Construction and Building Materials 25 (2011) 798–805

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Construction and Building Materials

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Production of rice husk ash for use in concrete as a supplementarycementitious material

M.F.M. Zain, M.N. Islam *, F. Mahmud, M. JamilFaculty of Engineering and Built Environment, National University of Malaysia (UKM), 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 11 March 2009Received in revised form 1 July 2010Accepted 18 July 2010Available online 14 August 2010

Keywords:Rice husk ash (RHA)By-productConcreteWasteCombustionGrindingPozzolanSupplementary cementitious materialX-ray diffractionScanning electron microscopy

0950-0618/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2010.07.003

* Corresponding author. Tel.: +880 1716539548.E-mail addresses: nazrul@eng.ukm.my, nazrul2100

Rice husk ash (RHA), rich in silica content, can be produced from rice husk using appropriate combustiontechnique for use in concrete as a supplementary cementitious material. This paper discusses productionprocess of RHA from rice husk and the quality of RHA produced using rudimentary furnace of the NationalUniversity of Malaysia (UKM). Three combustion methods and two grinding methods were used to inves-tigate physical characteristics and chemical aspects of RHA produced. Combustion temperature distribu-tion of the furnace, ash particle size, silica crystallization phase and chemical content of the producedRHA were studied using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Fromthe investigation, it was found that combustion period, chilling duration, and grinding process and dura-tion are important in obtaining RHA of standard fineness and quality. In addition, air ducts in the furnaceare very useful in order to supply air for proper burning of rice husk.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Rice husk can be burnt into ash that fulfils the physical charac-teristics and chemical composition of mineral admixtures. Pozzola-nic activity of rice husk ash (RHA) depends on (i) silica content, (ii)silica crystallization phase, and (iii) size and surface area of ashparticles. In addition, ash must contain only a small amount of car-bon. RHA that has amorphous silica content and large surface areacan be produced by combustion of rice husk at controlled temper-ature [1]. Suitable incinerator/furnace as well as grinding methodis required for burning and grinding rice husk in order to obtaingood quality ash.

Rice husk combustion technology has developed from open airburning in the field (around 1970s) to combustion using liquidizedlayers method (around 1990s). Temperature and combustionperiod can be controlled in liquidized layers combustion method[2]. Only moderate temperature and short period are required inthis method. However, researchers who study RHA usually buildtheir own incinerator/furnace or collect ash from rice mill.Although the studies on pozzolanic activity of RHA, its use as asupplementary cementitious material, and its environmental and

ll rights reserved.

@yahoo.com (M.N. Islam).

economical benefits are available in many literatures [3–54], veryfew of them deal with rice husk combustion and grinding methods.

In this study, rice husk combustion was performed using asimple furnace designed and built at the National University ofMalaysia (UKM), Malaysia. The design concept of this furnacewas based on those reported by Loo et al. [28] and Ramli [44]. Inthis research, three methods of rice husk combustion were usedbased on combustion fire, air supply, and cooling durations. Theproduced ash was ground to ensure that it meets the requirementsof BS 3892 standard [55]. In this study, two grinding methods weretried for grinding burnt ash. After grinding, physical characteristicsand chemical composition of the rice husk ash were determinedusing X-ray diffraction (XRD), scanning electron microscopy(SEM), and chemical analysis.

2. Experimental program

2.1. Furnace details

A simple furnace shown in Fig. 1 was designed and built after reviewing fur-naces reported by Loo et al. [28] and Ramli [44]. It has two sections: a ferrocementcylinder and a steel cylinder. Diameter of the ferrocement cylinder is 1030 mm,height is 1510 mm and wall thickness is 60 mm. The function of ferrocement cylin-der is to trap combustion heat within the furnace and preventing it from escapinginto air. Diameter of the steel cylinder is 760 mm, height is 1090 mm and thicknessis 5 mm. The steel cylinder acts as a container for burning rice husk.

Cap

Ferrocement cylinder Thickness 60 mm

Smoke Chimney

Small hole, Ø 5 mm

Finished section of furnace (Furnace opening closed by iron plate)

Door of steel cylinder

Air duct

Burnt ash collector

Plan (Steel Cylinder)

Rear View

Fig. 1. Furnace at UKM for burning rice husk.

Table 1Comparison of rice husk combustion methods A, B, and C.

