feasibility study of palm boiler ash as cement and …

Journal of Engineering Science and Technology Vol. 15, No. 4 (2020) 2361 - 2378 © School of Engineering, Taylor’s University

2361

FEASIBILITY STUDY OF PALM BOILER ASH AS CEMENT AND SAND REPLACEMENT IN CONCRETE

AZMI ROSLAN1,*, MOHAMED KHATIF TAWAF MOHAMED YUSOF2, SITI SHAHIDAH SHARIPUDIN2, ZENO MICHAEL3,

ILYA IZYAN SHARUL AZHAR3

1Faculty of Chemical Engineering, Universiti Teknologi MARA,

Pasir Gudang Campus, Jalan Purnama, Bandar Seri Alam, 81750, Masai, Johor, Malaysia 2Faculty of Civil Engineering, Universiti Teknologi MARA,

Pasir Gudang Campus, Jalan Purnama, Bandar Seri Alam, 81750, Masai, Johor, Malaysia 3Faculty of Mechanical Engineering, Universiti Teknologi MARA,

Pasir Gudang Campus, Jalan Purnama, Bandar Seri Alam, 81750, Masai, Johor, Malaysia

*Corresponding Author: [email protected]

Abstract

In this paper study the properties of Palm Boiler Ash (POBA) as sand and cement

replacement in concrete and identify optimum level of replacement. Besides that,

pozzolanic activity of the POBA to OPC also has been assessed. Samples were

tested to measure its compressive strength, ultra-pulse velocity (UPV), and

thermal conductivity of blended concrete. Three replacement level of sand and

cement has been employed which are 5%, 10%, and 15%. It is found that,

increased in the POBA replacement level in the cement mixture will linearly

reduce the compressive strength of concrete mixture. From perspectives of

pozzolanic activity, it is increases as amount of POBA replacement increases.

For UPV values, it is decreases as POBA replacement increases. Among those

three percentage of sand and cement replacement, 5% POBA replacement is the

most optimum quantity of the POBA replacement in the concrete as it shows a

high compressive strength when tested for compression strength, good quality of

concrete grade and thermal conductivity of 1.54 W/m.K.

Keywords: Cement replacement, Concrete, Palm oil boiler ash (POBA), Sand

replacement.

2362 A. Roslan et al.

Journal of Engineering Science and Technology August 2020, Vol. 15(4)

1. Introduction

In concrete, cement is used as a binder when to harden and adheres sands and

gravels to bind together. In the modern age, the demands trends of cement increase

proportionally due to the demands of the human population. It is around 7% of CO2

gas emissions are caused by cement production processes in factories, which

increases greenhouse gasses and causes other environmental problems [1].

According to Mangi et al. [2], around 400 kg/s of carbon dioxide released to the

atmosphere for 600 kg/s of cement production. Reliance on the cement in the

construction industry may lead to increase of greenhouse gas concentration such as

Carbon Dioxide (CO2) in the environment. There are two ways of greenhouse gas

emissions, directly and indirectly. First is the heating of limestone directly releases

the CO2, while the burning of fossil fuels is the indirectly release of CO2.

The direct emission occurs through a chemical process called calcination.

Calcination occurs when the limestone, which consists of calcium carbonate is

heated then breaks down into calcium oxide and CO2. Next, the indirect emissions

are produced by burning fossil fuels to heat the kiln. In the production of cement,

it requires the burning of limestone, coal, fossil fuels, and fuel oils. It is

approximately 0.97 tons of CO2 release for every ton of clinker produced [1].

Production of cement has been considered the second biggest contributor to CO2

gas emissions worldwide [3]. Consequently, one of the methods in reducing the

environmental impact on the cement production is by partial replacement of the

cement and sand with supplementary cementitious materials (SCMs) often sourced

from industrial or agricultural wastes.

SCMs will improve the environmental credibility of the concrete industry, but

also may enhance concrete strength and durability depending on the pozzolanic

activity of the SCM and the [4]. By incorporating SCMs in concrete will not only

reduce the demand for cement production but also reduces economic losses due to

repairs, maintenance and reduced serviced life [4]. Coal fly ash (CFA), granulated

blast furnace slag (GBFS), and the agricultural wastes such as rice husk ash (RHA),

sugarcane bagasse ash (SCBA) and Palm Oil Fuel Ash (POFA) and Palm Boiler

Ash (POBA) are industrial waste materials that are normally used as pozzolans.

