Materials Letters 64 (2010) 367–370

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Materials Letters

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Microwave (2.45 GHz)-assisted rapid sintering of SiO2-rich rice husk ash

Natt Makul ⁎, Dinesh K. AgrawalThe Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA

⁎ Corresponding author. Tel.: +1 814 863 8034; fax:E-mail address: num16@psu.edu (N. Makul).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.11.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 October 2009Accepted 9 November 2009Available online 16 November 2009

Keywords:MicrowaveSinteringGround rice husk ashCristolbalite

The as-received SiO2-rich material, rice husk ash (RHA), was ground to a specific surface area (Blaine) of4800 cm2/g and then was rapidly sintered by 2.45 GHz microwave energy using a multi-mode system. Thesintering temperatures were in a range of 800–1200 °C. The morphology, phase composition, microstructureand reactivity were investigated by SEM, XRD, and hydration heat evolution and strength index. Resultsshow that the sintered ground RHAs (SgRHAs) contain SiO2-cristolbalite and α-SiO2 as major phases. Thebulk density is up to 0.98 kg/l, and strength index at 28 days of the 800 °C microwave-sintered RHA paste areup by 30% when comparing to the as-received ground RHA (gRHA).

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1. Introduction

Rice husk/hull has been widely used as a fuel in the electricitygenerating power plant. Generally, in the production process, ricehusk is ground and fed by air to the combustion chamber of a boiler.However, practically even improper burning conditions i.e., higherburning temperature and short feeding rates bring about muchcrystalline silica. Therefore, various methods have been developedcontinuously such as grinding [1], chemical treatment [2], etc. toreduce the generation of silica phase during burning of the ash. One ofthese is re-heating for transforming the phases. Unfortunately, theheat slowly dissipates due to its high insulation [1–3] but volumetricheating by microwave energy can provide higher heating rates andefficiency.

Microwave sintering is the most popular energy source to heatdielectric materials in various industrial processes. With selective andvolumetric heating of microwave sintering, it can enhance theefficiency and reduce the processing time considerably thereby savingthe energy. In this work, we have applied microwave energy to sinterground RHA (gRHAs) as received from the power plant. Usingmicrowave we are providing an alternative cheap source of silica.Hereunder, we report the effect of microwave sintering on reactivity,microstructural development and heat evolution of hydrationreaction during sintering of gRHAs.

2. Experimental

2.1. Starting materials

The as-received RHA was obtained from a power plant inPathumthani, Thailand. It results in thermal energy used for boilingwater to generate steam of 360 °C with the pressure of 18 bars. Thesteam is used to move the turbines of a generator to generateelectricity. The electrostatic precipitator is used to collect ash. Theas-received RHA was then ground in a laboratory rod mill for 4 h intoreasonable fine particles This machine consists of two parts includinga cylindrical feature having 60.0 cm in diameter and three sizes ofrolled bar i.e., diameters of 0.9 cm, 1.2 cm and 1.5 cm in amounts of45, 45 and 35 bars, respectively. The chemical composition of gRHA is90.6% SiO2, 0.5% Al2O3, 1.4% Fe2O3, 0.9% CaO, 0.2% MgO, 1.9% K2O,0.01% Na2O, 0.03% SO3, 0.09% TiO2 and 0.06% Free CaO. On average, theparticle size of gRHA is 800 micron with a specific surface area(Blaine) of 4800 cm2/g.

2.2. Microwave sintering setup

Themicrowave sintering setup used in this study is shown in Fig. 1(a)[4]. An IR-pyrometer is used tomonitor the temperature of the sample inthe microwave chamber. By sequential as shown Fig. 1(b), displays theheating profiles of various samples sintered in the temperature range of800–1200C. The temperature was maintained at the sintering tempera-ture by controlling the microwave power.

2.3. Testing procedures

The ground RHA (gRHA) was first dried at 110±5 °C for 24 h,homogenized by hand mixing, and then transferred to the microwaveprocessing chamber for sintering. After sintering, the phases of SgRHAs

Fig. 1. (a) Microwave sintering setup and (b) temperature profiles.

368 N. Makul, D.K. Agrawal / Materials Letters 64 (2010) 367–370

were identified byXRD. A Scintag PADVdiffractometer (Cupertino, CA,USA) in conjunction with a computerized calculation program. Cu K1.54059 Å and a scanning rate of 2° perminutewere used to record thediffraction patterns. The morphology was observed by a scanningelectron microscope (SEM; HITASHI 3500). Moreover, theirs bulkdensities and strength index were also tested in accordance with theASTM C 311 and 109 [5], respectively.