Serial no. Combustion stage duration Combustion method

A B C

1 Fire duration (min) 30 60 302 Air supply duration (min) 90 105 603 Chilling duration (day) 1 1 2

M.F.M. Zain et al. / Construction and Building Materials 25 (2011) 798–805 799

Inside the steel cylinder, two units of steel air ducts are located. Diameter,height and thickness of each duct are 100 mm, 200 mm, and 5 mm, respectively.Air ducts play two roles: supply air to husk during combustion process and act aspassages for fire. Combustion at the lower and upper levels of the furnace takesplace at the same time, causing the husk to burn faster. Air ducts are also usefulin controlling combustion temperature. Combustion temperature is controlled bysupplying air through air ducts using electric fan. Surfaces of the steel cylinderand air ducts are perforated with small holes of 5 mm diameter. Both ferrocementand steel cylinders are covered at the top with steel cap equipped with smoke chim-ney as shown in Fig. 1.

2.2. Combustion of rice husk

Rice husk was put into the furnace through opening at the top. Gasoline burnerswere placed inside the furnace under the air ducts (Fig. 2a). Combustion was con-tinued for a preset duration. After that duration, gasoline burners were taken out.Electric fan was then switched on and placed near the door of the furnace. It sup-plied air and maintained embers of the burning husk. Air was supplied for a fewminutes and then ash was allowed to chill. When the ash was cold, outlet at the bot-tom of the cylinder was opened and the ash was allowed to fall into a container

Rice husk

Gasoline Burner

(a)

Fig. 2. Rice husk combustion process: (a) combustion using

(Fig. 2b). In this study, three combustion methods, labelled as A, B, and C were triedas shown in Table 1. The main differences among A, B, and C were durations of com-bustion, air supply, and chilling of ash. During combustion process, temperaturewas recorded using thermometers located along the height of the furnace (200,400 and 600 mm above the base of the steel cylinder) as shown in Fig. 3. The ther-mometers were set into the furnace through holes of the ferrocement cylinder(thermometer 1 at layer 1 and so on).

Ash collector

(b)

gasoline burner and (b) taking out the burnt rice husk.

Layer 1

Layer 2

Layer 3

Layer 4

Fig. 3. Positions of thermometers from the base of the steel cylinder.

Table 2Types of rod used in grinding burnt rice husk.

Types of rod used

A B C

Number 40 20 Combination of A and BDiameter (mm) 10 20Length (mm) 500 500

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2.3. Grinding of burnt rice husk

Grinding of burnt rice husk was done using a Los Angeles machine. Two typesand numbers of rods, rod A and rod C as shown in Table 2, were tried for grindingburnt rice husk in this study. About 5.0 kg of ash was placed into the Los Angelesmachine for each grinding.

2.4. Physical characteristics and chemical composition

The size of rice husk ash was determined using scanning electron microscopy(SEM). The chemical analysis was performed using X-ray fluorescence (XRF) tech-nique, and silica crystallization phase was determined using X-ray diffraction(XRD) technique by an automatic diffractometer. Carbon based contents were de-tected using CHNS-932 Elemental Analyser.

0

100

200

300

400

500

600

700

85 (1:25)

115 (1:55)

175 (2:55)

205 (3:25)

235(3:55

Tem

pera

ture

(o C)

Burning duration, min

Thermometer 4 Thermometer 3

Fig. 4. Variation of combustion temperature with burning durat

3. Results and discussions

3.1. Combustion stages

Fig. 4 shows the variation of combustion temperature withburning duration at different layers of the furnace. The followingobservation can be made from this figure.

3.1.1. First 2 hThe average temperature of all layers of the furnace (see Fig. 3)

was 500–600 �C during the first 2 h of combustion. When combus-tion temperature reached 100 �C, rice husk started losing itsweight as water started evaporating from rice husk. When temper-ature reached 350 �C, rice husk burned, water became vapour andloss of weight continued. In the temperature range of 400–500 �C,carbon became oxidised and rice husk lost its major part of weight.Combustion of rice husk took place at all four layers and burnt ricehusk looked black at this stage. Silica in the husk was still in amor-phous state.

3.1.2. 2–5 hDuring this period of combustion, ash was still burning. Tem-

perature at layer 1 (thermometer 1) increased uniformly from500 �C to 700 �C. Temperature at layers 2 and 3 decreased from500 �C to 200 �C. Temperature at layer 4 remained around 100 �Cto 150 0C.

3.1.3. 5–24 hCombustion temperature at layers 4, 3 and 2 started decreasing

uniformly until it was below 100 �C. Husk at these layers was com-pletely burnt. Temperature at layer 1 (containing embers) was stillhigh, between 200 �C and 250 �C.

