On blended cement and geopolymer concretes containing palm oil fuel ash (original) (raw)

On blended cement and geopolymer concretes containing palm oil fuel ash

Elnaz Khankhaje a { }^{\text {a }}, Mohd Warid Hussin b { }^{\text {b }}, Jahangir Mirza b,* { }^{\text {b,* }}, Mahdi Rafieizonooz a { }^{\text {a }}, Mohd Razman Salim c,** { }^{\text {c,** }}, Ho Chin Siong d { }^{\text {d }}, Muhammad Naqiuddin Mohd Warid a { }^{\text {a }}
a{ }^{a} Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310, UTM Skudai, Johor Bahru, Malaysia
b { }^{\text {b }} UTM Construction Research Centre (UTM CRC), Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310, UTM Skudai, Johor Bahru, Malaysia
c{ }^{c} Institute of Environmental and Water Resource Management, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310, Skudai, Johor Bahru, Malaysia
d { }^{\text {d }} Low Carbon Asia Centre, Faculty of Built Environment, Universiti Teknologi Malaysia, 81310, UTM, Johor Bahru, Johor, Malaysia

A R T I C L E I N F O

Article history:
Received 11 November 2014
Received in revised form 24 September 2015
Accepted 26 September 2015
Available online 30 September 2015

Keywords:

Palm oil fuel ash
Blended cement concrete
Aerated concrete
Geopolymer concrete
Compressive strength
Durability

A B S T R A C T

This article discusses the utilization of palm oil fuel ash (POFA) in normal and geopolymer concrete. Malaysia, one of the world’s largest producers of palm oil, produces more than 10 Mt/year of palm waste as ash, which is called POFA. Since 1989, extensive research has been conducted on its utilization in concrete. Several published studies have noted POFA’s enormous potential as a partial replacement of cement in concrete. This paper describes the effects of using POFA on different fresh and hardened properties of concrete. The latest studies on the use of ground POFA revealed that concrete made from this material possesses better fresh properties and medium to higher strength than ordinary Portland cement (OPC) concrete. One of the major findings is that concrete that incorporates 20%20 \% fine POFA by weight of cement showed better durability properties than OPC concrete. Because limiting CO2\mathrm{CO}_{2} emissions has become a matter of increasing importance in the construction industry, concrete that uses less cement in its production and utilizes an increased amount of waste, such as POFA, offers an environmentally viable solution. Moreover, 100%100 \% cement-free geopolymer concrete can be produced using blended ash, such as POFA and fly ash.
© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Palm oil is a well-known vegetable oil that is used for cooking and food processing [1]. In some countries such as Malaysia and Thailand, the palm oil industry is among the most important industries pertaining to agriculture. Huge amounts of solid waste by-products in different forms such as kernels, fibers and empty fruit bunches are produced along with the crude palm oil. It is estimated that to produce one kg of palm oil, approximately four kg of dry biomass is produced [2]. To save energy and fuels, these waste materials are often burned and used in heating up the boilers to generate power in palm oil factories. By weight, 5%5 \% of this waste includes ash from fibers and shells. This ash is known as palm oil fuel ash (POFA) [3-5].

Malaysia is one of the largest producers and exporters of palm oil worldwide [6]. An estimated 10 Mt/year of total POFA waste is produced in Malaysia alone [7]. In Thailand, it is estimated that more than 10410^{4} tons of POFA are produced annually, and this amount increases every year. Currently, POFA usage is very limited and unmanageable, and most of

[1]it is disposed of in landfills. Consequently, it has caused numerous environmental problems. Al-Oqla and Sapuan [8] claim that utilizing the waste from the palm oil industry as composite material will not only enhance sustainability but will also solve the huge issues resulting from waste problems.

Extensive research has been performed on the possibility of using POFA in blended cement. Tay [3] was the pioneer of using POFA as a cement replacement in concrete. He found that original POFA (GPOFA) possesses pozzolanic properties that can replace cement partially to be used in concrete because silicon dioxide is the main chemical constituent of POFA. Tangchirapat et al. [5] and Chindaprasirt et al. [9] reported that OPOFA should not comprise more than 10%10 \% of binder mass because of its low pozzolanic properties. Therefore, to enhance the pozzolanic reactivity in POFA, Tangchirapat et al. [10] and Awal and Hussin [11] used ground POFA (GPOFA). They demonstrated that GPOFA exhibits good pozzolanic features and can replace ordinary Portland cement (OPC) at levels up to 30%30 \% by binder weight. Chindaprasirt et al. [12] observed that the compressive strength of concrete that contains 20%20 \% GPOFA was similar to OPC concrete. However, they found that by increasing the amount of GPOFA, the compressive strength decreased because of the higher water demand. Sumadi and Hussin [13] reported that by replacing 20%20 \% GPOFA with an equal weight of OPC, durable concrete can be produced that is almost identical to OPC concrete. Johari et al. [14] and Sata et al. [4] stated that POFA has the potential to produce high-strength

1. a Corresponding author.
   ** Correspondence to: M.R. Salim, Institute of Environmental and Water Resource Management, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310, Skudai, Johor Bahru, Malaysia.
   E-mail addresses: j.mirza@utm.my (J. Mirza), mohdrazman@utm.my (M.R. Salim). ↩︎

concrete. Sata et al. [4] produced high-strength concrete by using superplasticizer (SP) to decrease the water/binder (w/b) ratio. They revealed that the concrete containing 30%30 \% GPOFA showed a higher strength than OPC concrete at 28 days. Johari et al. [14] also showed that by using heat treatment and regrinding GPOFA, high-strength concrete can be produced by reducing unburned carbon and smaller-sized GPOFA particles. In addition, the use of POFA, particularly in high volumes, results in more eco-friendly concrete, thus contributing to a greener and more sustainable environment by reducing cement consumption [15].

POFA’s chemical and physical properties are similar to fly ash, which attracted its use as a cement replacement in concrete. Extensive research has thus been performed to realize the possibility of using POFA as a partial cement replacement in concrete. The ways in which POFA affects the fresh and hardened types of concrete and their durability is discussed in this article.

2. Properties and characteristics of POFA

2.1. Physical properties

The operating system in a palm oil factory [16], the burning temperature [17] and treatment processes [14] all significantly influence the physical characteristics of POFA. POFA is used with a cementitious material such as fly ash.

Table 1 compares the physical properties (specific gravity, fineness and strength activity index) of POFA, cement.

