strength activity index and microstructural

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StrengthActivityIndexandMicrostructuralCharacteristicsofTreatedPalmOilFuelAshDATASETFEBRUARY2014

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International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 05 100

115905-7676 IJCEE-IJENS October 2011 IJENS I J E N S

Strength Activity Index and Microstructural Characteristics of Treated Palm Oil Fuel Ash

Nurdeen M. Altwair, Megat Azmi Megat Johari and Syed Fuad Saiyid Hashim

Abstract The strength activity index of mortar and microstructural characteristics of pastes containing treated palm oil fuel ash (POFA) have been investigated. POFA obtained from a palm oil mill was treated via sieving, grinding and heating at temperature of 450C for 90 minutes in order to improve the pozzolanic reactivity of the POFA. The pozzolanic reactivity of the treated POFA was evaluated by conducting strength development tests according to ASTM C311. The hydration products of hardened pastes were analyzed by means of thermogravimetric analysis (TGA), x-ray diffraction (XRD), and scanning electron microscopy (SEM) in order to quantify the influence of the treated POFA which was used at different POFA/cement ratio ranging from 0 to 0.8. After 28 days, the strength activity index of the treated POFA with ordinary Portland cement exhibited very good performance and was higher than 100%. At 90 days, the strength activity index increased to 101.72 %. Using TGA, XRD, and SEM, a significant reduction in Ca(OH)2 content was observed with increasing amount of treated POFA. The development of C-S-H gel was higher when POFA/cement ratio was raised up to 0.3.

Index Terms - Palm oil fuel ash; Strength activity index; Hydration products.

1. INTRODUCTION

Compounds present in ordinary Portland cement, such as C3S and C2S, are known to react with water and form calcium silicate hydrates (C-S-H) and calcium hydroxide (Ca(OH)2) [1]. Approximately 70% C-S-H, 20% Ca(OH)2, 7% sulfoaluminate, and 3% secondary phases are formed as a result of the aforementioned reaction [2].

Dr. Megat Azmi Megat Johari is currently and Associate Professor at the School of Civil Engineering, Universiti Sains Malaysia 14300 Nibong Tebal, P. Pinang, Malaysia (E-mail: [email protected]). Nurdeen M. Altwair is with the School of civil Engineering, Universiti Sains Malaysia 14300 Nibong Tebal, P. Pinang, Malaysia, (corresponding author phone: +60174158209; e-mail: [email protected]). Dr. Syed Fuad Saiyid Hashim is a Senior Lecturer at the School of School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang, Malaysia (E-mail: [email protected]).

Silica, alumina, and iron oxides are found in agricultural residues. To directly replace cement, agro-wastes require thermal treatment and are milled to small particle size to improve their pozzolanic reactivity. Burning such wastes produces crystalline phases or crystallization of amorphous material [3]. When an agro-residue is utilized in concrete, two important factors should be considered; namely ash content and chemical constituents [4].The silica content of the ash is another important factor because, when ash from certain agricultural byproducts (e.g., rice husk and bagasse) is added to cement, the silica reacts with Ca(OH)2 to form additional C-S-H in the hydrated cement matrix, which increases the density of the matrix, and refines the pore structure [5, 6].

