Advances in Chemical Engineering and Science, 2011, 1, 147-153
doi:10.4236/aces.2011.13022 Published Online July 2011 (
Copyright © 2011 SciRes. ACES
Study of Rice Husk Ash as Potential Source of Acid
Resistance Calcium Silicate
Muhammad Mansha, Syed Hassan Javed, Mohsin Kazmi, Nadeem Feroze
Department of Chemical Engineering, University of Engineering & Technology, Lahore, Pakistan
Received May 6, 2011; revised June 9, 2011; accepted July 4, 2011
In present work acid resistant calcium silicate has been synthesized from silica of rice husk and calcium ox-
ide of analytical grade. The silica from rice husk was extracted at 550˚C in amorphous form and then al-
lowed to react with calcium oxide in the presence of excess water by Sol-Gel technique to obtain calcium
silicate hydrate gels. The molar ratios of Si/Ca were adjusted each time to obtain silica rich calcium silicate
hydrates. The gels were dried in oven and calcined in muffle furnace at various temperatures to obtain acid
resistant calcium silicate. The products were tested by analytical technique and by FTIR and XRD machines.
Studies show that at higher molar ratio of Si/Ca, the heat treatment improves the acid resistivity of calcium
silicate whereas at lower molar ratios the heat treatment does not make it acid resistant.
Keywords: Calcium Silicate, Rice Husk, Amorphous Silica, Acid Resistance, XRD, FTIR
1. Introduction
Silica finds numerous applications as an important raw
material for assorted types of industries including ce-
ramics, electronics and polymers. It has different sources
such as quartz, flint, natural silicates and siliceous agri-
culture wastes. Rice husk (RH) is an abundantly avail-
able agricultural by product in rice producing areas. On
weight basis rice husk weighs ~20% of the paddy pro-
duced. Rice husk finds multiple applications in a wide
range of industries. It is employed as a fertilizer for crops
and supplementary material for cement and concrete
manufacturing units. It has high percentage of silicon
which makes it a valuable starting material for a variety
of compounds like silica and its carbides and nitrides
Another major application of rice husk is in the power
sector on account of its high calorific value (13,000 -
16,000 kj/kg) [4]. The combustion of RH results in pro-
duction of rice husk ash (RHA). RHA is light in weight
and its color varies from grey to white [5]. Its safe dis-
posal is a big liability as it signifies terrific generation of
waste, loss of energy and environmental pollution. Thus,
an optimum use of RHA has become a great challenge.
Silica and carbon are the major components of RHA.
Silica claims 80% - 85% in RHA produced by heat gen-
erating systems and thus it can suitably be used for many
applications [6]. One such application can be the produc-
tion of acid resistant calcium silicate (ARCS).
Acid resistant calcium silicate is a white powder, usu-
ally used as filter aid in acidic media. It is also used as an
ion exchanger and for immobilization or removal of fatty
acids and heavy metal ions from the wastewater. It has a
high melting point of 1540˚C and a very low material
density of 2.9 g/cm3. Because of low density it is being
used in refractory materials and fireproof cladding. The
existing method for preparing ARCS is high pressure
hydrothermal reaction between calcium oxide and sili-
ceous material having high silica contents [5]. Amor-
phous silica shows high reactivity with calcium hydrox-
ide in the presence of water and form calcium silicate
hydrates whereas crystal silica acts as an inert ingredient
under similar conditions. Amorphous silica can be ob-
tained from rice husk under controlled burning condi-
tions such as residence time, temperature and air flow
system in the furnace [7].
In present study Sol-Gel technique was used to syn-
thesize calcium silicate hydrates which were converted
into ARCS by heat treatment with higher molar ratios of
Si/Ca. The specific objectives of this investigation were:
1) to study the effect of calcination temperature and sil-
ica to calcium oxide ratio on acid resistance of calcium
silicate, 2) To characterize the acid resistant calcium
silicate through FTIR and XRD.
Copyright © 2011 SciRes. ACES
2. Materials and Methods
Rice husk was thoroughly washed with distilled water to
remove soluble impurities followed by drying in electric
oven at 110˚C for 10 hours followed by grinding and
finally screening through 20 mesh sieve. Rice husk at
this stage was termed as powdered rice husk. It was
burned in muffle furnace for 90 minutes at 550˚C in
china dish by gradual rise in temperature. The ash ob-
tained was cooled down at room temperature and con-
verted into fine powder. The grounded rice husk ash
(GRHA) was screened through 200 mesh sieve and
stored in plastic container to protect it from moisture.