Palm oil has wide spectrum of applicability that deals with our daily life usage.

Either consumable or non-consumable products are mostly derived from palm oil.

Malaysia is one of the largest exporters and producer of palm oil [5]. It is calculated

about 10 Million tons of palm oil boiler ash had been produced in Malaysia alone

in each year [6]. Hence, wasted generated from palm oil production such as Empty

Fruit bunches and palm kernel shell is abundantly available and sometimes it is

wasted. Some of the company utilised this by-product by burning it into boiler to

generated steam for plant electricity [7] or others heat transfer equipment such as

heat exchanger. Normally, combustion process of palm oil residue in the boiler

requires a palm oil to husk ratio of 2:8 for the best performance of boiler [8] and

were burned at high temperature at almost 1000 °C [9]. The combustion product

from boiler is known as Palm Boiler Ash (POBA). The product is mostly consisting

of different types of substances such as materials with crystal like structure and

some carbon blend. It is derived from the heating of significant amount of fibre,

shell and empty palm fruit bunches from the palm oil fruit [5].

According to Khankhaje et al. [10], for every kilogram of palm oil produced,

three kilograms of POBA is generated. POBA is not easily degraded by nature and

Feasibility Study of Palm Boiler Ash as Cement and Sand Replacement . . . . 2363


continuous supply of this product will arises different kind of problems such as

POBA mountains at disposal area. Furthermore, the disposal of palm oil boiler ash

(POBA) into the landfills and open fields without treatment may cause various

diseases and be uncomfortable for human life [5]. Moreover, all these waste

materials can potentially cause environmental problems, financial losses, and

health hazards. According to Tangchirapat et al. [11], POBA can be used as a sand

and cement replacement in making concrete. With this utilisation of POBA in

concrete, it is expected can reduce the dependence of sand or cement usage thus

will reduce carbon dioxide emissions to environment.

Apart from that, building is considered as one of greenhouse gasses

contributors. Thirty percentage greenhouse emissions are attributed to building in

most countries [12]. Nowadays, most of the people spent their times in indoors. It

is of paramount important to understand energy conservation and thermal comfort

of the building. This is highly depending on the thermo-physical properties of the

construction materials. One of important property is thermal conductivity of

concrete. Energy consumption of the building is dependent on this property [13].

Lower value of this property can contribute to low energy usage of the building.

Thermal conductivity is a measurement of materials ability to conduct heat.

According to Asadi et al. [13], there are seven factors involving in the changes of

the thermal conductivity of the concrete which are humidity of the specimen, age,

temperature, water-cement ratio (W/C), fine aggregates fraction, type of admixture

and total aggregate volume faction.

The objective of this paper is to evaluate the feasibility of Palm Boiler Ash

(POBA) as sand and cement replacement in concrete with reference to the hardened

concretes and identify optimum level of replacement. In present context, optimal

level refers to the amount of POBA requires as a replacement of sand and ordinary

Portland cement (OPC) up to which the compressive strength, ultra-pulse velocity,

and thermal conductivity of blended concrete are equivalent or more than that of

unblended OPC concrete. Besides that, pozzolanic activity of the POBA to OPC

also has been assessed.

2. Methodology

2.1. Materials and sample preparations

Table 1 shows concrete mix ratio of cement, sand and aggregate used in this

paper. Ordinary Portland cement (OPC) was used as a binder to bind sand and

aggregates together. The coarse aggregate or gravel was used in this research was

taken from a quarry around Johor and sieves into two different size range, 14 to

20 mm and 6 to 10 mm. The fine aggregates, sand are used for this research. The

palm oil boiler ash used for this experiment was taken from Teluk Sengat Palm

Oil Mill. POBA is combustion by-product of palm kernel shell and empty fruit

bunches in the boiler at palm milling plant. Two different sizes of POBA were

used in the experiment which are passing 5 mm and 90 µm for sand and cement

replacement, respectively Figs. 1(a) and (b). All materials were dried in the oven

at the temperature of 110 ˚C ± 5 for 24 hours in order to removes moisture in it.

After oven dried, sand and POBA were stored in the airtight container. All

materials were kept in the humidity-controlled room to isolate from the

atmospheric humidity.



Table 1. Concrete sample mixtures by weight percentage.