3.1.4. 24–48 hAt this stage, temperature at all layers was low i.e. about 52 �C.

A thin layer of burnt husk, black in color, was found at the surfacelayer around the cylinder wall and around the air ducts. Blotches ofblack ash were also found at areas far from the air ducts (Fig. 5).The ash of other parts was white in color. There is high possibilitythat the areas of black ash blotches (Fig. 5) did not get enough airduring combustion process because they were far from the airducts. The air ducts were found to be capable of supplying air tohusk close to the ducts and, therefore, combustion was completearound the ducts. It was felt that number of air ducts should be in-creased for thorough combustion of rice husk.

)

265 (4:25)

295 (4:55)

1158 (19:18)

1220 (20:20)

2685 (44:45)

utes (hours)

Thermometer 2 Thermometer 1

ion at various layers of the furnace (combustion method C).

Steel cylinder

Air ducts

Black ash blotch

White ash

Fig. 5. Areas of black ash blotches of the furnace.

60

70

80

90

100

Passing 45 µmsieve (%) RHA (A)-5 kg-Rod A

RHA (A)-5 kg-Rod CRHA (A)-11 kg-Rod C

M.F.M. Zain et al. / Construction and Building Materials 25 (2011) 798–805 801

3.1.5. After 48 hAt this stage, temperature of rice husk ash at all layers was

around 30 �C. Ash was predominantly white in color and fine intexture.

It may be relevant to mention here that loading capacity of theUKM furnace is 45–50 kg for each combustion session. After twodays of combustion, 5.0 kg of ash was produced, i.e. about 10% ofrice husk only. Cook et al. [15] and Mahmud et al. [29] found thatashes produced in their cases were around 25% and 15%, respec-tively. The reason behind the less amount of ash obtained in thisstudy was that only white ash was taken while black ash was dis-carded due to its high carbon content. As mentioned earlier, forproper burning and for getting more white ash, number of air ductsin the furnace should be increased.

40

50

15 30 60 90 120 150 180 210

Grinding duration (minute)

Fig. 6. Variation of fineness of rice husk ash with grinding duration for grindingmethods A and C.

3.2. Grinding process

Researchers [17,29,53] agree that finer pozzolanic ash is better.Fineness of rice husk ash is important because it influences the rateof reaction and the rate of gain in concrete strength. Other thaninfluencing the rate of reaction, fineness also influences water–ce-ment ratio, creep, shrinkage, and workability of concrete. Mahmudet al. [29] found that finer rice husk ash particle yields larger sur-face area and increases strength of concrete. Chemically reactivevery fine substance would fill empty columns in concrete in anoptimum manner [56]. According to the standard [55], particlesof RHA retained on 45 lm sieve should not be more than 12.0%.Therefore, to get the required fineness, proper grinding of burntash is very important.

As mentioned earlier, two types of rod (rod A and rod C of Table2) were used for grinding burnt husk in this study. The use of rod Chelped to shorten the grinding period compared to that of rod A asshown in Fig. 6. In 90 min of grinding, fineness of 5 kg burnt huskobtained by combustion method A met the standard requirements[55]. Grinding of burnt husk in large amount (11 kg) at a time wasfound to delay grinding period. This was proven when grinding hadbeen continued for 210 min (3:30 h) using rod C and ground ashonly marginally satisfied the required fineness (Fig. 6). Therefore,5 kg of burnt rice husk was ground at a time using rod C for all sub-sequent grinding.

Fig. 7 shows that time required for grinding burnt ash obtainedusing combustion method C was the shortest i.e. 30 min, and thiswas sufficient for ash to meet the standard. The ash obtained usingcombustion methods A and B required more time for grinding tomeet the required standard, i.e. 90 and 60 min, respectively. Inmethod C, chilling of burnt rice husk had been continued for 2 dayswhich yielded finer ash as compared to those obtained by chillingfor 1 day in combustion methods A and B (Table 1). This was be-cause, during 5–24 h period, layer 1 was containing embers with

temperature around 200–300 �C (Fig. 4). The burnt rice husk pro-duced at that stage was coarse and was not completely burnt.Self-chilling process for longer duration in method C was usefulin yielding finer ash. Therefore, chilling period of 2 days (as usedin combustion method C) instead of 1 day is recommended.

