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Dumping of palm oil fuel ash (POFA) not only occupies land but also creates environmental pollution and health hazard. These problems can be reduced to a large extent by using POFA in concrete. A number of research works have been carried out to investigate the potential use of POFA as a supplementary cementing material for normal, high-strength, and aerated concretes. This paper presents a review on the use of POFA in different types of concrete. It firstly discusses the physical and chemical properties of POFA. However, the emphasis has been given on the effects of POFA on the fresh and hardened properties and durability of concrete. This paper shows that POFA increases the water demand and thus decreases the workability of concrete when used in ground condition. However, ground POFA has shown a good potential in improving the hardened properties and durability of concrete due to its satisfactory micro-filling ability and pozzolanic activity. In addition, this study has identified certain gaps in the present state of knowledge on POFA concrete, and listed some research needs for future investigation. The findings of this study will encourage the use of POFA as a supplementary cementing material in concrete.
Keywords: Concrete, fineness, micro-filling ability, palm oil fuel ash, pozzolanic activity, supplementary cementing material.
Palm oil fuel ash (POFA) is a by-product of palm oil industry. It is resulting from the combustion of palm oil plant residues. The Elaeis Guineensis tree, commonly known as palm tree, was first introduced in Malaysia as an ornamental plant in 1970. It is now a leading agricultural cash crop in Malaysia and other tropical countries, such as Indonesia and Thailand. The palm oil industry has been developed rapidly in Malaysia since 1980 . At present, there are more than three million hectares of palm oil plantation in Malaysia . In total, about 90 million metric tons of trunks, shells, husks, palm press fibers, and empty fruit bunches are produced every year. After the extraction of the oil from the fresh palm fruit, both husk and shell are burnt in palm oil mill plants at a temperature of 800-10000C as boiler fuel to produce steam needed for electricity generation and palm oil extraction [3, 4]. The burning process results in an ash, which is referred to as palm oil fuel ash (POFA). After combustion in the steam boiler, about 5% POFA by weight of solid wastes is produced . This POFA is used to be considered as a nuisance to the environment. Since the tropical countries are continuously increasing the production of palm oil, the quantity of POFA is also increasing and thus creating a huge environmental load . In Malaysia, an investigation was carried out to examine the potential of POFA to be used as a fertilizer for the agricultural purpose . However, due to the absence of sufficient nutrients to be used as a fertilizer, POFA is mostly dumped in open field near palm oil mills without any profitable return, thus causing environmental pollution and human health hazard [7, 8]. As an effort to find a solution to these problems, several studies were conducted to examine the feasibility of using POFA in concrete. It has been found that the properly processed POFA can be used successfully as a supplementary cementing material for the production of concrete.
The use of POFA in Malaysia as a supplementary cementing material for concrete first started in 1990 . Tay  used unground POFA to partially replace ordinary portland cement (OPC) and showed that it had a low pozzolanic property, and therefore recommended that POFA should not be used with a content higher than 10% of cement by weight. Later many researchers showed that ground POFA can be successfully used as a supplementary cementing material in concrete due to its good pozzolanic property [9-12]. Tonnayopas et al.  used 5 to 30% ground POFA by weight of OPC and found that the incorporation of POFA in concrete decreased the concrete strength at early ages (3 to 21 days) but the strength achieved at and after 28 days for the concretes with 5 to 15% POFA met the ASTM C 618 requirement . Chindaprasirt et al.  used ground POFA in concrete and found that POFA has a good potential for concrete production. They observed that the partial replacement of OPC by ground POFA resulted in a higher water demand for a given workability of concrete. Moreover, they observed that the compressive strength of concrete with 20% ground POFA was as high as OPC concrete. The strength decreased when the POFA content became higher than 20%. A POFA content higher than 20% also increased the permeability of concrete. Hence, the optimum POFA content found by Chindaprasirt et al.  was 20%. In addition, Hussin and Ishida  used 20 to 40% ground POFA by weight of OPC in concrete. They determined the compressive strength, modulus of elasticity, poisson's ratio, shrinkage and creep of concrete, and found that, up to 30% POFA content, the aforementioned properties of hardened concrete are comparable to that of OPC concrete. Hussin and Awal [11, 15] also studied the strength properties of concrete containing ground POFA at various cement replacement levels of 10 to 60% by weight of OPC. They have shown that it is possible to use 40% POFA without affecting the concrete strength; however, the maximum strength gain occurs when the POFA content is 30%. Not only good strength, the POFA concrete has also shown adequate durability. Many laboratory investigations showed that POFA can be used in producing strong and durable concrete due to its pozzolanic property [15-17]. According to Sumadi and Hussin , POFA can be used up to 20% cement replacement level without any adverse effect on the strength characteristics and with a durability at least comparable to that of OPC concrete. It has shown good potential in suppressing expansion due to sulfate attack [18, 19] and alkali-silica reaction .