Sample Name OPC

(g)

Sand

(g)

Aggregates

(g)

Water

(g)

POBA (g)

90 µm 5 mm

CM 14.56 27.94 49.68 7.82 0.00 0.00

CM5 13.84 26.54 49.68 7.82 0.73 1.40

CM10 13.11 25.14 49.68 7.82 1.46 2.79

CM15 12.38 23.75 49.68 7.82 2.18 4.19

Total of 52 concrete cubes (each sample is produced with 13 concrete cubes)

containing various proportion of POBA/cement/sand/POBA were cast Figs. 2(a)-

(d). The replacement level of POBA for cement and sand being 5, 10 and 15% out

of cement and sand proportion on control sample. Control design concrete is the

concrete that have a basic mixture of water, cement and sand while the mix design

concrete has replacement of sand and cement by adding POBA.

In each mix design consist of 13 cast concrete cubes to be carried out for

different test. The mould of 100 mm × 100 mm × 100 mm (IS: 10086-1982) [7]

was checked, cleaned from impurities and all bolts tightened to prevent from any

leakage and grease was applied to each of the mould to provide a clearance of

mixture in the mould. T

able 1 indicates cement, sand, water and POBA proportion for every concrete

mix design. The mixture was mixed in the mixer and poured into the mould. The

mould was shaken by using shaker to prevent honeycomb occurrence in the

concrete mixture and promotes well segregation of concrete mixture.

The cast concrete will be retained in the mould for 24 hours before released and

transferred into curing pond. In the curing pond, cast concrete was immersed in the

water bath for 28 days, this could prevent losses of moisture from concrete cast due

to atmospheric ambient attack. The water content is of paramount important as to

provide well-structured of concrete and strong bind in between sand, POBA and

coarse aggregates.

(a) Passing 90 µm. (b) Passing 5 mm.

Fig. 1. OBA samples for cement and sand replacement.



(a) Cleaning the mould (150 mm×

150 mm×150 mm) and applied

grease on the inner part of mould.

(b) Mixing process of concrete

mixture in the mixer machine.

(c) Pointed-ogive cylinder. (d) Curing process of concrete

casting in curing pond.

Fig. 2. Concrete casting process.

2.2. Test method

2.2.1. Pozzolanic activity of POBA and OPC cement

Pozzolanic activity of POBA and OPC cement has been assessed based on the

method of assessing its electrical conductivity of POBA and OPC suspension in

water. The method was adapted from Payá et al. [14] and Velázquez et al. [15] by

measuring electrical conductivity of the sample’s mixture. Table 2 shows samples

name and its mixtures. All samples evaluated on its pozzolanic activity for a period

of 6000 s and POBA size of 90 µm was used. The temperature of the sample

solution was kept at 50 ˚C ± 4 ˚C. OPC and POBA were added simultaneously in

50 mL of distilled water. Experimental setup diagram as shown in Fig. 3.

Table 2. Mix proportion of pozzolanic activity test.

Sample Name OPC (g) OPC (g)

OPC 1 0

POBA5 0.95 0.05

POBA10 0.90 0.10

POBA15 0.85 0.15



(a) Conductivity meter (b) Schematic diagram of pozzolanic

activity test

Fig. 3. Experimental set-up for pozzolanic activity test.

2.2.2. Compressive strength

The All samples (CM, CM5, CM10, and CM15) were first tested for compressive

strength using manually operated compression testing machine (Fig. 4) at 7, 14 and

28 days. The machine follows accordingly to ASTM E4 standard [16]. Three

replicates of each age were used for determining the average and experimental

bounds of the data. The samples were dried at the ambient temperature after

releasing it from curing pond to prevent moisture or hydration process that might

affect the compression test results. In this test, the compressive strength limits for

all samples were assessed using compression strength machine. This compressive

strength machine can withstand and creates loads up to 1000 N/mm2.

Fig. 4. Compression testing machine.

OPC, POBA

and water

mixture sample

Conductivity meter

Conductivity

probe

Magnetic

stirrer



2.2.3. Scanning electron microscope (SEM) analysis

In this current study, microstructure of hardened concrete for POBA, CM, CM5,

CM10 and CM15 are subjected to SEM analysis. The analysis was carried out using

JSM IT-200 JEOL (Fig. 5) operating at accelerating voltage of 15 kV. The samples

(CM, CM, CM5, CM10 and CM15) were cross-sectioned and samples

microstructure was analysed.