2.1.1. Specific gravity

POFA has a varying specific gravity, but according to many researchers (listed in Table 1), its value has never exceeded 3.0. The OPOFA, as received from the factory, is a large particle-sized material with a porous texture. According to Table 1, the specific gravity range of OPOFA is

Table 1
Physical properties of POFA, OPC.

between 1.95 and 2.05. Several researchers [4], [10] found that after the grinding process, the specific gravity of POFA increased because of a decrease in porosity. The specific gravity of GPOFA is in the range of 2.05 to 2.78 , which is lower than the specific gravity of OPC (3.14) but is nearly identical fly ash’s specific gravity (2.06-2.40). Therefore, cement particles are heavier and denser than those of POFA and fly ash [18].

2.1.2. Fineness (Blaine surface area)

POFA fineness can be expressed in terms of median particle size, percent mass collected in a 45μ m45 \mu \mathrm{~m} sieve and the value of the Blaine surface area of particles. As shown in Fig. 1, OPOFA dumped in the factory yard is grayish in color and fine with a light density. After being collected from the mill, POFA is dried in an oven at 100∘C100^{\circ} \mathrm{C} for 24 h to remove moisture. Then, POFA is sieved through a 300μ m300 \mu \mathrm{~m} sieve to remove the larger particles that were left after incomplete combustion processes. Tay [3] and Tangirapat et al. [5] reported that OPOFA is a low pozzolanic material because of its large-sized particles. According to Table 1, OPOFA has a median particle size of 30.80 to 62.5μ m62.5 \mu \mathrm{~m} with 34.80 to 70.00%70.00 \% by weight retained on a 45μ m45 \mu \mathrm{~m} sieve. OPOFA has a higher content of residual organics, and it is coarser compared to fly ash [3].

Sata et al. [4] and Tangchirapat et al. [10] found that after the grinding process, the pozzolanic reactivity of POFA increased in response to the increased fineness of POFA. POFA grinding has been performed by a ball mill [4], [19] or by a modified Los Angeles machine [7], [17]. Moreover, the grinding process will increase the Blaine fineness of POFA while reducing the retained POFA particles on a 45μ m45 \mu \mathrm{~m} sieve (Table 1).

Because carbon particles absorb SP and water, the presence of unburned particles of carbon will decrease the workability and strength of concrete [20]. Johari et al. [14] reduced the carbon content of ground POFA by using heat treatment ( 500∘C500^{\circ} \mathrm{C} for 1 h ) to achieve treated POFA (TPOFA) with a median particle size of 2.99μ m2.99 \mu \mathrm{~m}. In addition, they reported that by regrinding TPOFA, ultra-fine POFA (UPOFA) can be produced with a 2.06μ m2.06 \mu \mathrm{~m} median particle size. Johari et al. [14] also compared different particle sizes of POFA with cement and demonstrated that the median size of OPOFA is larger than cement, whereas TPOFA and UPOFA have a higher fineness than cement, as shown in Fig. 2.

2.1.3. Strength activity index

The strength activity index is an indirect method of measuring pozzolanic activity [21]. Based on ASTM C618 [22], the specified minimum strength activity index of fly ash is 75%75 \%. Previous studies [21], [23], [24], [25] have used this requirement, but there are no guidelines for POFA. The strength activity index of POFA improved because of the increased fineness of POFA [26]. As shown in Table 1, the strength activity index for POFA varied between 80 and 112%112 \%, which is greater than the minimum requirement value of 75%75 \%, stated in ASTM C 618 [22].

2.1.4. Soundness

Limited research has been performed on the soundness of POFA. Tay and Show [27] and Awal and Hussin [7] found that the expansion of cement/POFA mixes was below the maximum specified value of 10 mm based on the Le Chatelier apparatus as prescribed in BS 12 [28]. According to Tay and Show [27], concrete incorporating POFA will be free from undue expansion. In addition, Awal and Hussin [7] reported that the grounded POFA was found to be equally sound compared to OPC.

2.2. Chemical compositions of POFA

Table 2 shows a comparative study of the chemical compositions of POFA as provided in different research papers. Similar to the case of fly ash, different sources produce POFA with different characteristics. The variation in the chemical compositions of POFA is a result of the different types of feeding materials (fruit bunches, shells, etc.) and the burning temperature in the boiler of the palm oil factory [35]. POFA is moderately rich in silica content (SiO2)\left(\mathrm{SiO}_{2}\right) with small amounts of calcium,

Fig. 1. Palm oil fuel ash dumped at the factory yard [5,18].
magnesium, and sulfate. However, because of its reduced amount of large-sized particles and unburned fibers, POFA must be sieved and grinded to increase its pozzolanic reactivity and silica content. Awal and Shehu [21] reported that the presence of a higher silica content influences the pozzolanic activity during its reaction with free lime; it therefore creates extra calcium-silicate-hydrate (C-S-H) gels, which enhances the strength of POFA concrete.

As mentioned earlier, the unburned carbon content in POFA has a negative effect on concrete [20]. Loss on ignition (LOI) is a test that is used to estimate the carbon content of POFA or any other ash. The LOI for GPOFA ranges between 10 and 21.6%21.6 \%, which is high. This is a result of the low burning temperature. Johari et al. [14] reduced the carbon content of POFA to 2.53%2.53 \% by heating it at a temperature of 500∘C500^{\circ} \mathrm{C} for 1 h in an electrical furnace.

Because there are no guidelines for classifying POFA, the “Standard Specification for Coal Fly Ash” [22] has been used. Based on POFA’s production process, it is classified with the chemical composition of a class N fly ash [10,17], a class F pozzolan [18], and a class C pozzolan [36] (Table 2).

2.3. Morphology characteristics of POFA

A scanning electron microscopy (SEM) analysis of POFA has been performed by many researchers [14,18,21,25,48]. Unground POFA particles have been shown to be larger in size and highly porous compared to GPOFA particles, which are also irregular. Figs. 3 and 4 present the SEM of unground and GPOFA.

2.4. Mineralogy characteristics of POFA

X-ray diffraction (XRD) analysis studies have been performed by several [14,20,29] researchers on GPOFA. They demonstrated that the major crystalline phase is alpha-quartz, and they also identified the minor crystalline phases of potassium aluminum phosphate and

Fig. 2. POFA and OPC particle size distribution. Note that TPOFA has been pre-ground and subjected to heat treatment. UPOFA symbolizes ultrafine POFA [14].
cristobalite. Fig. 5 demonstrates that XRD patterns of TPOFA and GPOFA are similar. Moreover, Johari et al. [14] compared XRD for GPOFA and UPOFA and found that the POFA phase was not modified by the treatment process, which included heat treatment and grinding.