POFA has been used not only in normal concrete but also in special concretes such as high strength and aerated concrete. Several researchers reported that the ground POFA can be used to produce high strength concrete [4, 5, 20]. Ground POFA provides much higher compressive strength than unground POFA due to significant differences in particle size and surface fineness. The ground POFA with high fineness is a reactive pozzolanic material and therefore can be used in making high-strength concrete [5, 20]. Sata et al.  made high-strength concrete with 10-30% POFA by weight of OPC and showed that the concrete containing up to 30% ground POFA provides a higher compressive strength than OPC concrete at 28-days. However, 20% POFA produced the optimum strength in concrete. In addition, Abdullah et al.  used POFA in aerated concrete and found that the increased ground POFA content decreases the compressive strength of aerated concrete. They also observed that the replacement of cement by 10 to 40% ground POFA exhibits a significant improvement in the compressive strength of aerated concrete from 7 days to 28 days. Hussin and Abdullah  also used ground POFA in aerated concrete and observed that POFA concrete can produce similar strength as OPC concrete at 30% replacement and the maximum strength occurred at the replacement level of 20%. Thus the published literature shows that POFA has a good potential for the production of different types of concrete. However, the use of POFA in high performance and self-consolidating concretes is very limited.
This paper reviews the potential use of POFA in concrete as a supplementary cementing material. It firstly discusses the physical and chemical properties of POFA. However, this paper highlights the effects of POFA on the fresh and hardened properties, and durability of concrete. Above all, the gaps in the current state of knowledge on POFA concrete were sought to identify the future research needs.
2. PROPERTIES OF POFA
2.1. Physical Properties
The physical properties of POFA are greatly influenced by the burning condition, particularly burning temperature . A number of physical properties of unground and ground POFA used in various studies are shown in Table 1. Some of those properties are briefly discussed below.
Generally, unground POFA is light grey in color. This is due to the unburnt carbon content left at relatively low burning temperature. The unburnt carbon content becomes very low when the burning temperature is high. Unground POFA can be whitish in the absence of unburnt carbon .The color becomes dark grey in case of ground POFA.
2.1.2. Relative Density
The relative density of unground POFA is about 60% lower than the relative density of OPC . After the grinding process, the relative density of POFA increases. This is because the grinding process decreases the porosity with reduced particle size.
2.1.3. Particle Shape and Size
From scanning electron microscopy (SEM), it was found that the unground POFA particles are large and porous, as shown in Figure 1(b). The particles of unground POFA are mostly spherical and possess smooth surface indicating a rather complete burning . In contrast, the ground POFA is generally made of crushed particles of irregular shape as shown in Figure 1 (c). Also, the particle shape of ground POFA is similar to that of portland cement as can be seen from Figures 1(a) and 1(c).
The unground POFA has particles larger than OPC. However, the ground POFA has particles smaller than OPC. The typical particle size distributions of POFA and OPC are shown in Figure 2. The median particle size (d50) of unground POFA varies in the range of 62.5 to 183 µm, which is larger than that of OPC (10 to 20 µm). After grinding, the median particle size of POFA can be reduced to 7.2 to 10.1 µm [5, 22].