3.3. Physical characteristics and chemical composition of producedRHA

3.3.1. Scanning electron microscopy (SEM)Fig. 8 shows scanning electron micrographs (SEM) of rice husk

ash obtained by combustion method C and ground for 15, 60,and 120 min. After grinding for 15, 60 and 120 min, the meandiameter of rice husk ash was 49.00 lm (Fig. 8a), 40.97 lm(Fig. 8b) and 16.63 lm (Fig. 8c), respectively. Fig. 8 also shows that,due to grinding for relatively longer period, cellular structure ofrice husk ash was progressively broken down. In Fig. 8a, the cellu-lar shape of rice husk ash could be clearly seen. But after grinding itfor 120 min (Fig. 8c), the cellular structure disappeared and be-came much smaller particles. The large surface area of rice huskash was caused by the coarse micro-structural shape like cubesof cellular beehives [29,53] as shown in Fig. 8a and b. The cellularshape creates water absorption problem in a damp surrounding.This problem could be overcome by breaking down cellular struc-ture and, thus, reducing the ability to absorb water [29]. Therefore,grinding of burnt husk for 60 min or more is recommended.

Fig. 8. Scanning electron micrograph (SEM) of RHA particles ground for: (a) 15 min,(b) 60 min, and (c) 120 min.

30

40

50

60

70

80

90

100

15 30 60 90 120

Passing 45 µmsieve (%)

Grinding duration (minute)

RHA (A)

RHA (B)

RHA (C)

Fig. 7. Variation of fineness with grinding duration for rice husk ash obtained usingcombustion methods A, B, and C.

802 M.F.M. Zain et al. / Construction and Building Materials 25 (2011) 798–805

The phenomenon of breaking down cellular structure and thusresulting reduction of fine pores is the main factor in increasingstrength and durability of concrete. In this aspect, uniqueness ofrice husk ash as super pozzolanic substance was proven in researchby Manmohan and Mehta [33]. They also found that rice husk ashwas the most active pozzolan compared to several other mineraladmixtures (e.g., silica fume, fly ash and strong furnace dross). Un-like rice husk ash produced at combustion temperature 600–700 �C, the above substances could be obtained from fast chillingprocess or from condensation process of liquid having high tem-perature. This would yield particles having surfaces without finepores. The effect is that a period of induction would be requiredto reactivate the surface of those mineral admixtures in an alkalienvironment (medium of hydration for Portland cement). Theadvantage of rice husk ash is that its surface has high fine poresand the period of induction is the least as compared to those ofmineral substances mentioned above [33].

3.3.2. X-ray diffraction (XRD) analysisXRD analyses were performed to identify differences in the for-

mation of amorphous or crystalline silica for different combustionduration, temperature and cooling regime. From the XRD analyses,a qualitative assessment of the crystallinity of the samples can beobtained from the intensity of the narrow reflections as comparedto the broad band around 22� (2h) [41].

The intense broad peak observed for the samples (Fig. 9) indi-cates the amorphous nature of silica at different combustion con-ditions. Fig. 9 shows that all methods of combustion (A, B, and C)produced amorphous silica. Thus, furnace of UKM is able to pro-duce amorphous silica. The constant furnace combustion tempera-ture at 500–700 �C for the first 2 h (Fig. 4) was useful for this case.Temperature in the furnace was almost constant at 500–700 �C forfirst 2 h and then decreased uniformly until burnt husk becamecold after 48 h. This is in line with the researchers [13,15,24]who agree that silica does not become crystalline if combustiontemperature is below 700 �C. In certain cases, Cook [17] foundcrystalline substance at temperature exceeding 600 �C. At 800 �Ctemperature, silica begins to change into crystalline phase[17,24]. At 900 �C temperature, all silica becomes crystalline andno more amorphous structure is available [24]. At this stage,

formation of crystalline crystobalite starts. If temperature furtherincreases, tridymite is produced. However, RHA produced at thefurnace of UKM did not show major crystallization form and con-tained mostly amorphous silica.

3.3.3. Chemical composition analysisTable 3 shows the chemical analysis results of RHA obtained

using combustion methods A, B, and C. The table also shows chem-ical analyses of RHA obtained by several other researchers[15,24,29,42,53]. It can be observed that the content of SiO2 ofRHA produced in this study was between 79% and 87% (Table 3).Alkaline substances like K2O were main foreign particles in RHAhaving content between 2% and 4%. According to Mehta [1], K2Ocontent depends on types and amount of fertilizers used duringgrowing period of the paddy plant. Content of foreign particles likeCaO, MgO, P2O5 and others were less than 1%. Loss on ignition (LOI)progressively decreased for combustion methods A, B, and C. LOI of

Fig. 9. X-ray spectrum of RHA obtained using combustion methods: (a) A, (b) B, and (c) C.