3. Properties of fresh POFA concrete

3.1. Workability

Several studies have reported the effects of POFA on the workability of fresh concrete. Tay and Show [27] investigated the workability of POFA concrete by compacting factor. They found that with the increasing POFA replacement level, workability decreased. The results showed that the compacting factor of concrete with a 50%50 \% replacement level of POFA was 0.93 in comparison to 0.99 revealed for the OPC concrete. Sata et al. [46] assessed the amount of SP and the workability of concrete with different replacement levels of POFA, in terms of slump with a constant w/b\mathrm{w} / \mathrm{b} ratio of 0.28 . They reported that by increasing the amount of POFA from 10 to 30%30 \%, the quantity of SP necessary increased from 8.5 to 16.9 kg/m316.9 \mathrm{~kg} / \mathrm{m}^{3}, whereas the slump value decreased from 200 to 185 mm compared to the control mix. This is because of the high porosity of POFA particles, which causes the absorption of more water, thus resulting in a higher SP dosage. Chindaprasirt et al. [12] compared the workability of POFA, fly ash and OPC concrete. They revealed that POFA concrete required a higher w/b ratio compared to OPC concrete to obtain the desired slump value because of the irregular and angular shape and the porous nature of POFA. They reported that fly ash concrete required less water compared to POFA concrete to achieve the required slump because of its spherical-shaped particles and the solid texture of fly ash.

Johari et al. [14] investigated the workability of high-strength concrete mixes containing UPOFA ( 2.06μ m2.06 \mu \mathrm{~m} ) by replacing 50, 60, and 70%70 \% of cement and adding up to 2%2 \% SP in terms of the slump test based on BS EN 12350-2 [49]. They concluded that the slump values increased by increasing the UPOFA replacement. In addition, by using UPOFA, the workability of high-strength concrete increased because of the lower specific gravity of UPOFA compared to OPC. The lower specific gravity of the UPOFA binder paste volume fills the gaps between aggregates and acts as a lubricant to increase workability.

According to Table 3, Johari et al. [14] achieved higher slump with lower w/b ratio in comparison to the findings by Tangchirapat et al. [5]. This could be due to the reason that Johari et al. [14] used SP in their mix design which reduced water demand of concrete.

3.2. Setting time

Singh and Siddique [50] reported that the addition of POFA as a cement replacement in concrete increases its setting time. This could be a result of the increased demand of mixing water to achieve the desired workability. The added mix water lowers the pH value of the concrete

Table 2

and increases the distance between cement hydration products, resulting in a delay or decrease in the hydration activities of the cement particles. This delay in cement hydration in concrete by incorporating POFA causes an increase in both its initial and final setting time.

According to Tangchirapat et al. [25], POFA concrete has the longest setting time because of the pozzolanic reaction between calcium hydroxide (Ca(OH)2)\left(\mathrm{Ca}(\mathrm{OH})_{2}\right) and the pozzolan; it is typically slower compared to the hydration of cement. Furthermore, they showed that the setting time of POFA concrete decreases with the increased fineness of POFA.

The American [51] and British [28] standards require the initial setting time to be at least 45 min . According to ASTM C150 [51], the final
setting time should not exceed 375 min ( 6 h 15 min ). This value is required not to be longer than 10 h as specified in British standards (BS 12 [28]). All of the setting times for concrete that has various percentages of ash must meet the prescribed limits as per both the American and British standards.

3.3. Heat of hydration

Sata et al. [4] found that increasing the replacement level of GPOFA can reduce the peak temperature rise in concrete. In addition, the use of 30% GPOFA as a cement replacement produced the lowest peak

Fig. 3. Particle morphologies of the unground POFA [18,48].

Fig. 4. Particle morphologies of the ground POFA [18,25].
temperature rise and resulted in a 15%15 \% reduction of temperature compared to that of the control concrete (Table 4). Awal and Hussin [7] investigated concrete’s heat of hydration by a single replacement of 30% POFA and found that POFA has great potential in controlling concrete’s heat of hydration. Later, Awal and Shehu [24] recorded the temperature rise in large volume POFA (up to 70%) and found that replacing cement by a greater amount of POFA is beneficial, particularly for mass concrete in which thermal cracking is a matter of concern because of excessive heat. Mirza et al. [52] and Naseer et al. [53] reported that when OPC is mixed with water, it forms a hydrated binding cement paste (hcp) of C–S–H, and it liberates calcium hydroxide (CH). This reaction is generally quite rapid. However, when a pozzolan is present, its silica component reacts with liberated CH in hcp (in the presence of water) to form secondary CSH. This reaction is quite slow, resulting in a slow rate of heat liberation and slow strength development. This indicates that the pozzolanic reaction consumes lime instead of producing it.

4. Properties of hardened POFA concrete

4.1. Compressive strength

In concrete technology, compressive strength is indeed the most valuable physical property that concrete possesses. All other properties including tensile strength, flexural strength and modulus of elasticity are dependent upon concrete compressive strength. The strength development of concrete is influenced by the porosity of hydrated paste. This depends on the water/cement ratio and the hydrated paste. As mentioned earlier, the water demand is higher when POFA is used in concrete. In addition, unground POFA shows low pozzolanic reactivity because of its coarser particles and higher water demand. Therefore, as the amount of POFA is increased as a

Fig. 5. XRD patterns of GPOFA and TPOFA [20].
replacement of cement in concrete, its compressive strength will decrease. However, by grinding POFA and subjecting it to heat treatment, the compressive strength of concrete will increase because of its finer particles. Depending on the fineness of the POFA and the amount of cement replacement, different grades of strength have been reported by different researchers.

Tay [3] investigated the compressive strength of concrete mixes containing 10, 20, 30, 40 and 50% OPOFA as a cement replacement, with a constant w/b ratio of 0.6. It was reported that compressive strength decreases by increasing the level of POFA replacement. Thus, he recommended a mix with a 10%10 \% replacement level of cement by OPOFA because it would not significantly affect the long term concrete strength (a 1%1 \% decrease of compressive strength in comparison to the control mix).

Sinsiri et al. [29], Kroehong et al. [19] and Sujivorakul et al. [37] used increasingly fine POFA (FP) with a median particle size less than 11μ m11 \mu \mathrm{~m}. They reported that FP is a very reactive pozzolanic material, and it can be used in making high-strength concrete. Concrete with up to 10% FP possesses 28 day compressive strength ranging between 80 and 91 MPa .

Johari et al. [14] found that UPOFA with particles of a median particle size of 2μ m2 \mu \mathrm{~m} provides greater strength improvement (Fig. 6). It also allows for higher POFA replacement levels (up to 60%), which yielded a higher compressive strength at 28 days compared to control concrete. Altwair et al. [54] reported that by increasing the POFA/cement ratio (up to 0.2), compressive strength increased at 28 and 90 days. It achieved 30 MPa at 28 days with a 1.2 POFA/cement ratio.

4.2. Flexural strength

The literature has revealed that the published data on the flexural strength of POFA concrete are limited. The published data show that when cement is replaced by POFA in concrete, its flexural strength decreases. This mainly depends on the quality of the cement paste in the concrete mix. Because POFA is a porous material, when it is used in concrete as a cement replacement, the paste becomes lean, which decreases the flexural strength of POFA concrete.