Malaysia is one of the largest producer of palm oil with around 41% of the total world supply in years 20092010[7]. After the extraction of oil from fresh fruit bunches, considerable amount of solid waste by-products in the form of nutshells, fibers, and empty bunches, (i.e., more than 70 % of fresh palm oil fruit) are discharged from the mill [8].This waste is reused by the same industry as fuel in boilers for the production of steam to generate electricity and run internal operations, leaving behind 5% ash [9, 10] known as palm oil fuel ash (POFA). However, because of the deficiency in nutrients required in fertilizers, POFA is thrown into wastelands surrounding the palm oil mill, causing environmental problems and health hazards [8, 11]. With the aim of finding a solution to this issue, various studies have been carried out to determine the feasibility of using POFA in concrete and mortar. These studies have revealed that POFA can be used as a supplementary cementitious material when properly processed [12]. Majority of these studies were conducted on ashes obtained directly from palm oil mills to establish the pozzolanic activity and suitability of POFA as binders after grinding [13-15]. The POFA contains high amounts of unburned matter, silicon, and aluminum. In general, unground POFA is light gray because of the unburned carbon content left at a relatively low burning temperature. The colour becomes dark gray in the case of ground POFA [16]. The unburned carbon is the most significant factor to consider. The unburned carbon particles result in an increase in water requirement and dosage of super plasticizer (SP) because SP is absorbed by carbon particles [13, 17].



Studies have been conducted on the effect of POFA on the engineering properties of mortar and concrete, including their mechanical and durability related properties. However, the influence of adding different amounts of POFA without unburned carbon on pozzolanic reaction has not been well established. Recognizing the potential technical benefit of treated POFA on pozzolanic activity and microstructure of cement paste, which could lead to the greater utilization of POFA in concrete and subsequently could be useful in protecting the environment by minimizing the volume of waste disposed on the wasteland, has initiated the current research work. Consequently, the aim of the present research work is to investigate the pozzolanic reactivity of treated POFA mortars and to study the microstructure of POFA-cement pastes through the analysis of hydration products by thermogravimetric analysis (TGA), x-ray diffraction (XRD), and scanning electron microscopy (SEM).

2. MATERIALS AND METHODS

2.1 Materials

Ordinary Portland cement (OPC) supplied by Cement Industries of Malaysia, Berhad was used as the main binder material. This cement has a specific gravity of 3.15 and a Blaine surface area of 340 m2/kg. Table 1 shows the chemical compositions of the OPC as well as the treated POFA.

Table1. Chemical Constituents of OPC and treated Palm Oil Fuel Ash.

Chemical constituents (%) OPC Treated POFA Silicon dioxide (SiO2) 20.9 66.91 Aluminum oxide (Al2O3) 5.27 6.44 Ferric Oxide (Fe2O3) 3.1 5.72 Calcium oxide (CaO) 62.8 5.56 Magnesium Oxide (MgO) 1.52 3.13 Sodium oxide (Na2O) 0.16 0.19 Potassium oxide (K2O) 0.63 5.20 Sulfur oxide (SO3) 2.73 0.33 Phosphorus oxide (P2O2) 0.13 3.72 LOI 0.87 2.3

POFA was collected from a nearby palm-oil mill, United Oil Palm Industries Sdn. Bhd. located in Nibong Tebal, Penang, Malaysia. The removal of excess carbon and other unburned organic materials contained in POFA is important to avoid their potential negative effect on hydration. Thus, the POFA was dried in an oven at 100C for 24 h and then sieved using a set of sieves (3 mm, 600 m, and 300 m sieves) to remove the particles coarser than 300 m. The average particle size of POFA before milling was around 74.29 m with the specific surface area around 540 cm2/g. The untreated POFA was then ground in

a ball mill to reduce the particle size to improve reactivity. The milling time was approximately 6 hours at 45 rpm. To prevent glassy phase crystallization and particle agglomeration both of which could affect the pozzolanic properties, untreated POFA was heated at low temperature of 450C for 1.5 hours in an electric furnace. After the heat treatment, the colour of treated POFA turned from light brown to grayish red after the unburned residue was removed. Under the said method of treatment and temperature conditions, the agglomeration and crystallization of glassy phase of POFA particles did not occur during the heating process (Fig. 1).