Fig. 5. Scanning electron microscope JEOL IT-200.

2.2.4. Ultrasonic pulse velocity (UPV)

Ultrasonic Pulse Velocity test is a technique used to assess the quality and condition

of casting concrete at age of 28 day for CM, CM5, CM10 and CM15. Three

replicates were used for determining the average and experimental bounds of the

data. In this test, ultrasonic generator (Fig. 6) will generate and passing a pulse of

ultrasonic through concrete and measuring the time taken for the pulse to pass the

concrete. Higher velocity indicates that the concrete materials structure is in the

continuity without presence of any voids. The portable Ultrasonic Non-destructive

Digital Indicating Tester (PUNDIT) was used to measure the speed of ultrasonic

pulse passing to the concrete.

Fig. 6. Ultrasonic pulse analyser.



2.2.5. Thermal conductivity

The measurement of thermal conductivity is conducted using KD2 Pro Thermal

Properties Analyser at ambient temperature and pressure. The device using the

transient heated needle to measure thermal properties of concrete samples. The

device measures the thermal conductivity by evaluating the time and temperature

response of the sudden electric signal. The device is equipped with handheld

controller and needle sensor TR-1. The range of thermal conductivity measured is

0.1 to 4 W/m.K with an accuracy of ±0.10%. The Delrin block is provided for

verification of the sensor, where the thermal conductivity for Delrin block at -20

W/m.K to 60 W/m.K is not greater than 0.004 W/m.K (~1%). The measurement of

the thermal conductivity for concrete requires to mould pilot holes during casting

process using supplied pilot pins furnished with KD2 Pro. Pilot pins must be coated

with grease before installed in the wet concrete for easy removal process when

concrete is dried. Thermal conductivity paste is applied on the surface of the sensor

before thermal conductivity measurement of dried concrete is conducted. All

samples (CM, CM5, CM10 and CM15) were measured their thermal conductivity

at the age of 28 day. Three replicates of thermal conductivity were measured to

determine its average. Figure 7 show a schematic diagram of thermal conductivity

measurement test.

Fig. 7. Schematic diagram of thermal conductivity test.

2.3. Results and discussions

2.3.1. Pozzolanic activity of POBA and OPC cement

Different cement to POBA ratio is tested for comparing their behaviour and

influence in conductivity of their water suspensions. POBA with percentage

replacement of 5%, 10% and 15% were tested. Figure 1(a) shows conductivity over

time curves of the suspensions. From Fig. 8(a), three distinguishable dormant

period can be observed, and it is differed from one sample to other attributes to

amount of POBA and cement used. First dormant period occurs in the range of 0

to 360 s, at this stage, the dissolution of the ions passing into the solution is

increased resulting to the instantaneous increase in conductivity of the solution

[17]. During the elapse time of 0 to 360 s, the electrical conductivity of POBA15



was found to be higher than POBA10 and POBA5. This indicated that, increasing

the amount of POBA in the solutions will increase the amount of liberated salt, and

increase conductivity of solutions [17]. After that, solutions become supersaturated

due to ettringite and C-S-H formation [18]. Second dormant period can be observed

when the reactions continue slowly due to slow nucleation mechanism of

portlandite [18]. At this period, conductivity increase slowly until it reached at

critical super saturation. As in Fig. 8(a), second dormant differed from one another

due to varied amount of POBA and OPC leads to different rate of portlandite

formations. High amount of POBA resulting to different values of conductivity.

Moreover, increase in POBA replacement level seems to reduce time for this

critical super saturation stage to achieve. This is due to amount of OPC has been

reduced equivalent to amount of POBA replacement level in solution. With less

amount of OPC, it requires shorter time to reach at its critical super saturation stage,

since, the only ions contributor in the solution is from OPC. Second dormant for

samples POBA5, POBA10 and POBA15 occur at range of 360 s to 4980 s, 360 s

to 5100 s, and 360 s to 5580 s, respectively. However, solution with OPC without

presence of POBA seems to achieve its critical super saturation at earlier stage

when trend of conductivity graph decreases after first dormant period had passed

and this led to third dormant period. According to Sinthaworn and Nimityongskul

[17], this behaviour attributed to interaction between both Ca2+ and OH- ions and

tested pozzolans to form reaction product resulting to reduce in ions concentration

level in the solution, and hence reduce conductivity. Based on Fig. 8, POBA15

suspension reached highest conductivity followed by POBA10 and POBA5. The

variation of this conductivity values is due to soluble saline content of POBA and

the pozzolanic activity of them [2]. Pozzolanic activity is difficult to assess due to

several reason such as alteration of fixation of lime by high ion releasing process,

time for glassy material in pozzolans to be reacted, and pozzolanic activity

normally occur at medium- and long-term [19]. Therefore, in this paper proposed

method of calculation of %LC for 6000 s in order to evaluate pozzolanic activity.