Altwair et al. [15] assessed the flexural strength performance of environmental-friendly engineered cementitious composites (ECC) that contained different levels of GPOFA. To investigate the flexural performance of the ECC mixes, a 4-point bending test was used. These mixes had variable w/b ratios of 0.33, 0.36 and 0.38 with different amounts of TPOFA of 0,0.4,0.80,0.4,0.8 and 1.2 by weight of cement. The mix containing 0.4 TPOFA and a w/b ratio of 0.36 showed the highest flexural strength at 90 days of age. They concluded that all ECC beams caused a decrease in the flexural strength by increasing the w/b ratio and the POFA content (Fig. 7). Moreover, they reported that increasing the POFA content could lead to more cracks, and the ECC crack width declined significantly.

Table 3
Slump, setting times and compressive strength of concrete containing different percentage of POFA.

4.3. Split tensile strength

Sata et al. [46] investigated the split tensile strength of high-strength concrete mixes with OPC by replacing 10,20 and 30% of FP ( 10.1μ m10.1 \mu \mathrm{~m} ) at a w/b ratio of 0.28 . They found that mixes containing 10 and 20% FP exhibited a split tensile strength that was higher than the control mix at the age of 90 days. This was because of the high pozzolanic reactivity and great fineness of FP. However, Awal and Neguong [43] stated that

Table 4
Variation in temperature due to heat of hydration in POFA concrete.

the tensile strength of mixes containing high percentages of POFA (up to 50%) were lower than that of the control mix. Sata et al. [46] and Awal and Shehu [21] indicated that tensile strength development in both OPC and POFA concrete is similar to compressive strength development. Sata et al. [46] observed that the ratio of concrete splitting tensile strength and compressive strength varies between 6.3 and 6.9%. This is smaller than the corresponding values found for concretes with normal and medium level strength (approximately 8-10%). According to Fig. 8, Awal and Shehu [21] showed that the compressive strength and tensile strength of high-volume POFA concrete ( 50%50 \% POFA replacement) were lower than those of normal OPC concrete.

Altwair et al. [54] investigated the tensile strength of concrete with 0,0.40,0.4, and 1.2 POFA/cement ratios and a w/b ratio of 0.33 . They reported that the tensile strength of a specimen with a POFA/cement ratio equaling 0.4 was approximately 3.52 MPa , which was 9%9 \% higher than that the control mix and 16%16 \% higher than the concrete with a 1.2 POFA/cement ratio.

4.4. Modulus of elasticity

One of the key factors in designing the structural members is the modulus of elasticity (MOE). Several studies have investigated the effects of POFA on the concrete mixes.

Awal and Hussin [55] measured the static MOE of OPC concrete and concrete containing 30% POFA at the ages of 7 and 28 days. They demonstrated that the MOE of POFA concrete along with its compressive strength was lower than that of OPC concrete because of the lower pozzolanic reactivity of POFA compared to OPC. Sata et al. [46] investigated the MOE of the mixes with a w/b ratio of 0.28 and replacement levels of cement by GPOFA ( 10μ m10 \mu \mathrm{~m} ) at 10,20 and 30%30 \%. They stated that mixes

Fig. 6. Influence of ultrafine POFA on compressive strength [14].

Fig. 7. Comparison of ultimate flexural strength of ECC mixes at different w/b and POFA contents after 3, 28, and 90 days [15].
containing 20% GPOFA exhibited similar MOE values as the control mix (44.6 GPa).

In addition, Tangchirapat and Jaturapitakkul [41] compared the MOE in mixes with replacement levels of coarse POFA (CP) (19.9μ m)(19.9 \mu \mathrm{~m}) and FP (10.1μ m)(10.1 \mu \mathrm{~m}) of 10,2010,20 and 30%30 \% by weight of cement. It was shown that the MOE of concrete mixes that incorporated FP and CP were almost similar to OPC concrete. The MOE ranged between 25.0 to 28.0 GPa for mixes incorporating FP and CP whereas it was 27.5 GPa for OPC concrete. Similar results were also reported for concrete containing fly ash [56]. This is because the MOE of concrete is usually related to the aggregate strength rather than the paste strength [57]. As shown in Fig. 9, Tangchirapat and Jaturapitakkul [41] compared the MOE of concrete containing POFA and its compressive strength with the values suggested by ACI 318 [58]. They found that the MOE of concrete was 7%7 \% higher than that predicted by ACI 318 [58]. This could be because of the differences in the size of the testing samples and the source or type of coarse aggregate [57].

4.5. Drying shrinkage

Limited studies have been conducted to examine the effects of POFA on drying shrinkage.

Tay [3] observed that the drying shrinkage of POFA concrete containing 10% POFA was the same as the OPC control concrete. However, by increasing the unground POFA percentage as a partial cement replacement, the shrinkage increased slightly when measured on day 28.

Awal and Hussin [55] reported the shrinkage strain of POFA concrete to be approximately 19% higher than that of concrete with OPC alone

Fig. 8. Development of tensile strength in OPC and high volume POFA concretes [21].

Fig. 9. Relationship between modulus of elasticity and compressive strength of concrete [41]
after 2 days. However, as shown in Fig. 10a., Tangchirapat and Jaturapitakkul [41] suggested that the use of CP (20μ m)(20 \mu \mathrm{~m}) as a replacement for Portland cement by up to 30% binder weight cannot reduce the drying shrinkage of concrete. However, the use of 10-30% FP (10μ m)(10 \mu \mathrm{~m}) as a cement replacement reduced the drying shrinkage of concrete (Fig. 10b.). The very fine POFA caused pore refinement, the transformation of large pores into fine pores, and reduced the loss of water, thus reducing the drying shrinkage.

4.6. Microstructure

In their investigation, Altwair et al. [39] investigated the paste with 0,0.3 and 0.8 TPOFA/cement ratios at the age of 9 days. They found that by increasing the amount of TPOFA in the paste, small pores form in the structure, and C−S−H\mathrm{C}-\mathrm{S}-\mathrm{H} covers almost the entire fractured surface. The majority of the large spaces were filled with C-S-H gel, thus creating a dense structure. Therefore, the microstructure of the paste became denser (Fig. 11). Furthermore, Kroehong et al. [19] investigated the

Fig. 10. Drying shrinkage of concretes containing (a) coarse POFA (CP) and (b) fine POFA (FP) [41].

Fig. 11. SEM photos of hardened pastes at 90 day curing time: (a) POFA/C = 0; (b) POFA/C = 0.3; © POFA/C = 0.8 [39].
microstructure of the paste with a 20% level of FP (2.1μ m)(2.1 \mu \mathrm{~m}) as cement replacement at the age of 7 and 28 days. They reported that at the age of 7 days, the paste was porous with many voids. This is because of the production of ettringite needles during the process of hydration. However, after 28 days, the paste was denser because of the increased pozzolanic reactivity and hydration rate.