The gRHA and SgRHAs pastes with Type I Portland cement [5] 20%by wt. and a water-to-cementitious materials ratio (w/c) at 0.45 wereprepared. The isothermal heat evolution at 25 °C was measured in aconduction calorimetry. A Thermonetics SEC calorimeter with arefrigerated bath and circulator was used for calorimetry experi-ments. Samples of the hand mixed paste, containing 1.0 gram ofcementitious material, were immediately placed inside the calorim-eter. The heat evolution was measured for 24 h.

3. Results and discussion

3.1. External appearances

SEM micrographs of the gRHA and SgRHAs at different tempera-tures are shown in Fig. 2. The gRHA has rough surface, lumps andplatelets. After sintering in a temperature range of 800–1200 °C, it wasfound that the color of the samples turnswhiter than that of the gRHA,and appears sintering had occurred on the surface of the SgRHAssample. This is due to the fact that high temperature can induce gRHAparticles to bind together. Besides, after eliminating some of the

unburnt carbon, the bulk density of SgRHAs has increased from 0.78 to0.98 kg/l when temperature was raised to 1200 °C.

3.2. Phase compositions

Fig. 3 shows the XRD patterns of the gRHA and SgRHAs samples.Both are composed of SiO2-cristolbalite as a major phase and someα-SiO2 phase. However, at the temperature of 1100 °C, the α-SiO2

(2θ=27.50) was absent. It may be due to decreasing of the unburntcarbon.

3.3. Strength index

Strength data was collected on the samples prepared by mixingthe gRHA or SgRHAs with Portland cement Type I at 20% wt. and usinga water-to-cementitious materials (w/c) ratio at 0.485 [5]. Fig. 4shows the strength indices of the gRHA and SgRHAs pastes. It wasfound that at 7 days first, the strength index of the 900 °C SgRHAspaste increases, and decreases with increasing sintering temperature.While, at 28 days, the strength index of 800 °C SgRHAs paste is highestand more than 30% when compared with the gRHA.

3.4. Heat evolution

Fig. 5 shows the hydration heat evolution of the gRHA and SgRHAspastes made with Portland cement. It is obvious that SgRHAs pasteshad not only a shorter induction period in early hydration while

Fig. 2. SEM micrographs of the gRHA and SgRHAs.

369N. Makul, D.K. Agrawal / Materials Letters 64 (2010) 367–370

maintaining the same total heat evolution but a lower maximumtemperature.

4. Conclusion

The as-received ground RHA (gRHA) can be sintered in a range of800–1200 °C in a short period of time by microwave energy. Thesintered ground RHAs (SgRHAs) are composed of SiO2-cristolbaliteandα-SiO2 as major phases. The amount of crystalline silica phase hasbeen increased after microwave treatment. And hence the perfor-

mance will be enhanced. Furthermore, heat evolution of the SgRHAsshowed a lower value than that of the gRHA. It indicates that themicrowave sintering process can enhance the properties of the gRHA.

Acknowledgements

The authors gratefully acknowledge the Thailand Research Fund(TRF) for supporting this research project under the Royal GoldenJubilee Program (RGJ) contract. No. PHD/0030/2549.

Fig. 4. Strength indices of the gRHA and SgRHAs pastes.

Fig. 3. X-ray diffraction patterns of the gRHA and SgRHAs samples.

Fig. 5. Hydration heat evolution of the gR

370 N. Makul, D.K. Agrawal / Materials Letters 64 (2010) 367–370

References

[1] Mehta KP. The chemistry and technology of cements made from rice husk ash. Proc.UNIDO/ESCAP/RCTT Workshop on Rice Husk Ash Cement, Peshawar, Pakistan;1979. p. 113–22.

[2] Juliano BO. Rice: Chemistry and Technology. St. Paul, Minneapolis: American Assoc.of Cereal Chemists; 1985.

[3] Cook D. Rice husk ash cements: their developments and applications. Australia:UNIDO Publication; 1984.

[4] Fang Y, Chen Y, Silsbee MR, Roy DM. Microwave sintering of fly ash. Materials Letter1996;27:155–9.

[5] American Society for Testing and Materials, Annual Book of ASTM Standard Vol. 4.01,Philadelphia, PA, USA, 2008.

HA and SgRHAs pastes at w/c=0.45.