Figure 8(b) shows the (%LC) curves for POBA5, POBA10 and POBA15.

Time (s)

0 1000 2000 3000 4000 5000 6000 7000

C (

µS

/cm

)

0

200

400

600

800

1000

1200

1400

1600

OPC

POBA5

Time (s)

0 1000 2000 3000 4000 5000 6000 7000

%L

C

-10

0

10

20

30

40

50

OPC

POBA5

POBA10

POBA15

POBA5

POBA10

POBA15

(a) (b)I II III

(a) Electrical conductivity. (b) Loss of Electrical conductivity.

Fig. 8. Time evolution of aqueous cement suspensions with simultaneous

addition of POBA 0% (1:0 OPC/POBA ratio), 5% (1:0.05 OPC/POBA

ratio), 10% (1:0.11 OPC/POBA ratio) and 15% (1:0.18 OPC/POBA ratio).



Table 3 shows calculated areas of the area under the %LC curve for POBA5,

POBA10 and POBA15. From this tabulated data, it is concluded that pozzolanic

activity increases as amount of POBA replacement increases and OPC cement

amount decreases.

Table 3. Calculated areas under the curve of %LC

from 10 to 600 s for cement and POBA suspensions.

Sample Name Cement (g) POBA (g) Area (%LC.s)x 105

POBA5 0.95 0.05 0.29

POBA10 0.90 0.10 0.96

POBA15 0.85 0.15 1.16

2.3.2. Compressive strength of CM, CM5, CM10 and CM15

Figure 9 show the variation of compressive strength of concrete mixture with

POBA replacement at the age of 7 to 28 days. At the age of 7 days, the compressive

strength of CM5 was 30.40 N/mm2 or 118% of the control concrete followed by

CM10 and CM15 with 29.93 N/mm2 or 116% and 24.40 N/mm2 or 94%,

respectively. It can be observed in Fig. 9, the compressive strength for all samples

at 7 days shows a decreasing trend as POBA replacement increased. The results

presented agrees well with the Sata et al. [12] where effect of oil palm ash (OPA)

with low replacement level results to higher compressive strength. The amount of

Portland cement in the concrete mixtures plays an important role in the variation of

concrete compressive strength. At high amount of Portland cement, it promotes

higher hydration reaction to the concrete mixture, thus, increased compressive

strength of concrete mixtures. As to compared ratio of cement to POBA in the

sample CM5, CM10 and CM15, CM5 has the highest ratio compared to CM10 and

CM15, which are 6.51:1, 3.08:1 and 1.94:1, respectively. CM5 contains high

amount of cement compared to others, and this promotes higher hydration reaction

that results to higher compressive strength than others [Fig. 9] at age of 7 days.

Fig. 9. Compressive strength development of CM,

CM5, CM10 and CM15 at 7, 14 and 28 curing days.

All samples show progressive development on the compressive strength until

28 curing days. It can be observed that CM5 and CM10 with POBA replacement

Age (Days)

0 5 10 15 20 25 30

Com

pres

sive

Stre

ngth

(N/m

m2 )

0

10

20

30

40

CM : 0% POBA

CM5 : 5% POBA

CM10 : 10% POBA

CM15 : 15% POBA



of 5% and 10% attained 28 days compressive strength above 30 N/mm2 of M30

concrete mixture and above control sample (CM) compressive strength. The

compressive strength of POBA replacement concretes varied from 29.53 N/mm2 in

sample CM15 to 34.27 N/mm2 in sample CM5, corresponding to 90.8% and

105.3% of control concrete, respectively.

The results in Fig. 9 clearly indicate the relationship of the POBA replacement

percentage to the concrete mixture compressive strength at 28 curing days. It is

found that, the increase in the percentage amount of POBA in the cement mixture

will linearly reduce the strength of concrete mixture and reached at allowable limit

of the POBA replacement before falling in the range below M30 design mix

standard compressive strength at 28 days. Among three different replacement

levels, the use of POBA at the replacement level of 5% performed the best, which

resulted in the highest strength increase over the control concretes at all test ages,

particularly at the age of 7 days.