M.F.M. Zain et al. / Construction and Building Materials 25 (2011) 798–805 803

rice husk ash obtained using combustion method C was the lowest(8.45%). Thus fire duration, air supply duration and chilling processfollowed in method C were reasonably appropriate.

RHA with low carbon content increases pozzolanic activity [15].Water requirement in concrete is usually less for ash with low LOI.Mahmud et al. [29] found that high LOI is caused by high moisturecontent and ash becomes darker in that case. In this study, ricehusk used was ensured to be dry before burning. Color of ash ob-served in methods A, B, and C was found to be progressively lighterwhich was consistent with the decreasing value of LOI. Zhang andMalhotra [53] reported RHA with a loss of ignition of 8.55% and

carbon content of 5.91%, resulting in a black color. Nehdi et al.[42] reported RHA with a loss on ignition of 1.80% and carbon con-tent of 1.00%. James and Rao [24] reported RHA with a loss on igni-tion of 3.00% resulting in a white color. It can be observed thatmethod C produced the best quality ash among the methods fol-lowed at UKM (see Table 3). Loss on ignition, carbon content andcolor of the ash produced in method C were 8.45%, 3.21% andwhite, respectively. These results are comparable to the results ofother researchers [15,24,29,42,53]. Thus the quality of ash pro-duced by burning of rice husk in the furnace of UKM was good en-ough to use in concrete as a supplementary cementitious material.

Table 3Comparison of chemical content in percent of RHA produced using different methods.

Item Combustion methods atfurnace of UKM

Results of other researchers

A B C Cook et al. [15] James and Rao [24] Mahmud et al. [29] Nehdi et al. [42] Zhang and Malhotra [53]

Silicon dioxide (SiO2) 79.84 80.72 86.49 93.15 94.43 92.70 94.60 87.20Aluminium oxide (Al2O3) 0.14 0.08 0.01 0.41 – 0.20 0.30 0.15Ferric oxide (Fe2O3) 1.16 1.10 0.91 0.20 1.30 0.40 0.30 0.16Calcium oxide (CaO) 0.55 0.56 0.50 0.41 0.90 0.80 0.40 0.55Magnesium oxide (MgO) 0.19 0.18 0.13 0.45 0.65 0.20 0.30 0.35Sodium oxide (Na2O) 0.08 0.00 0.05 0.08 0.55 0.20 0.20 1.12Potassium oxide (K2O) 2.90 3.14 2.70 2.31 1.32 – 1.30 3.68Phosphorus oxide (P2O5) 0.80 0.90 0.69 – – – 0.30 0.50Titanium oxide (TiO2) 0.01 0.04 0.00 – – – 0.03 0.01Sulphur trioxide (SO3) – – – – – – – 0.24Manganese oxide (MnO) 0.07 0.06 0.07 – 0.38 – – –Carbon (C) 7.75 6.55 3.21 – – – 1.00 5.91Color Black Light grey White – White – – BlackLoss on ignition (LOI) 14.26 13.22 8.45 2.77 3.0 4.40 1.80 8.55

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4. Conclusions

(1) Combustion of rice husk with fire duration of 30 min, airsupply duration of 60 min and chilling duration of 2 days(combustion method C) was found to produce good qualityash. Carbon content of the ash was 3.21% and color waswhite.

(2) Air ducts in the furnace were useful in supplying necessaryoxygen for proper burning of rice husk and, thus, helped inproducing good quality ash. Increased number of air ductsfor future furnace is, therefore, recommended.

(3) Grinding of burnt ash using combination of steel rods (rod C)expedited the grinding process. Although grinding for30 min produced good result, grinding for 60 min or moreis recommended for achieving standard fineness of RHA.

(4) A simple furnace similar to the one built at UKM can be usedto burn and produce rice husk ash of acceptable quality. Butrice husk ash produced at the furnace of UKM was only 10%of the rice husk. This limitation should be addressed beforeinstalling a similar furnace.

Acknowledgement

The research work reported in this paper was funded by theMinistry of Science, Technology and Innovation, Malaysia and Uni-versiti Kebangsaan Malaysia (UKM). Materials were supplied byBernas Nasional Malaysia and Ready Mixed (M) Sdn Bhd, Malaysia.The authors would like to express sincere gratitude for all the sup-ports provided. The second author expresses his sincere gratitudeto Dhaka University of Engineering and Technology (DUET), Gazi-pur, Bangladesh for granting him leave for the research.

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