THE EFFECTS OF TYPES OF RICE HUSK ASH ON THE POROSITY OF CONCRETE

M.F. NURUDDIN

Associate Profesor Department of Civil Engineering, Universiti Teknologi PETRONAS Malaysia

Email: fadhilnuruddin@petronas.com.my

N. SHAFIQ

Associate Profesor Department of Civil Engineering, Universiti Teknologi PETRONAS Malaysia

N.L.M. KAMAL

Research Assistant, Department of Civil Engineering, Universiti Teknologi PETRONAS Malaysia

ABSTRACT

Controlled burning can produce amorphous rice husk ash (RHA) with high silica content and this can significantly

enhance the porosity of concrete. This study has been undertaken to investigate the effects of replacing 5%, 10%,

15% and 20% of cement in concrete by both RHA and microwave incinerated rice husk ash (MIRHA) burnt at

800°C, 700°C and 600°C. The porosity of concrete mixes was determined at 28 days and in general showed that

5% replacement of MIRHA-800 provided the lowest percentage of porosity compared to all mixes. X-Ray Fluorescence (XRF) analysis was performed to determine the content of various chemical oxides in RHA and

MIRHA.

Keywords: amourphous, silica, porosity.

1. INTRODUCTION 1.1 Burning Combustion Uncontrolled burning or open-field burning is common

place in rice paddy fields in managing rice husk that is

a by product of rice production. This method is the

fastest, cheapest and most effective way of disposal but

these proven benefits are outweighed by the

environmental hazards imposed by open-field burning

which distinctly leads to smoke contamination. To address these environmental issues the use of controlled

burning method is proposed and it was found that under

controlled combustion condition, rice husk ash (RHA)

with high reactivity of amorphous silica can be

produced [1]. RHA has been used as a highly reactive

pozzolanic material to improve the concrete properties.

Recent researches have shown that, RHA, rich in silica

(about 85% to 90%), can be a green material and re-

utilized in construction materials, by controlling the

burning temperature to ensure it is in a non-crystalline

state. It has been reported that RHA can be added to

concrete mixtures to substitute the more expensive Portland cement to lower the construction cost while at

the same time protecting the environment. It also cites

that RHA is not only cheap but also can improve the

durability of concrete [2]. Some researchers found that

through pozzolanic reaction, the addition of pozzolanic

materials can affect the porosity of concrete by

strengthening the aggregate-cement paste and the

reaction can modify the micropores structure. The

products formed due to the pozzolanic reactions occupy

the empty spaces in concrete pore structures which thus

become densified. The porosity of cement paste is then

reduced, and subsequently the pores are refined.

1.2 Amorphous Silica Amorphous silica rice husk is burnt in controlled

temperatures which are below 700°C. The ash

generated is amorphous in nature. The highest amount

of amorphous silica occurs in samples burnt in the range of 500°C - 700°C [3]. Another researcher

reported that a highly reactive ash can be produced by

maintaining the combustion temperature below 500°C

[1]. It was also stated by Hamad that ash prepared at a

temperature of about 500°C to 600°C consist of

amorphous silica [4]. In addition, the amorphous state

can achieve with under oxidising conditions for

relatively prolonged period or up to 680°C provided the

high temperature exposure was less than one minute.

The transformation of this amorphous state to

crystalline state takes place if the ash is exposed to high

temperatures of above 850°C. It is reported that silica was predominantly in amorphous form that the crystals

present in the ashes grew with time of burning for

incineration temperature up to 700°C [5].

1.3 Crystallinity It is not suggested to burn rice husk above 800°C

longer than one hour, because prolonged heating above

this temperature may cause the material to convert (at

least in part) to crystalline silica; first to cristobalite and

then tridymite [3]. At 800°C, the ash will convert to

cristobalite and after burning at 1150°C both

cristobalite and tidymite will be formed. This

crystalline will becomes less reactive.

1.4 Pozzolanic Pozzolanic materials are the main siliceous that

produce calcium silicate hydrate after reacting with

water and lime. It has been studied that high content of

amorphous silica and large internal surface area can make RHA a highly reactive pozzolanic material that

can improve the strength and durability of concrete.

1.5 Porosity Concrete porosity is definitely stated in terms of

percentage by volume of concrete. The strength and

durability of concrete are also influenced by its porosity

characteristic [6]. Many researchers have same opinion

that porosity of concrete can be reduced using

pozzolana or supplementary cementing material. There

are two principal contributing attributes of pozzolana. Firstly, the pore structure in the cement paste matrix

can be more denser using a quality pozzolan, secondly

the chemical reaction of lime crystals that form binders

increase paste density, reduced porosity over time, and

enhance the matrix chemical resistance to many

aggressive attacks.