2.1. Theoretical Background of Sol-Gel
Calcium carbonate (Merk, Germany) was heated in a
furnace at 1000˚C for 4 hours to decompose into calcium
oxide and carbon dioxide:
3(S)(s) 2(g)
A calculated amount of calcium oxide was slowly
added to boiling water to prepare calcium hydroxide:
(s)2 (l)2(Aq)
Known amount of GRHA was made to react with cal-
cium hydroxide in the presence of excess water such that
each time the amount of water was 7 times of solids used
to form calcium silicate hydrate gels by the following
2(s)3(s)2 (l)
The mixture was boiled for one hour and then placed
into the oven for 10 hours to form calcium silicate hy-
drate gel. Fully developed gel was then dried in oven for
10 hours at 110˚C to make sure absence of physically
absorbed water. Several samples were prepared by chang-
ing molar ratios of Si/Ca from 1.4 to 2.4. The gels ob-
tained from each molar ratio were dried and stored in
plastic containers to protect them from atmospheric car-
bon dioxide and moisture.
2.2. Heat Treatment
Calcium silicate hydrate gels made with different Si/Ca
molar ratios were heated in muffle furnace from 500˚C to
1100˚C for four hours. The product so obtained after heat
treatment was termed as acid resistant calcium silicate
2.3. Acid Resistance Test (ART)
ART was performed by immersing 15 g of each ARCS
in 200 ml of hydrochloric acid (30%) and also with sul-
phuric acid (1M) solution for four hour [8]. It was then
filtered and repeatedly washed with distilled water to
make acid free solids. The wet solids were dried in an
electric oven at 110˚C for 10 hours. This procedure was
performed for every sample obtained with different Si/Ca
2.4. Characterization
X-ray diffraction (XRD) is a technique for the study of
crystal materials and atomic spacing. XRD analyses were
performed using Philips Model PANlytical x-pert ma-
chine having X-ray tube (Philips PW 3373/00) operated
at 40 kV and current Intensity 40 mA. Spectra were ob-
served from 0 to 90 degree at a step size of 0.052 degree
for 400 second scanning time. Fourier Transform Infra
Red (FTIR) spectroscopy was performed using KBr pel-
let technique on JASCO model 4100. Approximately 1
mg of sample was mixed with 100 mg of KBr and then
grounded and pressed to prepare the pellets. FT-IR spec-
tra were obtained in the range of 400 to 4000 cm–1.
3. Results and Discussion
3.1. Characterization of Ground Rice Husk Ash
Calcium hydroxide usually reacts with amorphous silica
in the presence of water and forms calcium silicate hy-
drates. The reaction of calcium hydroxide with crystal
silica is almost negligible whereas with amorphous silica
it is quite rapid [6].
At elevated temperature the amorphous silica of RHA
converts itself into crystal silica. Therefore, rice husk
was carefully burned in the muffle furnace at 550˚C. The
formation of amorphous silica was confirmed by XRD
machine and the pattern obtained is shown in Figure 1. It
shows the characteristic peak at around 22 degrees con-
firming the formation of amorphous silica.
In order to perform ART samples of ARCS made at
various molar ratios of Si/Ca were soaked in 200 ml of
HCl and H2SO4 each for a retention period of 4 hours. It
was found that weight losses of ARCS in case of HCl
were similar to H2SO4. Table 1 shows the ART results
for HCl.
It is interesting to note that all samples calcined at less
than 800˚C were dissolved in HCl solution nonetheless
those samples were calcined above 800˚C showed resis-
tivity against HCl at room temperature. It is also inter-
esting to note that by increasing the calcination tem-
perature the resistivity against acid increases. It can also
be seen from Table 1 that by increasing the molar ratio
of Si/Ca, the weight loss of ARCS becomes zero at and
Copyright © 2011 SciRes. ACES
Figure 1. XRD of rice husk ash.
Table 1. Results of acid resistance test.
SiO2:CaO Mole Ratio 500˚C - 800˚C 900˚C 1000˚C 1100˚C
1.4 4.86 4.02 3.2
1.5 3.17 2.7 1.98
1.6 2.34 1.55 1.19
1.7 1.48 0.89 0.74
1.8 0.98 0.63 0.59
1.9 0.71 0.52 0.51
2.0 0.43 0.36 0.29
2.1 0.40 0.26 0.0
2.2 0.0 0.0 0.0
2.3 0.0 0.0 0.0
Samples dissolved in acid
0.0 0.0 0.0
above Si/Ca 2.1.