Fineness is a vital property of cementitious materials. The rate of hydration depends on the fineness of particles. For the rapid development of strength, a high fineness is necessary. The unground POFA is coarser than OPC but the ground POFA is much finer than OPC, as evident from Figure 2. The particle size of POFA can be reduced by grinding process in ball mills [4, 20, 23]. POFA may also be ground in a Loss Angeles abrasion machine using mild steel bar (12mm diameter and 800 mm long) instead of steel ball [3, 15, 17]. The grinding process reduces not only the particle size but also the porosity of POFA . After grinding, it becomes less porous with smaller particles .
The fineness of supplementary cementing material is generally measured with respect to the specific surface area of particles. The fineness of POFA can also be expressed with respect to the percent mass passing through or retained on sieve No. 325 (45 µm opening). The specific surface area of ground POFA is higher than OPC, as can be seen from Table 1. In addition, the percentage mass retained on sieve No. 325 can be in the range of 41.2 to 94.4% for unground POFA whereas it can be 1.0% to 3.0% for ground POFA. Both specific surface area and percent passing sieve No. 325 reveal that the surface area of POFA becomes higher after grinding.
2.1.5. Pozzolanic Activity
The pozzolanic activity expresses the reactivity of pozzolan (pozzolanic supplementary cementing material) for pozzolanic reaction. It can be determined by testing the compressive strength of 50-mm mortar cubes with and without pozzolan . The pozzolanic activity of supplementary cementing material depends on its silica content, particle size distribution, and surface fineness. The pozzolanic activity of POFA can be improved by increasing the fineness of ash through a grinding process . Therefore, ground POFA possesses a good pozzolanic activity index, as can be seen from Table 1. The specified minimum pozzolanic activity index of highly reactive supplementary cementing material is generally 85% .
There is limited literature on the soundness of POFA. Based on the experimental investigations, Tay  as well as Tay and Show  found that the soundness of POFA blended cement increases with the increase in ash content as evident from Figure 3. The results of the soundness test for various percentages of unground POFA blended with cement were within the limit given by the British standard , thus indicating that the cement-based materials including POFA will be free from undue expansion [1, 28]. According to Hussin and Awal , and Awal and Hussin , the ground POFA has been found equally sound as OPC.
2.2. Chemical Composition
The chemical composition of POFA reported in various studies is summarized in Table 2. The major chemical component of POFA is SiO2, which is in the range of 44 to 66%. The other pozzolanic components are Al2O3 and Fe2O3. The loss on ignition (LOI) and SO3 are in the range of 0.1%-21.5% and 0.2%-3%, respectively. In most cases, the loss on ignition was much higher than the limit specified in ASTM C 618 . In all cases, the amount of SO3 was well below, and in some investigations, the amount of alkali (Na2O) was higher than the maximum limit as prescribed in ASTM C 618 . Sata et al. [5, 20] and Tangchirapat et al.  stated that chemical composition of POFA satisfies the requirement for class N pozzolanic materials stated in ASTM C 618 , since the sum of SiO2, Al2O3, and Fe2O3 is close to 70%, SO3 is not higher than 4%, and LOI is close to 10%. In contrast, Jaturapitakkul et al.  found that the total amount of SiO2, Al2O3, and Fe2O3 of POFA is lower than the minimum requirement for natural pozzolan as specified by ASTM C 618 . Therefore, they enforced that POFA cannot be classified as a natural pozzolan. According to Abdullah et al.  POFA satisfies the requirement to be a pozzolan, and may be classified under Class F. The justification was made based on the percentage of CaO content of POFA which they found as 4.12%. Nagataki  mentioned that fly ash under Class F should have a CaO content less than 5%. Moreover, the total amount of SiO2, Al2O3, and Fe2O3 was near to 70%, which is the requirement for pozzolanic Class F ash. In addition, Hussin and Awal , and Ahmed et al.  reported that POFA satisfies the requirement to be a pozzolan and may be classified under Class C based on the standard specification stated in ASTM C 618 . Thus, there are contradictory results in justifying the classification of POFA based on its chemical composition. Hence, more study is needed to avoid this contradiction.