Kroehong et al. [59] investigated the effects of FP (2.1μ m)(2.1 \mu \mathrm{~m}) and CP (15.6μ m)(15.6 \mu \mathrm{~m}) on the microstructure of the paste at 28 and 90 days. They noted that in the paste containing FP, the Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2} content decreased as a result of the pozzolanic reactivity compared to the paste with CP. This caused a lower total porosity and a higher compressive strength of the FP paste than the paste containing CP. Moreover, as the curing period increases from 28 to 90 days, there is a decrease in the formation of Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2}, which leads to an increased peak intensity of C∼S∼H\mathrm{C} \sim \mathrm{S} \sim \mathrm{H} and C2ASH8\mathrm{C}_{2} \mathrm{ASH}_{8} phases and a denser paste.

5. Durability properties of POFA concrete

5.1. Permeability

The studies showed that the permeability of POFA concretes is a function of the amount of cement replacement, the fineness of the POFA particles, and the concrete age. Because of the increased fineness of POFA particles and its level of addition in concrete, permeability decreases. Moreover, permeability of POFA concrete decreases by increasing the curing age and decreasing the water-to-cement ratio.

Sumadi and Hussin [13] found that the water permeability of POFA concrete decreased with increasing age ( 7 days to 360 days) because of the formation of additional gel from the pozzolanic reactivity of ash. Chindaprasirt et al. [12] investigated the water permeability of highstrength concrete mixes with 20,40 and 55%55 \% replacement levels of FP (8.0μ m)(8.0 \mu \mathrm{~m}) as OPC. They stated that concrete incorporated with 20 and 40%40 \% FP showed lower values of permeability compared to the OPC control concrete at the age of 28 days. The water permeability of the mixes containing FP at 90 days was even lower than that at 28 days. At both ages, the w/b ratios of the concrete mixes with FP were higher than that of the control mix. This could be a result of the secondary reaction between OPC and FP, thus forming additional gel at the later age [52,53]. In contrast, the permeability of concrete containing 55% POFA rapidly increased because of the low cement content and high w/b ratios.

Tangchirapat et al. [10] reported the lowest water permeability in high-strength concrete containing 20% GPOFA compared to other amounts of replacement. Moreover, it was demonstrated that replacing cement with GPOFA with a binder weight as high as 30% slightly increased the water permeability of concrete. Additionally, the water permeability of all high-strength concrete that contained GPOFA was almost half as much as that of the control OPC concrete. These researchers proposed that GPOFA can improve the porosity and pore size, thus producing highly impermeable and dense concrete.

Tangchirapat and Jaturapitakkul [41] investigated the relationship between the compressive strength and water permeability of concrete at 28 and 90 days. They found that as the compressive strength increased, the water permeability values of GPOFA and OPC concretes
decreased. This behavior has two explanations: (1) GPOFA had a pozzolanic reactivity that led to denser concrete; and (2) the specific gravity of GPOFA was much lower than that of OPC; thus, the former had a larger amount of paste with less aggregate compared to OPC concrete, which resulted in lower water permeability.

Johari et al. [14] investigated the water and gas permeability of the high-strength green concrete (HSGC) mixes by replacement levels of 0, 20, 40 and 60% UPOFA ( 2.06μ m2.06 \mu \mathrm{~m} ) as cement. They reported that the amount of gas permeability in the mixes incorporating UPOFA showed a constant decline. The mix containing 60% UPOFA exhibited a 76% reduction in the amount of gas permeability compared to the control mix. In addition, their study revealed that adding UPOFA to mixes as a cement replacement considerably reduces the water permeability of the HSGC, particularly at 28 days of water curing (Fig. 12). After 3 days of age, UPOFA lowered the coefficient of water permeability despite seemingly causing a trend of higher water permeability coefficients at higher UPOFA contents, which could contribute to the dilution effect (Fig. 12). They reported that HSGC with UPOFA replacement at 20, 40 and 60% as cement showed a reduction in the water permeability coefficient of 18, 24 and 33%, respectively, compared to the control mix at 28 days. This is because of the increased reactive nature of the UPOFA.

It has been reported [60], [61], [62] that with the substitution of OPC by POFA, in general, the water absorption decreases as the level of substitution of cement by ash in mortars increases, indicating their high durability against permeability. This decrease was because the pozzolans can fill the micropores in the cement matrix and can increase the durability of cements significantly by changing the framework of the matrix. It has also been reported [63] that the use of pozzolans in cement concrete reduced permeability and improved resistance against the corrosion of steel in concrete. Because of the continued formation of hydrates, the pores are filled. Furthermore, because of the absence of free lime, which could be leached out, the partial replacement of OPC by pozzolans reduces the permeability of concrete by 7 to 10%. Moreover, pore

Fig. 12. Coefficient of water permeability of HSGC with different ultrafine POFA contents [14].

Fig. 13. Impact of temperature rise on compressive strength of POFA concrete [70].
size distribution studies of hydrated pozzolanic cements have shown that the reaction products are very efficient in filling up the larger capillary pores, thus improving the strength and impermeability of the system [52,53].

5.2. Resistance to chloride attack

Chindaprasirt et al. [45] evaluated the resistance to chloride penetration of mortars containing 20 and 40%40 \% POFA by using the rapid chloride penetration test (RCPT), the rapid migration test (RMT) and the immersion test. As per ASTM C39 [64], 100×200 mm100 \times 200 \mathrm{~mm} cylinders were prepared to determine the rapid chloride penetration. After 27 days of water curing, the cylinders were cut into 50−mm50-\mathrm{mm} slices and coated with epoxy. According to ASTM C1202 [65], the epoxy-coated specimens were conditioned and tested at the age of 28 days. The RCPT results reported that the mortars containing 20 and 40%40 \% POFA reduced the charge passed to 1900 and 1050 coulomb ©, respectively, compared to 7450 C of the control mix. The results of the RMT using 30 V dc were the same as the RCPT results. They stated that the incorporation of 20 and 40%40 \% of POFA reduced the penetration depths to 7.0 and 4.5 mm , respectively, compared to the depth of 16.0 mm in the control mix. The immersion test confirmed the results of the RCPT and the RMT by showing that
the incorporation of POFA improves mortar’s resistance to chloride penetration. This is because of the increased pozzolanic reactivity, the reduced Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2}, and the improved permeability of the mortar.

