Synthesis and Characterization of Nano-Wollastonite from Rice Husk Ash and Limestone

Hamisah Ismail1,a, Roslinda Shamsudin1,b, Muhammad Azmi Abdul Hamid1,c

and Azman Jalar2,d

1School of Applied Physics, Faculty of Science & Technology, Universiti Kebangsaan Malaysia,

43600 Bangi, Selangor, Malaysia.

2Institute of Micro Engineering & Nanoelectronic, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

ahamisahismail@yahoo.com, blinda@ukm.my, cazmi@ukm.my, & dazmn@ukm.my

Keywords: nano-wollastonite, rice husk ash, limestone, autoclave, sol-gel method

Abstract. Wollastonite, CaSiO3 material was prepared from rice husk ash, as the source for SiO2

and limestone, source for CaO using sol-gel method. Rice husk ash and CaO powder was mixed

together in 100ml distilled water with the rice husk ash/CaO ratios of 45:55 and 40:60. The mixed

solution was place in the autoclave and heated at 135°C for 4 hours and calcined at 950°C for 1 and

2 hours. From the XRD results, ratio of 45:55 exhibited that β-wollastonite is the major phase

and the minor phase is only contributed by cristoballite and by calcining the mixture for 2 hrs would

yield better crystallinity. Both of the rice husk ash:CaO ratios produced wollastonite materials in

cylinder structures. Wollastonite with nano size grain was obtained for an hour calcination and 2 hrs

of calcination would increase the grain size over 100 nm for ratio, 45:55 and 40:60.Therefore in

order to get the nano size of wollastonite material, period of calcination process has to be

controlled.

Introduction

Wollastonite or CaSiO3 is one of a natural calcium silicate group and it has a single-chain of

silicate [1] and an important raw material in ceramic applications, where it exhibits good strength,

low dielectric loss, good bioactivity and biocompatibility [2,3]. Sol-gel process is a wet-chemical

technique widely used in the fields of materials science and ceramic engineering [4], for example,

the synthesized of a wollastonite-titania material [5]. Zhao and Chang was also synthesized

tricalcium silicate powders using so-gel method with Ca(NO3)2.4H2O and Si(OC2H5)4 (TEOS) as

the precursors and nitric acid as a catalyst [6]. Nevertheless, chemical or catalysts used are

dangerous to the well-being and surroundings. Most of the sol-gel method use chemicals and

catalysts as starting material. In this study, β-wollastonite was prepared using sol-gel method.

Agricultural waste, rice husk ash and local mineral resources, calcium carbonate powders were used

as starting materials. The advantage of this that a non dangerous chemicals or catalysts were used

which reduce the pollution due to open burning of rice husk. Furthermore no purification steps were

necessary which helps in reducing the costing, waste dumping problem and shorten the overall

process and also releasing of toxic gas and hazardous chemical due to burning of rice husk ash.

Experimental

Rice husk was collected from the rice mill in Penang and the local calcium carbonate powder

was purchased from Holy Mate (M) Sdn Bhd. Rice husk ash was produced by the combustion

process at 950°C for an hour with the heating rate of 5°C/min. While calcium oxide was obtained

through the calcination process of calcium carbonate at 1100°C for 5hrs with the heating rate of

10°C/min. No further purification process was done to the precursors. The method used for

Materials Science Forum Vol. 756 (2013) pp 43-47Online available since 2013/May/14 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.756.43

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fabrication of nano-wollastonite was altered from method used by Pei et al [2]. Two ratios of

SiO2/CaO were used, i.e. 45:55 and 40:60. 10gm of silica and calcium oxide powder mixture was

soaked in 100 ml of distilled water which manually stir for 10 mins, autoclaved at 135°C for 4 hrs

and then cooled to room temperature. The resulting of white precipitate with water solution was

dried at 90°C in an oven for a day. Then the white powder was calcined at 950°C for 1 and 2 hrs.

Both raw materials and the calcined powders were characterized using X–ray fluorescence (XRF),

particle seizer, X-ray diffraction (XRD and variable pressure scanning electron microscopy

(VPSEM).

Results and Discussion

Chemical composition of the raw materials, rice husk ash after firing at 950°C for one hour and

calcium oxide obtained after calcining for 5 hrs at 1100°C is shown in Table 1. Silica, SiO2 is the

main element in rice husk ash, which is similar to that reported by Lim and co-workers [7], while

calcium oxide, CaO is in calcium carbonate with the percentage of 89.50% and 97.22%,

respectively. CaO obtained from the calcination of calcium carbonate is nearly pure. Therefore we

can assume that the starting materials for synthesizing the wollastonite material is rice husk ash and

CaO powder.