3. EFEECT OF POFA ON THE PROPERTIES OF CONCRETE
3. 1. Fresh Properties
In various experimental studies, it was found that POFA does not cause any severe adverse effect on the workability of concrete. However, the workability decreased with the increase in POFA content [1, 22, 31, 32], as can be seen from Table 3. The higher replacement of POFA exhibits lower slump and consequently, lower degree of compaction . POFA concrete needs more water than OPC concrete for lubrication to maintain the same workability . This is due to high porosity of POFA particles which absorb some water, and thus reduce the free water content needed for workability. In addition, the water demand of ground POFA becomes greater than that of unground POFA due to increased specific surface area (surface fineness). The angularity and irregularity of ground POFA with some porous particles also contribute to increase the water demand of concrete for a given workability.
3.1.2. Setting Time
Several studies showed that the addition of POFA delayed the setting of concrete, and therefore the initial and final setting times increased with the increase in POFA content [1, 24, 28], as can be seen from Table 4. Tay , and Tay and Show  reported that the setting times of POFA concrete, though increased, still fulfilled the ASTM requirement . In contrast, the other studies showed that the setting times of POFA concrete with various ash contents did not conform to the ASTM requirement . Nevertheless, the long setting times of POFA concrete are due to the pozzolanic reaction (reaction between POFA and calcium hydroxide), which is usually slower than the hydration of cement . In addition, porous POFA particles absorb some water , which cannot readily participate in hydration reaction, thus increasing the setting time of concrete. The setting time of POFA concrete varies with the degree of ash fineness and replacement level of cement. The ground POFA decreases the setting time of concrete as compared with the unground POFA , since it enhances the pozzolanic reaction due to increased surface fineness. Also, the higher replacement level of cement with POFA results in the reduction of C3S, thus increasing the setting time of concrete .
3.1.3. Segregation and Bleeding
Limited studies investigated the effect of POFA on the segregation and bleeding of concrete. Few investigations reported that there was no segregation in all the concretes with various POFA contents [1, 28]. It was observed in the research carried out at the University of Technology, Malaysia that the use of POFA not only improved the workability with no segregation but also reduced the bleeding significantly . However, no studies were conducted to examine the effect of POFA on the segregation and bleeding in case of high performance and highly flowing or self-consolidating concretes.
3.1.4. Other Fresh Properties
Limited studies have been conducted to examine the effects of POFA on the plastic shrinkage, slump loss, and air content of concrete. The plastic shrinkage can cause early-age cracking in concrete, thus aggravating many durability problems. The slump loss significantly decreases the workability of concrete before it is properly placed. Both plastic shrinkage and slump loss may cause difficulties for concreting in hot countries. Also, the air content is an important factor for the freeze-thaw durability of concrete. Hence, more studies are needed to investigate how these properties, particularly plastic shrinkage and slump loss, are affected in the presence of POFA. Furthermore, no studies have been carried out to examine the effect of POFA on the two fundamental rheological properties, yield stress and plastic viscosity of concrete. These two properties should be investigated if POFA is intended to be used in highly flowing or self-consolidating concrete.
3.2. Hardened Properties
3.2.1. Heat of Hydration
Limited literature is available on the heat of hydration of POFA concrete. According to Sata et al. , the increased content of ground POFA can reduce the peak temperature rise of concrete. The use of 30% ground POFA as a partial replacement of cement produces the lowest peak temperature rise and gives 15% lower temperature than OPC concrete. This is due to the reduction in cement content in the presence of POFA. The partial replacement of cement by ground POFA decreases the total heat released . According to Awal and Hussin , the partial replacement of OPC by POFA is advantageous in controlling the temperature rise, particularly for the mass concrete where the thermal cracking due to excessive heat rise is of great concern.