Fig. 1. XRD patterns of POFA before and after treatment (Q-Quartz; C-Cristobalite; S-Amorphous silica; K-potassium aluminum phosphate K3Al2(PO4)3). The specific surface area of the treated POFA was around 6200 cm2/g. The average particle sizes of untreated and treated POFA were approximately 2.87 and 2.99 m, respectively. The treated POFA particles were irregular in shape and having porous texture. In addition, there was no agglomeration of POFA particles after the heat treatment as can be seen in Figs. 2 and 3. The main component of the treated POFA is SiO2, and the total amount of SiO2, Al2O3, and Fe2O3 is 79.07 (Table 1), which indicates that the chemical compositions of the treated POFA was well within the specifications set by ASTM C618-05[18].

Fig. 2. Cumulative particle size distribution curves of POFA before and after treatment.

0.1 1 10 100

Cum

ula

tive

pass

ing

(%)

Particle size (m)

Ground POFA after treatment

Ground POFA before treatment



Fig. 3. SEM photographs of treated POFA.

2.2 Pozzolanic activity method

The pozzolanic activity of the POFA has been determined based on compressive strength according to ASTM C311. Mortar containing 20 % POFA partially replacing the OPC was tested. The control mixture was prepared with 1500 g of OPC, 4125 g of graded sand and 726 g of water. The test mixture was prepared with 1200 g of OPC, 300 g of POFA, 4125 g of graded sand and the required quantity of water to obtain a flow of 110% of the control mixture. Fifty-millimeter cubes were cast for the present study. After moulding, the specimens and moulds were placed in the moist room at maintained 23 2C for 24 hours. Then the cube samples were removed from the moist room, and demoulded from their respective moulds. The cubes were then placed and stored in saturated lime water. The compressive strength was determined from the average of three specimens at the ages of 3, 7, 14, 28, and 90 days. The cubes were tested using a universal testing machine (Model UH-F1000kNI) test system with a 1000 kN capacity loading frame. The compressive strength was measured using a constant loading rate of 21 MPa/min.

2.3 Microstructural characteristics

A total of seven POFA-cement pastes and a control paste were prepared to study the hydration products (Table 2). The water/binder ratio was fixed at 0.29.The cement paste and POFA-cement pastes were tested and analyzed using TGA (LINSEIS THERMOWAAGE L81, XRD (RIGAKU D/Max 2000), and SEM (KEYENCE, VE-9800). The tests were conducted after 90days of curing to estimate the hydration products. Analyses using TGA, XRD and SEM were performed to assess the reaction with calcium hydroxide. XRD analysis was carried out in terms of qualitative values. The analysis was based on the intensity of the peak corresponding to Ca(OH)2 in the samples.TGA is defined as the technique whereby the mass of a substance in a heated environment is recorded at a controlled rate as a function of time or temperature[19]. In addition, TGA is more suitable for studying the hydration or pozzolanic

reaction that takes place at later stages [20]. Thermo-grams after 90-days of curing were obtained at a temperature of 251000C at a rate of 6C/min. Ca(OH)2 can be measured by the amount of water loss, which is very close to its water content and hence proportional to the amount of Ca(OH)2 [21, 22]. The hydration products were also examined by SEM to understand the paste morphology.

Table 2. Paste mix proportions.

POFA/C W/B C/C Symbols 0 0.29 1 Mc

0.1 0.29 1 M1 0.2 0.29 1 M2 0.3 0.29 1 M3 0.4 0.29 1 M4 0.5 0.29 1 M5 0.6 0.29 1 M6 0.8 0.29 1 M7

The pastes were cast in 403030 mm cube moulds, compacted using a tamping rod, and sealed in plastic sheets to prevent water evaporation. The samples were cured in water at 23 1C for 90 days. The moulds were then removed and the samples were immediately immersed in acetone for 24 hours to stop the hydration process. The testing was performed on powder samples (95% passing 45 m sieve) obtained by grinding the hardened pastes. The grinding process was carried out by crushing the hardened pastes into smaller pieces, followed by milling in a grinding bowl. A small amount of the sample was used to determine the Ca(OH)2 content by TGA and XRD analyses. The powder sample was divided into two parts of 10 g each. An XRD test was conducted on one of the samples, whereas a TGA test was conducted on the other. Before grinding, small pieces of hardened paste with an average diameter of approximately 1 cm were taken to determine paste morphology using SEM.