2. MATERIALS AND EXPERIMENTAL DETAILS

2.1 Materials MIRHA was obtained by burning rice husk in UTPMI

(Figure 1) with a controlled temperature in order to establish the optimum burning temperature.

Fig. 1 UTP Microwave Incinerator (UTPMI)

The UTPMI used for the burning process had the

temperatures set at 800°C, 700ºC and 600°C to produce

good quality MIRHA. The source of rice husk was

taken from rice milling plant, Bernas-Malaysia. One

type of RHA was obtained from rice mill in Sungai

Ranggam, Perak (assigned as SG-RHA) and it is a

waste product of rice husk that was burned as fuel for

boilers at 1000°C for 1 minute. While another type of

RHA was obtained from rice mill in Sungai Manik,

Perak (assigned as SM-RHA) which has been burnt to

get the energy from the burning and supplied it for parboiling of rice. It was burnt in the range of 600°C to

900°C. Grinding of MIRHA, SG-RHA and SM-RHA

are conducted using a Los Angeles abrasion machine

with 3000 cycles. The cement used in this investigation

was ordinary Portland cement. The fine aggregate used

was natural sand with the fineness modulus 2.7 and

classified in Zone 3. The coarse aggregate used was

crushed aggregate with the maximum size of 20 mm

according to BS 812-103.2 1989. The absolute volume

method adopted in calculating the mixture proportions.

MIRHA, SG-RHA and SM-RHA were incorparated as a replacement of cement on a weight basis. A number

of mixes have been chosen so that the performance of

concrete with different admixtures can be compared.

MIRHA, SG-RHA and SM-RHA is then used to

replace 5%, 10%, 15% and 20% of cement content in

concrete with 0.45 w/c. The control concrete was

designated normal concrete (NC) without any addition

of MIRHA, SG-RHA and SM-RHA as a comparison.

Superplasticizer was used in concrete containing

MIRHA to increase its workability. The mixture

proportion for 0.45 water cement ratio is shown in

Table 1.

Table 1 Mixture Proportions of Concrete

MIRHA/

SG-RHA/SM-RHA SP Cement

(%) (kg/m³) (%) (kg/m³)

0

5

10

15

20

0.00

25.00

50.00

70.00

95.00

0.0

0.4

0.8

1.5

2.0

475.00

450.00

430.00

405.00

380.00

2.2 Casting, Curing and Testing of Specimens The concrete specimens were prepared in the laboratory

using wooden moulds of size 400x400x40mm and

compacted on table vibrator. After casting, the concrete

planks were covered with plastic sheet and left in the

casting room for 24 hours. After that all specimens

were demoulded and put into the curing tank at room

temperature until the desired age of testing. At the

defined ages for testing: three 50mm diameter discs

were cored-out from the slab of concrete. Total porosity

of the samples was determined according to the vacuum

saturation method that was developed by RILEM [7]

using the Eq. 1 stated below:

100

watersat

ovensat

WW

WWP

(1)

Where, P is the total porosity in percentage, Wsat is the

weight of saturated samples measured in the air; Woven

is the weight of oven dried samples measured in the air,

and Wwater is the weight of saturated samples measured

in water, all weight measurement are in g.

2.3 X-Ray Fluorescence Figure 2 shows the X-Ray Spectrometer used in this

research.

Fig. 2 Bruker Axs S4 Pioneer X-Ray Spectrometer

X-Ray Fluorescence (XRF) analysis was performed to

determine the content of various chemical oxides in MIRHA. The analysis was carried out using

spectrometer of Bruker Axs S4Pioneer. The powder

sample of MIRHA was compacted in a specific

container using hand compactor and brought into the

spectrometer. The analyzed result was captured using

the installed software.

3. RESULTS AND DISCUSSION

3.1 X-Ray Fluorescence (XRF) X-Ray Fluorescence (XRF) analysis is proficient in

analyzing material contents inside MIRHA, hence the

amount of SiO2 can be observed. The presence of

various materials within MIRHA sample can be seen in

Table 2. This table shows the result of XRF analysis of

MIRHA burnt at 800°C, 700°C, 600°C, RHA collected

from Sg. Ranggam mill (SG-RHA) as well as from Sg.

Manik mill (SM-RHA). It is evident that burning RHA with higher temperature will give higher silica content.

Despite of no significant different of SiO2 content

between MIRHA burnt at different temperature but

MIRHA burnt at 800°C showed a lower result on

porosity. The oxide content of MIRHA burnt at 800°C

was the optimum composition that could give

significant improvement to the concrete porosity. The

lower percentage of porosity of MIRHA burnt at 800°C

compared to SG-RHA as well as SM-RHA revealed

that burning temperature can influence the quality of

RHA. It is shown that even the percentage of SiO2 is higher with high burning temperatures; it is believed

that the RHA have partially converted to crystalline

silica which is could not react with Ca(OH)2. It has

been revealed that at higher temperatures with longer

burning times, a crystalline structure is formed and this

can lowers the pozzolanic activity [8].