3.2.2. Compressive Strength
Many studies were carried out to examine the effect of POFA on the compressive strength of concrete. Some studies [1, 28] revealed that the compressive strength of concrete decreases as the POFA content is increased. In contrast, some other researchers found that the concrete made with POFA exhibits a higher compressive strength than OPC concrete. According to Tay , and Tay and Show , the compressive strength of concrete decreased for the unground POFA content in the range of 20 to 50%. But the compressive strength of POFA concrete was similar to that of OPC concrete for 10% unground POFA as shown in Figure 4. The decrease in the compressive strength of concretes containing unground POFA was due to the large POFA particles with high porosity. The porous POFA particles increase the actual water-binder (w/b) ratio in concrete due to absorption, and thus results in a lower compressive strength [19, 23].
Tonnayopas et al.  also showed that there can be a decrease in the concrete strength at the early age due to the slow pozzolanic activity of ground POFA. However, the latter strength of POFA concrete was higher than that of OPC concrete. They also concluded that the optimum ground POFA content was 20% to obtain satisfactory concrete strength. Chindaprasirt et al.  found that the compressive strength of concrete with 20% ground POFA was higher than OPC concrete. In contrast, they obtained that the compressive strength of concrete at 40% ground POFA was less than that of OPC concrete. Hussin and Awal [11, 15] reported that it is possible to use 40% ground POFA in concrete without any adverse effect on strength although the maximum strength gain occurs at 30%. Sata et al.  observed that the compressive strength of concrete at the early ages (≤ 7 days) was higher for 10% ground POFA than 20-30%. But they found that the compressive strength of concrete at later ages (> 28 days) was higher for 20% POFA after 28 days. According to Ahmed et al. , the optimum content of ground POFA was 15% to achieve the maximum gain in compressive strength.
The above-mentioned studies indicate that the effect of POFA on the compressive strength of concrete largely depends on its fineness. Tangchirapat et al.  found that the concrete containing 10-30% ground POFA exhibits a higher compressive strength than OPC concrete at 28 days, as evident from Figure 5. Also, Sata et al.  observed that the concrete with 10-20% ground POFA provides a greater strength than OPC concrete, as obvious from Figure 6. This is because of satisfactory micro-filling ability and pozzolanic activity of ground POFA. The ground POFA particles fill the micro-voids between cement particles due to smaller particle size . The micro-filling ability mostly contributes to increase the compressive strength of concrete at the early ages. In addition, the SiO2 of ground POFA reacts with the Ca(OH)2 liberated from cement hydration in the presence of water (pozzolanic reaction, and forms additional or secondary calcium silicate hydrate (C-S-H). The pozzolanic reaction mainly contributes to increase the compressive strength of concrete at the later ages by improving the interfacial bond between paste and aggregate . However, both micro-filling ability and pozzolanic activity of POFA may depend on the water-binder (w/b) ratio of concrete. Alike other supplementary cementing materials, POFA can be more effective in these two mechanisms when used in a concrete with relatively a low w/b ratio .
3.2.3. Flexural Strength
Limited literature has been found on the flexural strength of concrete containing POFA. Eldagal  used 20% and 30% POFA passing through 10 micron sieve along with 20% and 30% POFA passing through 45 micron sieve for determining flexural strength of high-strength concrete and shown that POFA concrete exhibits lesser flexural strength than OPC concrete, but higher replacement gave higher flexural strength. For structural application, more research is necessary to examine the flexural capacity of concrete containing POFA.
3.2.3. Splitting Tensile Strength
Few studies focused on the splitting tensile strength of concrete incorporating POFA. Sata et al.  made high-strength concretes using 10%, 20%, and 30% ground POFA and tested their splitting tensile strength. They found that the splitting tensile strength of concretes with 20% and 30% POFA was slightly higher than that of OPC concrete as shown in Figure 7. The highest value of splitting tensile strength occurred at 20% POFA content. The increase in the tensile strength of concrete is possibly due to the pore refinement resulting from the micro-filling ability and pozzolanic activity of ground POFA. In contrast, Eldagal  have shown that POFA concrete exhibits tensile strength less than OPC concrete like flexure and the value is higher for higher replacement. However, more investigation is necessary to examine how POFA influences the tensile strength of concrete.
