Determination of amorphous silica in rice husk ash by a rapid
*, J. Monzo
, M.V. Borrachero, A. Mellado, L.M. Ordon
Grupo de Investigacio
n en Quõ
mica de los Materiales (GIQUIMA), Departamento de Ingenierõ
a de la Construccio
n, Universidad Polite
cnica de Valencia,
Camino de Vera s/n, Valencia 46071, Spain
Received 3 January 2000; accepted 30 October 2000
A rapid analytical method for evaluating amorphous silica in rice husk ash (RHA) is provided and therefore an assessment of its potential
pozzolanic activity. The method is based on bringing the siliceous non-crystalline fraction of the pozzolan into solution as glycerosilicate by
treating the test material with glycerol. The titration of the resulting solution is performed with an aqueous glycerol solution of barium
hydroxide. Data are compared to those obtained using a standard method and a good concordance is achieved. It is demonstrated that the
proposed method can be a useful tool for evaluating amorphous silica in RHA. D 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Amorphous material; Reaction; Pozzolan; Titration
Rice husks are a residue produced in significant quan-
tities on a global basis. While they are utilized as a fuel in
some regions, in other countries they are treated as waste,
causing pollution and disposal problems. Due to growing
environmental concern, and the need to conserve energy and
resources, efforts have been made to burn the husks under
controlled conditions and to utilize the resultant ash as a
building material [1±3].
Rice husk ash (RHA) is a highly reactive pozzolanic
material suitable for use in lime±pozzolana mixes and for
Portland cement replacement. RHA contains a high amount
of silicon dioxide, and its reactivity towards lime depends
on a combination of two factors, namely the non-crystalline
silica content and its specific surface. The non-crystalline
phase in RHA obtained from combustion at temperatures
below 600°C consists primarily of a disordered Si±O
structure which is the product of decomposition and sinter-
ing of opaline or hydrous silica without melting. Occasion-
ally, a small amount of crystalline impurities may be
present, including quartz, cristobalite and/or tridymite.
When RHA is produced by uncontrolled combustion, the
ash is generally crystalline and presents poor pozzolanic
properties. However, by burning the rice husks under
controlled temperature and atmosphere, a highly reactive
RHA can be obtained .
Thus, the reactivity of RHA as pozzolanic material
depends on the crystalline/amorphous ratio. Therefore, for
RHA characterization, the evaluation of the amount of
amorphous silica becomes very important. For this purpose,
there are some specific methods in the literature. One of
them, proposed by Mehta , established that the degree of
amorphousness of silica is estimated by the ``silica activity
index,'' which is determined by calculating the percentage
of available silica that is dissolved in an excess of boiling
0.5 M sodium hydroxide in a 3-min extraction period.
Another simpler method is to titrate a suspension containing
1 g of the ash in 25 ml of distilled water against methylene
blue. The amount of methylene blue required to produce a
color change could also be used as an index of the state of
the silica in the ash .
Standard test methods for evaluating reactive silica (also
called soluble silica) are somewhat tedious, since these
include stages of filtration, calcination, etc.
Therefore, the interest in having rapid analytical meth-
ods that can provide a measure of the amount of
amorphous silica in RHA (and in other pozzolans) is
justified, mainly to provide an assessment of the pozzo-
lanic activity of these materials.
* Corresponding author. Tel.: +34-963-87-75-64; fax: +34-963-87-75-
E-mail address: email@example.com (J. Paya
Cement and Concrete Research 31 (2001) 227 ±231
0008-8846/01/$ ± see front matter D 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0008-8846(00)00466-X