Hussin et al. [44] investigated the chloride penetration of concrete containing different fineness levels of POFA at 7, 28 and 90 days. They observed that a higher fineness of POFA accelerates the pozzolanic reaction, resulting in the formation of extra C-S-H gel, thus creating denser concrete and enabling it to exhibit better resistance to chloride attack. Johari et al. [14] investigated the RCPT on concrete containing 20, 40 and 60%60 \% UPOFA as a cement replacement. The results reported that the total charge passed (TCP) in the UPOFA concrete was reduced compared to the control mix. High-strength concrete with 60%60 \% UPOFA reduced 84%84 \% of the TCP compared to the control mix.

5.3. Resistance to sulfate attack

Several research studies have evaluated concrete’s resistance to sulfate attack by conducting two tests to evaluate the expansion and reduction in compressive strength. Tangchirapat et al. [10] and Jaturapitakkul et al. [47] assessed the expansion and compressive strength loss of concrete bars with different replacement levels and particle sizes of POFA that were exposed to 5%MgSO45 \% \mathrm{MgSO}_{4} solution for 2 years. They reported that increasing the fineness of POFA resulted in a lower loss in compressive strength and expansion than the control mix. Hussin et al. [44] investigated the effects of different fineness levels of POFA particles on the expansion and compressive strength loss of concrete in a 10%10 \% Na2SO4\mathrm{Na}_{2} \mathrm{SO}_{4} solution for 3 months. The losses in compressive strength of mixes containing 45μ m45 \mu \mathrm{~m} and 10μ m10 \mu \mathrm{~m} POFA and the control mix that were immersed in 10%Na2SO410 \% \mathrm{Na}_{2} \mathrm{SO}_{4} solution were approximately 0.77,0.640.77,0.64 and 7.00%7.00 \%, respectively, compared to the same mixtures that were cured in water. The results of the expansion in mortar bars were similar to the losses in compressive strength. They revealed that mortar bars containing 10μ m10 \mu \mathrm{~m} POFA showed a reduced expansion compared to both the control mix and mortar bars containing 45μ m45 \mu \mathrm{~m} POFA. This is because of the FP particles ( 10μ m10 \mu \mathrm{~m} ), which hasten the pozzolanic reactivity by forming secondary C-S-H gel in a short time, thus creating denser concrete that is more resistant to sulfate attack compared to the concrete containing CP particles ( 45μ m45 \mu \mathrm{~m} ). Tangchirapat et al. [10] assessed the sulfate resistance of concrete that incorporated 10, 20 and 30% UPOFA in terms of expansion and strength reduction at 180 days. These mixes of UPOFA were immersed in a 10%MgSO410 \% \mathrm{MgSO}_{4} solution and expanded by 0.016,0.0150.016,0.015, and 0.017%0.017 \%, respectively; these values were lower than that of control mix ( 0.021%0.021 \% ). The same findings were also obtained in a loss of strength test. The specimens containing UPOFA exhibited a lower percentage reduction in compressive strength compared to the control mix. The lower expansion and loss of strength in concrete containing UPOFA was because of the high pozzolanic

Fig. 14. Effect of cooling system on cracks pattern POFA concrete [70].

Fig. 15. POFA aerated concrete at the magnification of 2000×2000 \times magnification [76].
reactivity and the reduction of Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2} in concrete, which led to the reduction in the formation of gypsum and ettringite.

5.4. Resistance to carbonation

It is also known that carbonation, which is a consequence of the transformation of Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2} to CaCO3\mathrm{CaCO}_{3}, alters the microstructure of cement paste by decreasing porosity [66].

Chindaprasirt and Rukzon [33] reported that the total porosity of mixed cement pastes can be increased compared to that of OPC paste when POFA is employed. When these pastes were subjected to 50%50 \% relative humidity (RH) and 5%CO25 \% \mathrm{CO}_{2} for 28 days, the pore voids were filled, and attacks on C-S-H were likely to occur. The overall porosity declined after carbonation, and there was a decrease in the specific surface areas of the cement pastes mixed with POFA, which indicated the void infilling. The increase in the large POFA pores signified that the pores were widening, mainly because of the possible attack on C-S-H.

Chindaprasirt and Rukzon [33] indicated that the incorporation of FP caused the carbonation depth to decrease compared to cases in which coarse particles were used. They demonstrated that the amount of SP required was reduced by FP and that the mortar strength was improved compared to coarser POFA concrete. As a result of the drop in the amount of Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2} of the cement hydration products, the partial substitution of OPC with POFA augmented mortar carbonation. Moreover, they noted that because of the enhanced dispersion and filler effect,

Fig. 16. OPC aerated concrete at the magnification of 2000×2000 \times magnification [76].
using FP caused a fairly low carbonation compared to the use of CP, even with acceleration in the pozzolanic reaction. POFA can serve as a favorable pozzolan replacing part of Portland cement in producing mortar that exhibits a low carbonation depth and a high strength.

5.5. Performance of POFA concrete at elevated temperatures

Fire resistance is an important issue because concrete must be able to preserve its structural actions for a prescribed timespan; this is known as its fire rating [67]. The type of aggregates, cement used, moisture and porosity affect the fire resistance capacity of normal concrete [68]. According to Khoury [69], the judicious choice of materials can allow the compressive strength of Portland-based cement paste to be maintained without a major loss at temperatures up to 550∘−600∘C550^{\circ}-600^{\circ} \mathrm{C}.

Ismail et al. [70] studied the effects of elevated temperature, which involved water spray and air-cooling systems, on the properties of POFA concrete. In their investigation, 28-day concrete samples were exposed to 800∘C800^{\circ} \mathrm{C}, and a greater amount of mass loss was observed. At temperatures lower than 500∘C500^{\circ} \mathrm{C}, a greater mass loss occurred in moisture than in OPC. At 800∘C800^{\circ} \mathrm{C}, the mass loss of concrete containing 20%20 \% POFA was lower than OPC concrete. This can be attributed to the smaller Ca(OH)2\mathrm{Ca}(\mathrm{OH})_{2} content in the POFA mixture. As observed in Fig. 13, residual compressive strength continuously declines as the temperature increases, except for the case of air-cooling samples. These authors associated the upsurge in POFA concrete residual strength between 100∘C100^{\circ} \mathrm{C} and 500∘C500^{\circ} \mathrm{C} via air-cooling to several different factors: the fineness of the POFA used in the mix, changes in thermal strain, and the hydration of unhydrated cement. In POFA concrete, more cracks appeared in the air-cooled samples than in the water-cooled surface, as shown in Fig. 14. Furthermore, Ismail et al. [70] reported that at a temperature of approximately 350∘−550∘C350^{\circ}-550^{\circ} \mathrm{C}, residual performance enhancements may be achieved in favor of POFA concrete compared to its original performance, particularly when water has not been employed in the cooling process. They investigated the surface discoloration of firedconcrete and observed that the penetration of the surface color into the concrete depended on the amount of inward heat. Clear surface discoloration appeared as deep as 10 mm , specifically in samples that were subjected to 500∘C500^{\circ} \mathrm{C} or lower temperatures; the color entered deeper in the specimens that were heated up to 800∘C800^{\circ} \mathrm{C}.