3.2.4. Modulus of Elasticity
Some research works reported the effect of POFA on the modulus of elasticity of concrete. According to Hussin and Ishida , the modulus of elasticity of concrete containing ground POFA was lower than that of OPC concrete at the early age (7 days). At later ages, the modulus values were comparable to those of OPC. Moreover, 20% POFA produced a higher elastic modulus than OPC at the age of 365 days as illustrated in Figure 8. This is mainly due to the improvement of interfacial transition zone between aggregate and cement paste caused by the pozzolanic activity of ground POFA. However, the effect of POFA on the modulus of elasticity also depends on the aggregate content of concrete. According to Sata et al. , the ground POFA content from 10 to 30% slightly decreases the modulus of elasticity of concrete due to a reduction in coarse aggregate content . They also mentioned that POFA had little effect on the modulus of elasticity of high-strength concrete as compared with OPC concrete.
3.2.4. Drying Shrinkage
Drying shrinkage is caused by the evaporation of internal water from hardened concrete. According to Tay , the drying shrinkage of concrete with unground POFA increases slightly after 28 days if the ash content is increased. It was also found that the drying shrinkage of concrete with 10% POFA is comparable to that of OPC concrete. Moreover, Hussin and Ishida  produced concretes with 10 to 40% ground POFA and found that 40% POFA exhibits the highest shrinkage, while 20% and 30% POFA provide a similar shrinkage developed in OPC concrete. In contrast, Tangchirapat et al.  observed that high-strength concrete with ground POFA produced lower drying shrinkage than OPC concrete for any amount of POFA added, as shown in Figure 9. The lower value of drying shrinkage in POFA concrete is due to the densification of concrete pore structure. The addition of ground POFA decreases the pore sizes due to pore refinement . The transformation of large pores into fine pores decreases the evaporation of water from concrete surface and thus reduces the drying shrinkage.
The creep of concrete refers to the deformation of hardened concrete caused by a long-term sustained load. Limited studies have been conducted on the creep of concrete containing POFA. According to Hussin and Ishida , the specific creeps of concrete with or without ground POFA were almost the same. After 180 days, the specific creep of POFA concrete was only about 5% lower than that of OPC concrete. They mentioned that a similar creep occurred for both OPC and POFA concretes due to the equivalent improvement in strength under the same curing condition. However, more research is needed to investigate the effect of POFA on the creep of concrete.
3.2.6. Water Absorption
Limited research has been conducted on the water absorption of POFA concrete. Tay , and Tay and Show  found that the water absorption of concrete increases with the increase in unground POFA content, as shown in Figure 10. They mentioned that the POFA concrete exhibits a more porous nature with higher unground POFA content. It indicates that the concrete with a higher unground POFA content tends to absorb more water due to greater porosity . But the water absorption of concrete can be reduced in case of ground POFA because of its satisfactory micro-filling ability and pozzolanic activity leading to a pore refinement. However, more research is required to investigate the water absorption of concrete containing ground POFA.
3.2.7. Water Permeability
The water permeability of concrete containing POFA depends on the content and fineness of POFA. Sumadi and Hussin  investigated the water permeability of concrete with ground POFA. They found that the permeability of POFA concrete decreased with increased age due to the formation of additional gel from the pozzolanic reaction of ash. Chindaprasirt et al.  made concrete incorporating ground POFA at the levels of 20, 40 and 55% by weight of cement and found that the water permeabilities of POFA concrete at both 28 and 90 days were lower than that of OPC concrete, except the concrete made with 55% POFA. Their results also showed that the concrete made with 20 and 40% ground POFA provided the lower permeability than OPC concrete even though the W/B ratios of these two concretes were higher than OPC concrete. In contrast, the permeability of concrete made with 55% ground POFA rapidly increased and was higher than that of OPC concrete. This is attributed to the low cement content and high W/B ratio of the concrete made with 55% ground POFA. According to Tangchirapat et al. , the concrete containing 20% ground POFA produced the lowest water permeability as compared to the other ash contents, as shown in Figure 11. In addition, all high-strength concretes containing ground POFA provided 50% lower water permeability than OPC concrete . This is due to the reason that the ground POFA increases the impermeability of concrete through pore refinement and porosity reduction.