3. RESULTS AND DISCUSSION 3.1 Strength activity index

Strength activity indices for all mortars are shown in Fig. 4. The strength activity index is the ratio of the strength of the POFA-cement mortar to the strength of the reference (cement mortar) at each specific curing time. The rate of strength development of cement mortar relies principally on its hydration rate. In contrast, the said rate relies on the cement hydration and rehydration caused by the pozzolanic reactivity of POFA in POFA-cement mortar. Figure 4 shows that the strength activity indices at 3, 7, 14, 28, and 90 days were higher than the minimum requirement of 75% as specified in ASTM C 618-05.The POFA cement presents a strength activity index of 97.3%, 97.6%, 99.3%, 100.7% and 101.6% of the reference cement strength at 3, 7, 14, 28, and 90 days, respectively. At the early ages of 3

International Journal of Civil & Environmental Engineering IJCEE

and 7 days, replacing OPC with 20% POFA was found reduce the compressive strength in comparison to the reference mortar. This could be attributed to dilution effect and delayed onset of pozzolanic reaction of POFACa(OH)2.

Fig. 4. Strength activity index of POFA mortar at 3, 7, 14, 28, and 90 days.

Nonetheless, the high fineness of the POFA could have contributed as fillers, filling the voids between the pates and the sand and contributing to greater than 97 % strength activity index [15]. At 14 days, the strength activity index increases to more than 99 %, which could have contributed by the pozzolanic reaction of POFA with Ca(OH)producing C-S-H and increasing the strength. days, the compressive strength of the POFAwas higher than that of the reference cementcould be attributed to pozzolanic reaction of POFA. results generally agree with the experimental results obtained by Sata et al. (2004), who found thatdays of curing time, the concretes with 20% of lower compressive strength than concretes POFA. The same results were observed after 7 concretes with 20% POFA gave higher compressive strength than concretes with 10% and 30% POFA.

At longer curing period of 90 days, the POFAmortar recorded a strength activity index of greater than 101 %. Hence, at this later age, the amorphous aluminous and siliceous minerals could have still actively reacted with Ca(OH)2, producing C-S-H and hydrated calcium aluminates, improving interfacial bonding between the sand and pastes. These characteristics have been shown to improve, thereby the increasing the compressive strength and density of mortar [1,23,24].

3.2 POFA-cement hydration

Since the POFA contains high amount of amorphous SiOit is expected to have pozzolanic properties and will chemically react with Ca(OH)2 produced from the hydration reaction of cement within the POFApaste. In general, TGA has been widely accepted as a

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3 7 14 28

Stre

ngt

h ac

tivity

in

dex (%

)

Age (days)

Reference Treated POFA

International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: 0

115905-7676 IJCEE-IJENS October 2011 IJENS

, replacing OPC with 20% POFA was found to in comparison to the

reference mortar. This could be attributed to dilution effect and delayed onset of pozzolanic reaction of POFA with

activity index of POFA mortar at 3, 7, 14,

Nonetheless, the high fineness of the POFA could have filling the voids between the pates

contributing to greater than 97 % strength strength activity index

increases to more than 99 %, which could have contributed by the pozzolanic reaction of POFA with Ca(OH)2,

H and increasing the strength. After 28 POFA-cement mortar

the reference cement, which again could be attributed to pozzolanic reaction of POFA. These

experimental results found that, before 7 20% of POFA gave

essive strength than concretes with 10% of after 7 days. The

concretes with 20% POFA gave higher compressive 30% POFA.

longer curing period of 90 days, the POFA-cement mortar recorded a strength activity index of greater than 101 %. Hence, at this later age, the amorphous aluminous and siliceous minerals could have still actively reacted with

ydrated calcium improving interfacial bonding between the

These characteristics have been shown to compressive strength

high amount of amorphous SiO2, it is expected to have pozzolanic properties and will

produced from the hydration reaction of cement within the POFA-cement

widely accepted as a

suitable technique to assess Ca(OH)hydration products, including C-Spaste. Figure 5 shows the TGA curves which indicate the changes in mass of pastes due to heating from room temperature to about 1000C.