2.3.3. Microstructure of POBA samples

The microstructure of the POBA was examined with a Scanning Electron

Microscope JSM IT-200 JEOL, as shown in Fig. 10. It shows POBA samples which

is angular, cylindrical and irregular in shape. Fineness of pozzolans [20] and shape

[17] are important to the rate of hydration and pozzolanic reaction.

(a) POBA with size variation at x85

mag.

(b) POBA sample view enlargement of

surface at (i) x1500 mag.

(c) POBA sample view enlargement of

surface at (ii) x1500 mag. (d) POBA sample view enlargement of

surface at (iii) x1500 mag.

Fig. 10. SEM image of POBA.

(ii)

(i)

(iii)



This is due to total surface area that it can contribute. Large surface area

increased the interfacial bonding between particles which results to the increased

in the concrete compressive strength. Besides that, high surface area will increase

siliceous and aluminous materials chemical reactivity with calcium hydroxide in

presence of water to form compounds possessing cementitious properties.

According to Sulaiman [21], angular and irregular shape has large total surface area

compared to rounded surface. In Fig. 10, POBA shows a rigid microstructure

without presence of porosity. The presence of porosity of the pozzolans is to be

avoided as it will reduce concrete compressive strength [14]. This is due to water

in the concrete design mixed has a greater tendency to fill-up porosity before

hydration reaction takes place. Therefore, it will reduce the demands of the water

for the hydration reaction resulting to the decrease in hydration products that

requires binding aggregates together. Moreover, smaller pozzolans size seems

beneficial to concrete mixture as it can act as micro-filler to the concrete design

mix which will distributes evenly across mixture, resulting to increase in

pozzolanic reaction and contributes to increase in the strength of the concrete [8].

2.3.4. Microstructure of concrete samples cross-sectioned

An increase in W/C from 0.54 to 0.63 decreases the strength consistently and nearly

4.3 N/mm2 for all samples. The high W/C ratio increases capillary porosity of the

hardened paste and affects the width and microstructure of aggregate-paste interfacial

transition zone [21]. Capillary pores are a part of the concrete gross volume which is

not filled by the hydration products. It is highly depended on the water to cement ratio

and degree of hydration. In this study, it shown that by using W/C ratio of 0.54 is

enough to fill all the space available in the concrete mixture [Fig. 11(a)]. Increase in

W/C greater than 0.54 will increase the volume of the capillary pores as the hydration

proceeds. The arrow in Fig. 11 shows an increase in pores generation from 0% to

15% POBA replacement in the concrete mixture. Consequently, it will affect to the

densification of concrete samples as shown in Table 4. Not only that, according to

Mo et al. [18], delayed of the pozzolanic reactions also effects to the pore generation

in the concrete mixtures results to slow C-S-H formation and incomplete reactions.

There is a significant relationship between density and pores volume in the concrete

samples. As more pores in the concrete samples, density of the concrete will be

reduced [22]. It can be observed that, the density of the concrete mixture gradually

decreases for CM, CM5, CM10 and CM15.

These results are further verified through microstructure observation of

concrete samples of CM, CM5, CM10 and CM15 (Fig. 11). It can be noticeable

that the numbers of pores and sizes are increases as the POBA replacement

increases from 5% to 15%.

Despite of W/C factor, fine aggregates (sand) plays an important role on the

strength development of the concrete. According to Bu et al. [5], sand plays an

important role in the concrete strength due to its function as filler in between coarse

aggregates. They found that, increasing sand contents reduces the total porosity of

cement mortar. Moreover, pores size distribution becomes finer as sand amount

increases. In this study, variation of the fine aggregates amounts proportionally to

POBA replacement was carried out. Therefore, presumably POBA can replace sand

and acts as filler in the concrete mixture. It can be observed, pores size distribution

increases as sand replacement increases, as shown in Fig. 11. The most significant



probability of this happened is due to size of the POBA used as sand replacement.

POBA with size of passing 5 mm seems to promote increase in pores size distribution.

Table 4. Summary of concrete mixture samples

properties using UPV and compressive strength test.