3.2.8. Chloride Penetration
Limited experimental investigation  revealed that the ground POFA can be used as a partial replacement of portland cement to produce concrete possessing a good resistance to chloride penetration. The depth of penetration of chloride ions into POFA concrete was much lower than that in OPC concrete as illustrated in Figure 12. The POFA particles increase the nucleation sites for the production of hydration products, consume Ca(OH)2, and produce pozzolanic products. The pozzolanic products fill in the pores in bulk binder paste and transition zone. Consequently, POFA decreases the permeability of concrete, and thus increases the resistance to chloride penetration.
3.2.9. Porosity and Density
Concrete with a greater POFA content may possess a higher porosity because of the porous nature of POFA. The density of concrete can be decreased due to the absorption of water by porous POFA particles. According to the investigations of Tay , and Tay and Show , the oven-dry, saturated surface-dry and air-dry densities of concrete made with unground POFA decreased with the increase in ash content, as shown in Figure 13. However, Tangchirapat et al.  reported that the ground POFA refine the pore size and reduce the porosity in concrete, and thus results in a dense concrete. Jaturapitakkul et. al.  also mentioned that the use of ground POFA decreases the Ca(OH)2 content of hydrated cement and reduces the voids between aggregates and hydration products, thus producing a denser concrete.
3.2.10. Other Hardened Properties
In published literature, there are limited studies on several important hardened properties of POFA concrete such as shear, bond, impact and fatigue strengths, oxygen permeability and diffusion, autogenous shrinkage, and electrical resistivity. The bond and impact strengths are vital properties of concrete. The POFA concrete should also be examined for its shear, impact and fatigue strengths before applying in structural member. The autogenous shrinkage and electrical resistivity are also important for the durability of POFA concrete. In addition, the oxygen permeability and diffusion of POFA concrete should be assessed to ensure adequate corrosion resistance leading to a good durability.
3.3.1. Resistance to Sulfate Attack
A few research investigated the effect of unground and ground POFA on concrete's resistance to sulfate attack [19, 24, 40]. The sulfate attack was simulated using 10% MgSO4 solution. In general, the expansion of concrete due to sulfate attack decreased with the increased content of unground POFA , as shown in Figure 14. However, high-strength concrete containing ground POFA showed a better resistance to sulfate attack , indicating that the fineness affects the sulfate resistance of concrete - the finer the POFA, the lower is the expansion of concrete due to sulfate attack. However, more research is needed to investigate the sulfate resistance of POFA concrete.
3.3.2. Resistance to Acid Attack
Limited research was carried out to evaluate the acid resistance of POFA concrete. Hussin and Awal , and Awal and Hussin  determined the weight loss of the concrete containing 30% ground POFA along with non-POFA concrete continuously submerged in 5% hydrochloric acid solution to measure the resistance to acid attack. They found that the weight loss of POFA concrete after 1800 hours was less than that of OPC concrete, as shown in the Figure 15. It was also observed that POFA concrete showed a better surface condition than OPC concrete after exposure to acid solution. The high acid resistance of POFA concrete was attributed to the pozzolanic property and low lime content of POFA. The amount of porous Ca(OH)2 was less due to a low lime content. Moreover, secondary hydration product (additional CSH gel from pozzolanic reaction) was produced at the expense of Ca(OH)2. As a result, the microstructure of concrete became dense with a reduction in porosity. This led to a reduced penetration of acid solution into the interior of concrete. Nevertheless, the above studies neither differentiated the effects of unground and ground POFA nor determined their optimum contents. Hence, more research is required for different contents of unground and ground POFA.