Fig. 5. TGA curves of the hydrated powder pastes ratio of 0.29: (a) POFA/C= 0; (b) POFA/C= 0.8.Three considerable endothermic shifts slope of each curve; at approximately and 6801000C. These changes take place mass loss. The first abrupt shift is broad and attributed to the removal of water molecules and decomposition of C-S-H, ettringite,second weight loss at approximately attributed to the Ca(OH)2 corresponding endothermic shift is reaction, which reveals the presence of Ca(OH)

Ca(OH)2 CaO + 2OH

The third shift at 680 C may be caused bycarbonate decomposition. The carbonation of the paste may occur during the preparation or grinding of TGA [26]. The endoshift at 680C is following reaction:

CaCO3 CaO + CO

As shown in Table 3 and Figure was determined from the changes in by the decomposition of Ca(OH)between 415515C in TGA

90

No: 05 103

I J E N S

Ca(OH)2 content and other S-H in hydrated cement

shows the TGA curves which indicate the changes in mass of pastes due to heating from room

hydrated powder pastes at w/b ratio of 0.29: (a) POFA/C= 0; (b) POFA/C= 0.8.

shifts take place in the approximately 100400, 415515,

take place as a result of shift is broad and can be

removal of water molecules and , and C2ASH8 [25]. The

approximately 415C can be dehydroxilation. The

corresponding endothermic shift is caused by the following reaction, which reveals the presence of Ca(OH)2:

CaO + 2OH (1)

may be caused by calcium The carbonation of the paste may

or grinding of the paste before The endoshift at 680C is caused by the

CaO + CO2 (2) 6, the Ca(OH)2 content

from the changes in mass of water caused )2 at temperature range

curves. TGA allows



estimation of the content of Ca(OH)2 present in the hardened pastes. The TGA results show a clear relationship between the reactive SiO2 content of POFA and the amount of Ca(OH)2 consumed by the pozzalanic reaction at 90 days. The DTG curve peak for Ca(OH)2 of the pastes containing POFA decreases with increasing POFA content. As shown in Figure 6, Mc (POFA/C= 0) contained the highest value of Ca(OH)2 loss which is approximately 6.39% in comparison to all pastes containing POFA. The increase in the Ca(OH)2 of the OPC paste was caused by the hydration of the cement. Addition of POFA by 0.1in term of POFA/cement ratio leads to a significant loss of Ca(OH)2 [i.e., approximately 4%]. The mass loss due to decomposition of Ca(OH)2 reduces only slightly at POFA/cement ratios between 0.1 to 0.3. Hence, Ca(OH)2 consumption due to pozzalanic reaction is nearly at the same level when POFA/cement ratio is between 0.10.3.

Table 3. Mass loss of calcium hydroxide according to TGA.

Mass loss of Ca(OH)2 by TGA, (% mass/mass) Symbols 6.39 Mc

4.096 M1 4.003 M2

3.8006 M3 3.148 M4 2.196 M5 2.056 M6 1.96 M7

Fig.6. TGA analysis on hydrated POFA/cement paste at 90 days.