6. POFA as a cement replacement in aerated concrete

6.1. Aerated concrete

According to Tam et al. [71] and Ramamurthy et al. [72], aerated concrete refers to either a mortar or cement paste, which is categorized as light weight concrete. They conducted the first comprehensive review on the properties, composition, and worldwide use of cellular concrete. Following Tam et al.'s [71] work, others used aerated concrete with a wide range of density (between 300 and 1800 kg/m31800 \mathrm{~kg} / \mathrm{m}^{3} ), which shows greater flexibility for its potential uses.

6.2. Mechanical properties of POFA aerated concrete

Abdullah et al. [16] studied the potential of POFA to replace cement in aerated concrete. It was revealed that a high percentage of POFA in aerated concrete causes the compressive strength to decline. Nonetheless, remarkable improvement in aerated concrete strength from 7 to 28 days can be realized by concrete containing as high as 40%40 \% POFA. It was concluded that replacing 10 to 35%35 \% of cement with POFA is likely to allow for the production of aerated concrete.

Abdullah et al. [74] investigated aerated concrete that was incorporated with 0 to 50%50 \% of POFA as cement replacement at the age of 28 days. They stated that aerated concrete incorporating 10, 20 and 30%30 \% POFA produced lightweight concrete that exhibited a strength that fulfills the requirement of ASTMC129-85 [75]. Nevertheless, the

highest strength was obtained at 20% replacement by POFA. Gradual reductions in strength were observed beyond the replacement level of 20% POFA. This is because higher amounts of POFA replacement result in a lower amount of calcium oxide because of the lower cement content.

Hussin and Abdullah [76] demonstrated that up to 30% OPC replacement with POFA was able to produce an aerated concrete panel exhibiting strength equivalent to that of an aerated concrete panel containing 100% cement. The maximum strength for aerated POFA concrete occurred at the replacement level of 20%. Moreover, they found that curing the panel in 100% saturated conditions, such as water curing, led to lower strength development and exhibited very high deformation under the continuous load increment compared to other curing methods that minimized the contact of water. Thus, air curing could be a good method to achieve a higher strength performance for aerated concrete panels and lower deformation when load is applied.

Hussin et al. [18] investigated drying shrinkage and MOE of aerated concrete containing 20% POFA, 0.30% Aluminum powder and 0.75% SP. They reported that POFA-aerated concrete is able to exhibit lower drying shrinkage values compared to that of OPC aerated concrete as light weight concrete. In addition, they reported that POFA-aerated concrete demonstrated higher MOE values than the aerated OPC concrete. This is because of that pozzolanic reaction of POFA that produces extra C–S–H gels contributing toward densification of the internal structure of this material and thus making this lightweight concrete stiffer.

6.3. Microstructure study of POFA aerated concrete

Hussin and Abdullah [76] revealed that the microstructure of POFAaerated concrete is denser than OPC concrete (Figs. 15 and 16). Moreover, at 20% cement replacement with POFA, a dense structure is formed as a result of the crowded C–S–H produced in this agro-blended cement-based aerated concrete. This structure also increases the strength of this environmental friendly concrete.

6.4. Fire resistance capacity of POFA aerated concrete

Abdullah and Hussin [77] studied the fire resistance properties of POFA cement-based aerated concrete. They reported that POFAaerated concrete can be classified as non-combustible material because of clause 204 in the Malaysia Uniform Building By-Laws [78] and that it can be used for application in wall construction.

Fig. 17. Compressive strength of BAG and OPC concretes exposed to elevated temperature [90].

7. Utilization of POFA in blended ash geopolymer concrete

7.1. Geopolymer concrete

To achieve an environmental-friendly concrete, several studies are on-going regarding the utilization of waste materials to produce green concrete. Among the many research studies, a successful study developed geopolymer concrete to eliminate the use of cement [79]. In terms of reducing global warming, Gartner [80] reported that the geopolymer could reduce CO2\mathrm{CO}_{2} emissions to the atmosphere caused by cement and aggregate industries by approximately 80%.

Davidovits [81] first used geopolymers to produce alkali aluminosilicate binders generated through the alkali silicate activation of aluminosilicate substances as an alternative binder to OPC. Van Jaarsveld et al. [82] reported that geopolymer synthesis depends on the use of materials rich in aluminum-silicate glass activated with alkaline solutions. Several research studies have investigated the effects of using various agro-industrial wastes, such as ground granulated blast furnace-slag (GGBS), fly ash (FA), rice husk ash (RHA) and POFA, as binder ash on the properties of blended ash geopolymer (BAG) concrete. Lately, thermal conductivity and the mechanical transport properties of lightweight aggregate foamed geopolymer concrete that incorporated POFA was assessed [83].

7.2. Mechanical properties of geopolymer concrete using POFA as blended ash

Ariffin et al. [40] used pulverized fuel ash (PFA) and POFA from agroindustrial waste as a 100% replacement of cement with a combination of sodium silicate and sodium hydroxide solution used as alkaline liquids to manufacture geopolymer concrete. They reported that the compressive strength increased gradually with the increase in the PFA:POFA ratio. The highest compressive strength of 25 MPa was achieved at the blended ash ratio of 70:30 (PFA:POFA). In addition, the compressive strength of geopolymer concrete also increased gradually with a prolonged curing period. The strength improved significantly (up to 25 MPa) at a curing period of 28 days.

Karim et al. [84] fabricated a non-cement composite binder (NCB) from slag, POFA and RHA with NaOH as the activator. They found that the combination of 42%42 \% slag, 28%28 \% POFA and 30%30 \% RHA with 5%NaOH5 \% \mathrm{NaOH} at 28 days achieves the highest compressive strength of 40.68 MPa and a flexural strength of 6.57 MPa .

Liu et al. [85] used POFA and FA as binder in lightweight oil palm shell geopolymer concrete (OPSCC). They revealed that the compressive and tensile strength of OPSCC increased with increases in POFA content up to 20%; however, a higher replacement of POFA resulted in a significant reduction in strength.

Mijarsh et al. [86] investigated the main parameters affecting the compressive strength of geopolymer concrete using a large amount of TPOFA based on the Taguchi method. They found that the optimum amount of TPOFA used to synthesize the optimum geopolymer mortar is 65%65 \% with 35%35 \% additive material (Ca(OH)2\left(\mathrm{Ca}(\mathrm{OH})_{2}\right., silica fume and Al(OH)3)\left.\mathrm{Al}(\mathrm{OH})_{3}\right), resulting in a compressive strength of 47.27±5.0MPa47.27 \pm 5.0 \mathrm{MPa} at 7 days of curing.