3.3.3. Resistance to Alkali-Silica Reaction
Limited research has been carried out to investigate the effect of POFA on concrete's resistance to alkali-silica reaction. Awal and Hussin  used ground POFA in concrete as a supplementary cementing material and showed that a reduction in expansion occurred with an increased ash content. After 12 days of exposure, about 25% reduction of expansion was obtained in concrete containing 10% POFA. In addition, they reported a substantial reduction in expansion for 50% POFA. Furthermore, the total alkali content (as Na2O equivalent) in their study was much higher than that specified in ASTM C 150 . Despite the higher alkali content, ground POFA was very effective in reducing the expansion due to alkali-silica reaction. The reason is that the pozzolanic POFA particles react very rapidly with the alkalis present in cement because of their reactive nature, thus leaving very little unreacted alkalis for the later reaction with reactive aggregate. However, more research is needed to confirm the beneficial effect of POFA in reducing alkali-silica reactivity.
3.3.4 Resistance to Carbonation
Few studies focused the effect of POFA on concrete's resistance to the carbonation of concrete. Awal and Hussin  investigated concrete's resistance to carbonation with and without ground POFA. They found that there is a little difference between the carbonation values of OPC and POFA concretes. They also mentioned that the results are not truly conclusive because POFA concrete appears to be more sensitive to the exposure condition - the dryer the concrete, the deeper the carbonation. Nevertheless, further research is needed to investigate the resistance of POFA concrete to carbonation.
3.3.5. Others Durability Properties
In published literature, there is no information about the effects of POFA on the freezing and thawing resistance, de-icing salt scaling resistance, and corrosion resistance of concrete. In addition, no study was conducted to examine the abrasion resistance of POFA concrete. The performance of POFA regarding these durability properties needs to be investigated before using it in concrete.
4. RESEARCH NEEDS
POFA can be used as a supplementary cementing material up to a certain replacement level of cement without causing any adverse effect on the strength and durability of concrete. However, more research is needed to confirm the beneficial effects of POFA on several concrete properties and durability issues. In this context, the following research needs have been identified for further investigation to encourage the use of POFA in concrete:
Investigation of the effects of POFA on the plastic shrinkage, slump loss, and air content of concrete.
Investigation of the effects of POFA on the two rheological properties, yield stress and plastic viscosity of concrete.
Examination of the effects of POFA on the tensile, flexural, shear, impact, fatigue, and bond strengths of concrete.
Investigation of the effects of POFA on the creep, autogenous shrinkage, water absorption, electrical resistivity, and oxygen permeability and diffusion of concrete.
Further investigation of the effects of POFA on concrete's resistance to sulfate attack, acid attack, alkali-silica reaction, and carbonation.
Assessment of the effects of POFA on the durability performance of concrete with respect to the resistances to freeze-thaw, de-icing salt scaling, corrosion, and abrasion.
Investigation on the potential use of POFA to produce high performance and self-consolidating concretes.
The following salient conclusions can be drawn based on the findings from the review on the utilization of POFA in concrete:
· The use of POFA as a supplementary cementing material in concrete can solve the disposal and health problems caused by the ash generated in palm oil industry, decrease the environmental pollution caused by the cement factories, and reduce the cost of concrete.
· The physical and chemical properties of POFA are favorable for concrete production. POFA can be used to substitute a significant amount of portland cement without affecting the properties and durability of concrete.
· POFA can be used as a supplementary cementing material with a content up to 40% by weight of cement. However, the optimum POFA content is 20 to 30%. A POFA content higher than 40% may adversely affect the properties of concrete.
· The fineness of POFA plays an important role in concrete. The high fineness of POFA improves its micro-filing ability and pozzolanic activity, and thus contributes to improve the hardened properties and durability of concrete.
· POFA concrete shows a comparable and sometimes a better performance than OPC concrete in resisting acid attack, sulfate attack, and carbonation.
· Further research should be carried out to confirm the beneficial effects of POFA on several concrete properties and durability issues, and thus to encourage the use of POFA in concrete.
· Additional research should be conducted to extend the use of POFA in high performance and self-consolidating concretes.