POFA content of up to 0.8 (POFA/cement ratio) leads to loss of Ca(OH)2 of approximately 2%. Hence, the higher the content of POFA, the higher the consumption of Ca(OH)2 as a result of pozzolanic reaction, which concurs with the observation from TGA analysis in Fig. 6. In addition, the low Ca(OH)2 concentrations with increasing POFA contents may have been also caused by the dilution effect at higher POFA/cement ratio. With the inclusion of POFA at POFA/cement ratio of 0.8, the amount of CaO in

the Portland cement is reduced, resulting in low C-S-H and Ca(OH)2. The reduction of Ca(OH)2 content at 90 days in the TGA results explains the role of POFA in reducing Ca(OH)2 via the pozzolanic reaction. The observation is generally in agreement with previous findings of Chandara [7].

The XRD patterns of hydrated powder pastes with and without POFA at 90 days are shown in Figure 7. The XRD analysis was performed on prepared powder pastes. The XRD pattern at Theta angle from 1090 was studied to identify the peak patterns of Ca(OH)2 and C-S-H. The Ca(OH)2 and C-S-H peaks after 90 days of curing were identified and presented. The concentrations of hydration products were determined through the length intensity of Ca(OH)2 and C-S-H collected by X-ray scans recorded as intensity in unit counts.

Ca(OH)2 could be detected at several locations along the 2 Theta; specifically, at peaks of 2 Theta of 18.6, 34.4, and 47.5. A comparison of patterns clearly show that the intensity of CH peaks representing Ca(OH)2 was significantly reduced with the increase in the POFA/cement ratios, as demonstrated by the TGA results. Thus, the XRD patterns support the TGA results and provide additional information about the mineral phases existing in the hydrated cement pastes.

Fig.7. XRD pattern of hydrated powder pastes containing POFA at age of 90 days.

The reduction of Ca(OH)2 in the pastes indicates its consumption in the pozzolanic activity. More Ca(OH)2 is consumed during hydration as the POFA contents increase (i.e., the higher addition of POFA), the higher the content of amorphous SiO2 available to react with Ca(OH)2. This reaction is more effective with the largest amount of Ca(OH)2, which comes from the hydration of OPC to produce C-S-H. The POFA reaction can be expressed as

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0 0.2 0.4 0.6 0.8 1

Mas

s lo

ss o

f Ca(O

H)2

(% )

POFA/C



Ca(OH)2 (from the hydration of OPC) + SiO2 (POFA) C-S-H (calcium silicate hydrate) (3)

The incorporation of POFA at 0.1 to 0.3 (POFA/cement ratio) resulted in a remarkable increase in C-S-H, detected along the 2 Theta axis between 13.755.5C of 2 Theta. This is confirmed by the XRD pattern and SEM images. However, the high intensity and broad peaks of C-S-H at these ratios could be attributed to pozzolanic reaction between Ca(OH)2 and amorphous SiO2, indicating that the reaction at these ratios was extremely active due to the presence of a large amount of Ca(OH)2. This active reaction matches with the result of mass loss of Ca(OH)2 by TGA [the Ca(OH)2 losses for 0.1, 0.2, and 0.3 POFA/cement ratios were approximately 4.09, 4.0, and 3.8, respectively]. In addition, the fineness of the POFA could have affected the pozzolanic reaction rate [25] (i.e., the fineness of POFA affected the increase in production rate of C-S-H). The C-S-H of pastes with POFA/cement ratio of 0.10.3 increased significantly than OPC paste which may be attributed to the particle sizes being smaller than OPC (the average particle size of POFA was about 2.99 m). Moreover, the fine POFA has a larger surface area to provide the silica for pozzolanic reaction and probably also could have some accelerating effect on OPC hydration. From Figure 8, the intensity of C-S-H in the POFA/cement ratio up to 0.8 is lower than the others because of the dilution effect. The degree and the rate of hydration provided by the OPC is reduced when OPC is fixed at constant weight and POFA is increased in hardened pastes because of the increase in the amount of POFA in the total mass of paste. This, in turn reduces the amount of Ca(OH)2 gradually, resulting in less amount of C-S-H. Besides, from the C-S-H and Ca(OH)2 formed during the hydration process, the XRD profiles indicate the presence of a small peak representing calcium aluminates hydrates (C4AHx). The POFA contains Al2O3 in an amorphous form (more than 5% of Al2O3 in amorphous form) [27], which plays a very important role in pozzolanic reaction. Thus, the pozzolanic reaction of POFA with Ca(OH)2 released from hydration of cement leads to the production of a greater amount of calcium aluminate hydrate (C-A-H).

