mixtures containing different percentages of POFA as cement replacement as reported in the literature are shown in Table 4. It may be disadvantageous to use POFA in excessive proportions if the structure is expected to face adverse conditions such as earthquakes [96]. In a recent study by Zeyad et al. [2], it was shown that ultrafine POFA replacing cement can achieve compressive strength higher than control samples, which may reach more than 90 MPa at 28 days. In 2007, Tangchirapat et al. [3] used three types of POFA in concrete; the first type was original POFA called OP, the second type was median particles (15.9 m) called MP, and the third type was small particles (7.4 m) called SP. They reported that the compressive strength of concrete containing OP was much lower than in case of OPC, while compressive strength of concrete containing 10% MP, and concrete containing 20% SP was better than normal concrete at 90 days. Application of Nano POFA with particle size less than 100 nm has better characteristics than normal cement mortar as filler and binder in cement mortar, in addition more than 80% cement replacement by Nano POFA in concrete mix can be used to achieve compressive strength more than normal concrete [63]. Another study investigated the potential of using high volume of POFA and OPC in sustainable concrete with various proportions in addition to the effects of these quantities on the chemical and physical properties [99]. Incorporation of 10–20% of POFA as filler in lightweight foamed concrete improves the compressive, flexural, and tensile strength if compared with lightweight foamed concrete containing 100% sand [87]. On the other hand, Awal and Mohammadhusseini [123] conducting their research by incorporating POFA and waste carpet fiber (WCF) as replacement of cement in various proportions of WCF and 20% POFA. The compressive strength was reported to range between 38.1 and 49.1 MPa at the age of 91 days. Some researchers have conducted their studies by adding Nano silica to the unground POFA, for example Noorvand et al. [20], to improve the mechanical properties of concrete, such as increasing the compressive strength and decreasing water absorption of cement mortar. More research is required in this regard to improve mechanical properties of concrete using waste materials, which are freely available and cost less, such as combination of Nano POFA as cement replacement with egg shells [152]. 6. Discussion and conclusions A review of the literature on the use of POFA as cement replacement in the concrete production emphasizes the importance of this practice towards sustainability. On the one hand, cement and concrete industry has been reported to consume large amounts of energy, utilize great quantities of natural resources and generate significant proportion of the CO2 in the atmosphere. Any technology aimed at reducing the use of cement in the preparation of concrete is going to be beneficial on all these three fronts i.e. energy economy, resources sustainability, and environmental friendliness.

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On the other hand, large quantities of agricultural/industrial waste byproducts are being dumped in the landfill sites without treatment or re-use. The management of these wastes is a significant environmental challenge. The use of these waste materials in the production of concrete can not only result in the efficient solid waste management of the same but it will also help reduce the use of cement realizing all the benefits mentioned above. Many researchers have studied the potential of using POFA as partial cement replacement in concrete. The significant information gathered from reviewing the present state of this practice can be summarized as:
Palm oil fuel ash (POFA) is obtained as a byproduct when waste materials are burnt in palm oil mills to produce electricity.
Grinding procedures can be applied to obtain finer varieties of POFA such as Ground POFA (GPOFA), Ultrafine POFA (UPOFA), and Nano POFA.
In its original size, the microstructure composition of POFA is weak and highly porous. Reducing the particle size to micro and nano, however, significantly improves the performance of POFA. The finer varieties of POFA react well with the other constituent materials and produce stronger concrete.
POFA satisfies the ASTM C618 requirements to be used as a binder pozzolanic material in concrete production.
POFA is rich in SiO2 and therefore, is a good pozzolanic material. With the addition of Calcium, it can produce extra CalciumSilicate-Hydroxide (C-S-H) gels, which increase the density and durability of the cement mortar.
Addition of Nano Silica along with POFA increases the compressive strength of concrete and reduces the water absorption.
The use of POFA in concrete improves the resistance to the chloride and sulfate attacks.
The compressive strength of concrete containing POFA decreases at temperatures in excess of 500 °C.
The specific gravity of POFA ranges between 2.6 and 1.89, which is less than the specific gravity of cement resulting in the production of lighter concrete when cement is partially replaced by POFA.
POFA has a high content of unburnt Carbon resulting in decreasing the workability when high amount of POFA is used as cement replacement. This necessitates the use of a super plasticizer.
The use of POFA as a secondary cementitious material can help reduce the drying shrinkage in concrete.
POFA can also be used as partial cement replacement to produce self-compacting concrete.
The finer the particle size of POFA, the higher is the compressive strength of the resulting concrete. However, the finer varieties of POFA have higher specific gravity than the coarser varieties.
The use of POFA tends to delay the hydration process and increase the absorption of water.
A high volume of Nano POFA reduces the heat of hydration of cement mortar. It can also be used to treat thermal cracking resulting from large temperature increase in mass mortar.
Researchers have reported various proportions ranging from 10% to 20% of cement replacement by POFA to be optimum for the compressive strength of concrete.
The use of ultrafine POFA has been reported to produce concrete strength up to 90 MPa at 28 days.
The use of Nano POFA has also been reported to produce strengths more than normal concrete.
Incorporation of 10–20% of POFA as filler in lightweight foamed concrete improves the compressive, flexural, and tensile strength compared with lightweight foamed concrete containing sand only.

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Based on the above review of the literature on the use of POFA as partial cement replacement in concrete production, the following directions are suggested for the future research. 1. A comparative study can be carried out between concrete containing POFA with micro particles and concrete containing POFA with Nano particles, in terms of resistance for marine environmental conditions. 2. Compressive strength of concrete samples prepared with the POFA obtained from various sources such as different factories should be compared in order to determine if the source from where the POFA has been obtained is significant for concrete properties. 3. The use of Nano POFA along with Nano-Alumina in various proportions needs to be studied in order to find out the physical and chemical properties of the resulting concrete.

Conflict of interest There is no conflict of interest.

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