Figure 2 presented the concrete porosity characteristic

of concrete with different binder types. The porosity of

concrete containing RHA was found to be lower than

normal concrete regardless of different burning

temperatures and percentages inclusion of RHA. When

subjected to increasing of MIRHA and SG-RHA inclusion, the concrete exhibits lower performance due

to declining of OPC amounts in mixtures. This could be

due to the un-hydrated MIRHA, SG-RHA and SM-

RHA that absorb more water. Nonetheless, the SM-

RHA concretes show that only 5% of inclusion gave a

better performance compared to normal concrete while

10%, 15% and 20% inclusion of RHA leads the

increasing of porosity. Its show that, addition of

MIRHA which is more than 5%, to the concrete

mixture absorbed water in large amount and cause the

mixture to be dry. As shown in Figure 3, there is only slight difference in porosity between all percentages of

MIRHA-800 in concrete. This shows consistency in the

amorphousness and highest amount of SiO2. It is

evident from the slight difference of other percentage

inclusion of MIRHA. This confirm that’s 5% is the

best. Nevertheless, MIRHA-700 and MIRHA-600

concretes show significant difference between all

percentages of inclusion. It is believed that with lower

burning temperature the ability to extract the optimum

amount of silica in rice husk is hampered. Even though

RHA burnt at 600°C, open burning doest not improve

the quality of RHA. Therefore microwave burning contributes significantly.

Table 2 Chemical composition of OPC, MIRHA, SG-RHA and SM-RHA

Oxide

Weight %

OPC MIRHA 800 MIRHA 700 MIRHA 600 SG-RHA SM-RHA

Na2O 0.0164 0.1215 0.0725 0.0195 0.0203 0.0371

MgO 1.4334 0.4864 0.5036 0.5885 0.3562 0.5696

Al2O3 2.8357 0.4473 0.4181 0.3572 0.4889 0.9607

SiO2 20.4449 89.3430 87.2145 86.3115 90.3289 90.3615

P2O5 0.1023 2.5792 3.0006 3.008 2.5118 2.3052

K2O 0.2646 4.9756 6.1856 6.3366 4.5351 4.2964

CaO 67.7341 0.7584 0.8106 0.9996 0.5634 0.8627

TiO2 0.1701 0.0183 0.0184 0.0191 0.0843 0.0278

Fe2O3 4.6352 0.3971 0.3957 0.7227 0.3900 0.2907

SO3 2.2020 0.8952 1.3131 1.5145 0.6458 0.2038

MnO 0.1614 0.0807 0.0836 0.1301 0.0667 0.0844

Fig. 3 Porosity of OPC, MIRHA, SG-RHA and SM-RHA concretes

This phenomena had interrupted the cement hydration

process to produce Ca(OH)2. Lower content of

Ca(OH)2 produced also affected the pozzolanic reaction

with SiO2 [9].With insufficient amount of Ca(OH)2,

pozzolanic reaction could not occur properly and

resulted in lower calcium silicate hydrate (C-S-H) gels

being produced. While the first provides a denser

interface by acting as a filler and providing secondary

hydration products, the second helps towards

deflocculation of the cement and MIRHA particles, and

reduction in the water content of the mix as well as

providing extra consistency. The increased rate of

hydration may be attributable to the ability of MIRHA

to provide nucleating sites to precipitating hydration

products like lime, CSH, and ettringite. It has been

revealed that with correct mix proportion, MIRHA

concrete can achieve early strength higher than normal

concrete as recommended by many researchers [10].

4. CONCLUSIONS

This research was carried out to identify the effect of

burning temperature between control combustion and

uncontrolled combustion of RHA on concrete porosity.

This study also was carried out to identify the optimum

burning temperature and percentage inclusion of

MIRHA, to obtain quality rice husk ash that can

significantly improve the concrete porosity. The

utilization of Microwave Incinerated Rice Husk Ash

(MIRHA) into the concrete mix proportions has given various effects to the concrete properties. The

improvement of porosity characteristic results of

MIRHA concrete samples was influenced by the

quality of MIRHA and mix proportion that were used.

The following conclusions can be drawn from the

study:

1. The percentage porosity of MIRHA concretes are

lower compared to plain cement mortar regardless of

burning temperature of MIRHA.

2. The percentage porosity of MIRHA concretes are

lower compared to plain cement mortar regardless of percentage inclusion of MIRHA of each different

temperature.

3. This research shows that increased burning

temperature for MIRHA produced concretes with

decreased porosity.

4. Porosity of MIRHA concretes are found lower

compared to RHA burnt at rice mills under high

temperature.

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