Table 1: Compositions of the raw materials.

Raw materials Elements Weight percentage

(wt %)

Rice husk ash

SiO2 89.50

K2O 3.61

P2O5 3.36

MgO 1.24

Al2O3 0.58

CaO 0.57

Others 1.14

Calcined calcium

carbonate

CaO 97.22

MgO 2.38

Others 0.4

The mixture of rice husk ash and CaO powder before calcination has average particles size in

micron and became smaller after calcining, which is reducing to nano size (refer Table 2). Ratio of

45:55 has smaller particles size, before and after calcinations compare to ratio of 40:60 and both

ratios show that by prolong the calcination period would make the particles became bigger. Ratio of

40:60 possesses bigger particles size because of the uncompleted reacted calcium oxide and it was

agglomerated.

44 ISESCO Conference on Nanomaterials and Applications 2012

Table 2: Particle size analyses for raw and calcined samples.

Sample ratio

Particle size, nm

Before calcination

(raw material)

After calcination

1 hr 2 hrs

45:55 281.3 1.9 9.4

40:60 342.0 15.7 72.9

From the phase analysis, the XRD results (Figure 1) show that both ratios exhibit β-

wollastonite as the major phase where all the peaks totally match the standard values of the

wollastonite phase (JCPDS card No. 43-1640) together with cristoballite as the minor phase.

Wollastonite phase has been detected in both ratios because the calcining temperature used was

950°C and according to Lee et al. [8], β-wollastonite occurred at a relatively low temperature, i.e.

around 900°C. Another minor phase, calcium oxide has been detected in 40:60 ratio for 1 and 2 hrs

soaking period indicates that the CaO has not been completely reacted to produce wollastonite.

Fig. 1: X-Ray diffraction patterns of the raw and calcined sample for both ratios.

(w:β-wollastonite, c:cristobalite & co:calcium oxide)

40SiO260CaO – 2h

45SiO255CaO – 2h

40SiO260CaO – 1h

45SiO255CaO – 1h

45SiO255CaO – raw

40SiO260CaO – raw

2 theta, ϴ

Inte

nsi

ty,

cou

nts

c w w co

w

Materials Science Forum Vol. 756 45

Morphologies of the raw and calcined samples for both ratios are shown in Figure 2. All the

samples for both ratios exhibited irregular grain shape. The raw samples had grain size more than

100 nm and after 1 hr calcining, the size reduced to less than 100 nm. Reducing in grain size was

observed for both ratios. Lengthen the calcining period to 2 hrs would not reduce more the grain

size yet the size is increased again. The same condition was also reported by Zhu et. al. [9], soaking

time of the calcination process affected the size grain. From SEM micrographs, 45:55 ratios had a

grain size of ∼164.1nm for uncalcined CaSiO3, decreased to ∼80.4 nm after 1hr calcination and after

2 hrs of calcining the size increased again to ∼171.9nm. Ratio of 40:60 also exhibited the same

pattern; the uncalcined CaSiO3 had a grain size of ∼150.72nm, decreased to ∼73.3nm after 1hr

calcining and increased again to ∼112.7nm after 2 hrs calcining.

Fig. 2: SEM micrographs of CaSiO3 material with 2 different percentage; 45% SiO2 :55% CaO, (a)

raw (b) calcined at 950°C 1 hr (c) calcined at 950°C 2 hrs and 40% SiO2 :60% CaO (d) raw (e)

calcined at 950°C 1 hr (f) calcined at 950°C 2 hrs

Conclusions

Wollastonite, CaSiO3 material has been successfully synthesized from rice husk ash and CaO

powders using sol gel method with well crystalline structures of diameter 30-100 nm. The source

materials used are from waste and local mineral material that are easily obtained, non-toxic and give

rise to no unwanted by-products after complete reaction, which make this green method feasible to

be realized for large-scale production of CaSiO3 with no further purification carried out to the raw

materials. The grain size of the obtained wollastonite was very much influenced by the period of

calcination process.

Acknowledgements

The authors would like to thank research grants DLP-2011-017, GUP-2012-077 and the Ministry of

Higher Education Malaysia through MyBrain Scholarship for supporting the research.

46 ISESCO Conference on Nanomaterials and Applications 2012

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