Ranjbar et al. [87] evaluated the compressive strength of geopolymer concrete containing POFA:fly ash at ratios of 0–100%. They reported that fly ash-based mixes with replacement of 0 to 25%25 \% POFA obtained approximately 70 and 98%98 \% of its 28 -d compressive strength at days 3 and 7. POFA-based mixes with contents of 50, 75 and 100% POFA ( 50 to 0%0 \% fly ash) gained only approximately 40 and 62%62 \% of their 112-d strength just after days 3 and 7 . This could be because of the high SiO2/Al2O3(Si/Al)\mathrm{SiO}_{2} / \mathrm{Al}_{2} \mathrm{O}_{3}(\mathrm{Si} / \mathrm{Al}) aspect ratio in POFA-based geopolymer mixes. The increase in the Si/Al\mathrm{Si} / \mathrm{Al} ratio results in the reaction of Al content and advanced stages, providing more silicate for condensation and reaction between silicate species, which causes the dominance of oligomeric silicates. The domination of Si content reduces the rate of condensation,

resulting in the gradual hardening of the geopolymers. As a result, an increase in the POFA/fly ash ratio in geopolymer mortars delays the ultimate compressive strength.

Islam et al. [79] investigated the development of the compressive strength of geopolymer concrete incorporated with fly ash, POFA and GGBS as binders at the age of 28 days. The compressive strength of a mix containing 100% POFA showed a 50% lower compressive strength compared to a mix containing 100% fly ash. Any increase beyond a 30% increase in POFA content decreases the strength. Because of the cohesive characteristics of coarser particles, the POFA could not be mixed properly, and hence the strength development was poor. The mix containing 30% POFA and 70% GGBS exhibited the highest strength of 66 MPa .

7.3. Geopolymer concrete durability with POFA as blended ash

7.3.1. Resistance to sulfate and sulfuric acid attack

Ariffin et al. [88] and Bhutta et al. [89] studied blended ash geopolymer (BAG) concrete by incorporating PFA and POFA with alkaline activators in contact with 2%2 \% sulfuric acid and 5%5 \% sodium sulfate solutions for 18 months. They concluded that geopolymer concrete exhibited higher resistance to acids compared to OPC concrete because of the complete elimination of cement in the mixture. After 18 months of exposure to 2%2 \% sulfuric acid solution, geopolymer concrete containing POFA and PFA experienced an 8%8 \% mass loss, which was much less than the OPC concrete mass loss (20%). Moreover, geopolymer concrete showed a 35%35 \% loss in its compressive strength whereas OPC concrete lost 68%68 \% of its strength after 18 months. Bhutta et al. [89] reported that there was no significant damage to the surface of geopolymer concrete after exposure to 5%5 \% sodium sulfate solution for one year.

7.3.2. Performance of geopolymer concrete using POFA as blended ash at elevated temperatures

Hussin et al. [90] investigated the influence of high temperatures on the properties of geopolymer concrete containing POFA and PFA. They observed that geopolymer concrete showed a better structural stability than OPC concrete after exposure to 800∘C800^{\circ} \mathrm{C}. Surface cracks appeared at temperatures between 600 and 800∘C800^{\circ} \mathrm{C} for geopolymer concrete whereas they were observed on OPC concrete at a temperature of 200∘C200^{\circ} \mathrm{C}. Fig. 17 shows the compressive strength of BAG concrete compared to OPC concrete exposed to different elevated temperatures. The geopolymer concrete containing POFA showed an average strength loss of 16%16 \% compared to the 50%50 \% strength loss in OPC concrete and exhibited the best performance. Furthermore, they also observed that the strength of BAG concrete increased as the temperature increased.

Ranjbar et al. [91] investigated the effects of fly ash-based geopolymers containing POFA:fly ash ratios of 0−100%0-100 \% at elevated temperature of up to 1000∘C1000^{\circ} \mathrm{C}. Increasing the POFA content in the fly ashbased geopolymer mortars reduced the initial pore formation upon exposure to elevated temperatures. This decreased the compressive strength and density of the mixture. The pore size became significantly larger by increasing the temperature beyond 800∘C800^{\circ} \mathrm{C}. The high POFA content specimens deformed once they were subjected to temperature beyond 800∘C800^{\circ} \mathrm{C}. In contrast, the fly ash-based geopolymers had more thermal stability and maintained their shapes up to 1000∘C1000^{\circ} \mathrm{C}. As mentioned earlier, this could be because of the high silica content in POFA causing a higher Si/Al\mathrm{Si} / \mathrm{Al} ratio as the POFA content is increased.

8. Conclusions

The following conclusions are drawn:

1. Concrete compressive strength decreased up to 50%50 \% by the addition of OPOFA as partial replacement of cement. This is because of the low pozzolanic reactivity and high water demand of OPOFA that reduced the workability and strength of POFA concrete.
2. After grinding process, fineness and pozzolanic reactivity of POFA significantly increased. By using 20%20 \% fine POFA by weight of cement, high-strength concrete can be produced with lower drying shrinkage, lower water permeability and higher resistance to chloride and sulfate attack than OPC concrete.
3. Heating ( 500∘C500^{\circ} \mathrm{C} for 1 h ) and regrinding GPOFA produced ultra-fine POFA. As the ultra-fine POFA increased 20 to 40%40 \% by weight of cement, compressive strength of POFA concrete improved up to 14%14 \% relative to OPC concrete.
4. Geopolymer concrete containing blended fly ash and POFA or slag and POFA in a 70:30 ratio resulted in 25 and 66 MPa compressive strength respectively at 28d; similar or much higher than OPC concrete.

9. Recommendations

1. Very few studies have reported on the use of POFA in mortars and concrete for properties such as plastic shrinkage, bleeding and flexural strength. Therefore, there is a definitive need to study these properties.
2. A thorough evaluation of OPC mortars and concrete that contain POFA should be performed regarding their long-term performance and durability characteristics.
3. The optimum mortar and concrete mixtures containing OPC and POFA that display satisfactory durability properties in the laboratory or in small-scale tests should be tested in large-scale field tests to determine their performance.
4. The pozzolanic reactivity of ternary blends, i.e., mixtures of OPC, POFA and other abundantly available local pozzolanic materials, such as fly ash and rice-husk ash, must be studied.

Acknowledgments

Thanks for the financial support provided by Universiti Teknologi Malaysia through the Ministry of Education Malaysia under OTR grant, Vot. R.J1300000.7301.4B145. Appreciation is also to the support given by the Japan International Cooperation Agency (JICA) under the scheme of SATREPS Program (Science and Technology Research Partnership for Sustainable Development) for the project Development of Low Carbon Society Scenarios for Asian Region.

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