Sample

Name

W/C

ratio

Average Density

of Concrete (kg/m3) Average

Velocity

(km/s)

Average

Compressive

Strength (N/mm2)

Day 7 Day

14

Day

28

Day

7

Day

14

Day

28

CM 0.54 2436.8 2429.2 2383.3 3.9 25.83 31.33 32.53

CM5 0.57 2404.5 2401.6 2374.3 3.6 30.40 33.97 34.27

CM10 0.60 2368.3 2359.9 2337.2 3.0 29.93 31.47 32.53

CM15 0.63 2354.3 2389.0 2329.0 2.9 24.40 26.47 29.53

(a) 0% POBA replacement at x37 mag. (b) 5% POBA replacement at x37 mag.

(c) 10% POBA replacement at x37 mag. (d) 15% POBA replacement at x37 mag.

Fig. 11. SEM image of micropore of sample concrete design.

POBA during hydration process will losses some part of its element and diluted

in water as ion. Hydration reaction takes place consume these ions and formed

ettringite and C-S-H. During dilution process, POBA size will be reduced and has

a greater tendency to leave void space. With replacement level of POBA in the

concrete mixture presence in excess (cement and sand), unreacted ion will be leave

unreacted and remain as ion. This will be causing difficulties during compaction

[13], since, addition of POBA will delayed the formation of the portlandite as

compared to OPC mixed design, as shown in Fig. 8. The reaction temperature has

been increased up to 50 ˚C ± 4 ˚C as to accelerate reaction of POBA and OPC.

Pores

Pores

Pores

Pores

Pores



Therefore, at temperature below than 50 ˚C ± 4 ˚C (ambient or curing pond

temperature), reaction time will be expected to be longer. CM is highest sand

content percentage followed by CM5, CM10, and CM15.

The high the W/C, the more water separates from the fresh paste forming a thinner

film on the aggregate’s grains, thereby decreasing the interfacial paste strength and

consequently decreasing the properties of the concrete. The effect of water to cement

ratio can be confirm in this study as the cement to water ratio of CM is the lowest

water to cement ratio (0.54) followed by CM5 (0.57), CM10 (0.60) and CM15 (0.63).

An additional impact of this ratio can be further verified by compressive strength

development of concrete samples as shown in Fig. 9 when compressive strength of

CM5, CM10 and CM15 show a decreasing trend as ratio of cement to water increases.

This is due to excess water has develop a thin film around aggregates grain during

hydration reaction and lowering the interfacial strength that binds hydration product

and aggregates when hardened. In Fig. 12 shows that the microstructure of aggregate-

paste interfacial zone of the sample concrete design with different POBA

replacement. It can observe that, the separation of the hydration products from the

aggregates linearly increases as the amount of W/C ratio increases. The arrow in Fig.

12 shows aggregate-paste interfacial transition zone for 0% to 15% of POBA

replacement in the concrete mixture. CM15 shows the highest separation of the

distance between hydration product to aggregates with 31.46 µm followed by CM10

and CM5 with 29.83 µm and 20.63 µm, respectively.

(a) 0% POBA replacement at x37 mag. (b) 5% POBA replacement at x37 mag.

(c) 10% POBA replacement at x37 mag. (d) 15% POBA replacement at x37 mag.

Fig. 12. SEM image of microstructure of aggregate

paste interfacial transition zone of sample concrete design.

These results are further verified through microstructure observation of

concrete samples of CM, CM5, CM10 and CM15 (Fig. 11). It can be noticeable

Aggregate-paste interfacial

transition zone


transition zone


transition zone



that the numbers of pores and sizes are increases as the POBA replacement

increases from 5% to 15%.

2.3.5. Ultra-pulse velocity

Figure 13 shows a result of ultra-pulse velocity (UPV) of concrete samples CM,

CM5, CM10 and CM15 at 28 curing days. The UPV values obtained was found in

the range of 2.9 km/s to 3.9 km/s (Table 4). Based on the replacement level of

POBA in the concrete mixture, it was found that, CM5 performed the best, which

indicates replacement level of 5% in concrete mixture. It appears that UPV values

decreases as POBA replacement increases compared to control sample (CM). This

attributes to the increment of W/C ratio which gives an impactful result of UPV

values as it can create an interfacial spacing between aggregates and hydration

products, and pores generation during hydration reaction. The interconnected pores

inside the concrete provided free space which causes UPV values to be reduced.

Fig. 13. Ultra-pulse velocity of concrete samples.