The above results were also confirmed by the SEM images. The typical microstructures of fractured surface of hardened pastes at 90 days with or without POFA are shown in Fig.9. Pastes with 0, 0.3, and 0.8 of POFA/cement ratios were chosen for SEM because they were clearer in terms of change in the amount of hydration products. OPC paste had been hydrated in a wide zone, and cement particles had been coated by C-S-H. The remaining calcium hydroxide appears brightest followed by C-S-H and large dark pore sand cracks revealing particle boundaries (Fig. 9a).

Fig. 8. XRD patterns of calcium silicate hydrate (C.H.S) formed by adding different POFA/C at age of 90 days.



Fig. 9. SEM photos of hardened pastes at 90 days curing time: (a) POFA/C= 0; (b) POFA/C= 0.3; (c) POFA/C= 0.8.

The absence of the pozzolanic material (i.e., POFA) causes the lime to remain as it is since there are no pozzolanic activities that can improve the structure through formation of additional C-S-H gel [28]. For the microstructure pattern of POFA paste when POFA/C is 0.3, the structure contained small pores, and the fractured surface was almost completely covered by C-S-H. Most large spaces had been filled with C-S-H gel, forming a dense structure. The hydration reaction was very active, and a huge amount of gel was formed (Fig. 9b). Therefore, the microstructure of the paste became denser. Figure 9c shows the SEM image of a sample with 0.8 POFA/cement ratio. Residual POFA grains appear brightest with the products of reaction. Unreacted particles of POFA could be observed. These particles exist because they contain POFA on some particles in the form of crystalline phase. The amount of crystalline particles increases when POFA is increased. In addition, reducing Ca(OH)2 and increasing POFA leads to dilution effect. Most voids had been filled by small unreacted POFA particles made it serving as inert filler.

4. CONCLUSIONS Based on the experimental studies presented in this paper, the following conclusions can be drawn:

1. Ground POFA obtained by heating at 450C for 1.5 hours resulted in loss on ignition significantly lower than that of the untreated POFA (LOI= 2.3). Hence, the treated POFA was free from carbon and other organic matter. In addition, the treatment process yielded POFA with high specific surface area, preserving the amorphous characteristic related to pozzolanic activity of POFA and free from particle agglomeration.

2. Strength activity index of POFA/cement mortar fulfilled the requirements of pozzolanic materials as per ASTM C 618-05. Compressive strength test confirmed that after 28-days of curing time, the strength of treated POFA was

greater than that of the reference cement. This increase was larger at 90 days. Hence, strength increased as curing time progressed because of the consumption of Ca(OH)2 by the POFA via the pozzolanic reaction.

3. At 90 days curing time, TGA data and interpretations of XRD diagrams of the POFA/cement pastes confirmed that Ca(OH)2 gradually decreased with an increase of POFA content. C-S-H compound was identified as the main product of the reaction between POFA and Ca(OH)2. This product was higher with the addition of POFA up to 0.3 from the mass of cement. The results of SEM analysis of the POFA/cement pastes confirmed that there was an increased pozzolanic reaction when POFA/C ratio was up to 0.3.

ACKNOWLEDGEMENT The authors gratefully acknowledge the Universiti Sains Malaysia for providing the financial support through the Research University and Short-Term Grant Schemes for undertaking the research work. The support provided by the technical staffs from the School of Civil Engineering as well as the School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia is greatly appreciated. Special thanks are due to United Palm Oil Industries for providing the palm oil fuel ash.

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strength activity index and microstructural

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