In FTIR analyses (Figure 2(a-c)) the bands observed
at about 3440 cm–1 show the chemically or physically
absorbed water. Peaks in the range of 750 to 900 cm–1
are the characteristic of O-Si-O bond as described by
Colthup spectra-structure correlation charts for infrared
frequencies [8]. These peaks confirm the presence of
silicate in the samples. Bands observed in the range of
900 - 1100 cm–1can are attributed due to Si-O-Ca bonds
The peaks in the range of 1600 - 2500 cm–1 show the
presence of unreacted calcium oxide in the samples. In
Figure 2(a) high intensity peaks can be seen in the range
of 1600-2500 cm–1, indicating presence of unreacted
calcium oxide. In Figure 2(b) low intensity peaks can be
seen in the same range because of more quantity of silica
and less quantity of calcium oxide. In Figure 2(c) no
peak has been observed in the same range indicating ab-
sence of calcium oxide. Disappearance of peaks repre-
sents the disappearance of calcium oxide or coating of
silica on calcium silicates.
In all three spectra the peaks of calcium silicate are
different indicating presence of various types of calcium
silicates but exact type can not be identified only by
Same three samples as have been analyzed in FTIR
machine were scanned in XRD machine and the results
are shown in Figure 3. It is interesting to note that as
well as general pattern is concerned all three patterns are
similar; however intensity and number of peaks are
changing. In Figure 3(a) the number of peaks in the
range of 20 to 70 degrees is 15. In Figure 3(b) the num-
ber of peaks becomes 18 in the same scanning range
where as in Figure 3(c) the number of peaks increased to
21. Increasing numbers indicate the temperature effect
on calcium silicate however the general pattern remains
the same indicating that type of calcium silicate is same
with same molar ratio of 2.1.
From XRD, FTIR and ART results it is evident that
free calcium oxide reacts with silica at elevated tem-
peratures and the excess silica probably surrounds the
calcium oxide and eventually calcium silicate becomes
acid resistant.
Three more samples having molar ratios of SiO2:CaO
= 1.8, calcined at 900˚C, 1000˚C, and 1100˚C were
scanned under same conditions to compare the effect of
temperature. These patterns follow the β-wollastonite
pattern however increasing intensity of peaks and more
number of peaks have been observed and are shown in
Figure 4.
In Figure 4(a) the number of peaks are 9 where as in
Figure 4(b) 13 peaks have been observed and in Figure
Copyright © 2011 SciRes. ACES
Figure 2. FTIR Spectrum of calcium silicate with SiO2:CaO = 2.1 and Heat Treated at (a) 900˚C, (b) 1000˚C (c) 1100˚C.
Copyright © 2011 SciRes. ACES
Figure 3. XRD pattern of calcium silicate with SiO2:CaO = 2.1, calcined at (a) 900˚C (b) 1000˚C and (c) 1100˚C.
Copyright © 2011 SciRes. ACES
Figure 4. XRD pattern of β-wollastonite with S iO 2:CaO = 1.8, calcined at (a) 900˚C (b) 1000˚C (c) 1100˚C.
4(c) these peaks rose to 14. The β-wollastonite is one
form of calcium silicate which showed less resistance
against HCl during reaction.
4. Conclusions
Acid resistant calcium silicate can be synthesized from
rice husk by extracting silica in amorphous form. At
lower molar ratios of Si/Ca the calcium silicate reacts
with strong acids such as HCl and H2SO4 whereas at
higher molar ratios, calcium silicate shows acid resistiv-
ity. Heat treatment improves the acid resistivity of cal-
cium silicate. At molar ratios of Si/Ca 2.1, the acid
resistivity improves remarkably after heat treatment.
Higher molar ratios and higher calcination temperatures
both improves the acid resistivity of calcium silicate. It
has also been observed that other types of calcium sili-
cates appear at lower molar ratios. An important ceramic
raw material β-wollastonite was obtained at 1.8 molar
ratio of silica and calcium oxide.
5. Acknowledgments
The financial and technical support provided by Univer-
sity of Engineering and Technology Lahore Pakistan, is
highly appreciated.
Copyright © 2011 SciRes. ACES
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