2.3.6. Thermal conductivity of concrete samples

It can be observed that, for the concrete mixture without presence of POBA (CM),

the thermal conductivity is 1.62 W/m.K whereas for the CM5, CM10 and CM15 are

1.54 W/m.K, 1.38 W/m.K and 1.23 W/m.K, respectively. On average, the reduction

in the thermal conductivity was about 4 to 14% from control sample. The decrease in

thermal conductivity as POBA replacement level increases could be attributed to the

presence of the porosity in the concrete samples [21, 23] and interfacial distance

between hydration products and aggregates. Increasing voids in the concrete mixtures

will reduce its density. This voids initially filled up by water and during the hydration

process, this water will be consumed as to proceed with the reactions. Drying of the

concrete will removes its water content will create voids and entrapped air. The

presence of entrapped air will promote an additional resistance to the thermal

resistance network of the concrete, and thus, decrease thermal conductivity of the

concrete mixture. The results of this experiment agree well with Misri et al. [24],

where thermal conductivity is a function of density. Low density will show low

thermal conductivity of the concrete mixture. Figure 14 shows the thermal

conductivity results of CM, CM5, CM10 and CM15 with replacement level of sand

Sample Name

Ult

ra-P

uls

e V

elo

cit

y (

km

/s)

0

1

2

3

4

5

CM CM5 CM10 CM15

3.9000

3.6000

3.00002.9000



and cement. Materials with high thermal conductivity normally used as heat sink,

whereas, low value suitable for thermal insulation materials.

Fig. 14. Thermal conductivity of concrete samples.

3. Conclusions

From the present study on the feasibility of POBA as cement and sand replacement,

it is shown that percentage replacement of the POBA contributes a significant effect

to properties of concrete. The following conclusion can be drawn.

• The pozzolanic activity Thermal conductivity decreases as POBA replacement

level increases as percentage level of POBA increases. Pozzolanic activity

closely related to the amount of OPC used. This is due to the hydration reaction

will occurs higher when OPC cement percentage in the suspensions increases,

thus, established more of formation of ettringite and C-S-H. The POBA15

shows %LC.s maximum value of 1.16×105 followed by POBA10 and POBA5

with 0.96×105 and 0.29×105, respectively. In general, based on the %LC, the

pozzolanic reactivity of the POBA samples tested from high to low is

POBA15>POBA10>POBA5.

• Compressive strength of concrete samples is decreases as replacement level is increases.

• Water to cement ratio is an important parameter in concrete design. Increased in

the W/C ratio will increase number of pores and create interfacial spacing

distance between aggregates. With angular, cylindrical and irregular shape of

POBA structure, it attributes to increase in the compressive strength of concrete

samples at 28 days. Inconsistent of the water to cement ratio in the concrete

design gives an impact on the strength development of the concrete samples and

through observation under SEM, it shown that, an increase of number and size

of pores, and interfacial spacing distance between aggregates and paste.

• Ultra-pulse velocity shows that, increase in POBA replacement level will

decrease UPV value. This attributes to the increment of the interfacial spacing

distance and number of pores in the concrete samples.

• Thermal conductivity decreases as POBA replacement level increases.

• Therefore, POBA is seems gives an advantage to the concrete properties as it

provides pozzolanic reaction and formed a product that possessing

Sample Name

Th

erm

al

Con

du

cti

vit

y (

W/m

.K)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

CM CM5 CM10 CM15

1.61751.5400

1.3815

1.2315



cementitious properties that will increase the compressive strength of concrete.

With a constant W/C ratio of 0.54 promotes a beneficial impact on the concrete

in terms of pores size and number, interfacial spacing distance ultra-pulse

velocity and thermal conductivity. In this research paper, CM5 performed the

best compared to CM10 and CM15 with less pore’s generation, high thermal

conductivity and 5.2% improved compressive strength compared to CM.

Furthermore, it is recommended for future research to study the effect of

different POBA particle size to the compressive strength of the concrete mixture,

and stability of concrete to the temperature treatment.

Acknowledgement

The research work was supported by the Faculty of Chemical Engineering

University Technology MARA (UiTM) and Faculty of Civil Engineering

University Technology MARA (UiTM), Johor Branch, Pasir Gudang Campus.

Nomenclatures

OPC Ordinary Portland Cement

POBA Palm Boiler Ash

SEM Scanning Electron Microscope

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