Rice husk ash (RHA), used as a pozzolanic admixture in
cement and concrete, is obtained from the combustion of rice husk (RH) under
certain conditions of the surrounding environment, temperature and residence
time in a combustor, and subsequent size reduction. It contains low unburned
carbon, and silica is mostly in amorphous form. RHA manufactured in the modern
fluidized or cyclonic bed reactors has high surface area of the order of 20–40
m2/g, comparable with that of silica fume (SF).
It is a material with proven pozzolanic characteristics
and is added to cement and concrete as a partial replacement of Portland
cement. However, its application has not been widely commercialized as yet,
mainly on account of the non-availability of RHA of the desired pozzolanic
characteristics on a large scale on the one hand and the lack of awareness
about the potential for RHA as a mineral admixture on the other.
The RH produced in farms and rice mills is conventionally
employed as a fuel or dumped as waste. Many researchers found that the use of
properly manufactured and treated RHA improves the performance and the
durability of concrete.
RHA possesses the potential to replace SF in high-strength
and high-performance concrete. RHA manufactured through controlled burning of
RH shows performance comparable with that of SF, when added to concrete in
binary (Portland cement [PC] + RHA) or tertiary (PC + RHA + FA) blends, in
terms of strength and reduced permeability toward the external deteriorating
agents.
The major characteristics of RHA are its high water demand
and coarseness in comparison to SF. In order to improve these characteristics,
RHA needs to be ground finer into particle size range of 4–8 μm (1 μm = 10-6 m) and a superplasticizer is added to
reduce water requirement.
RH is presently considered as an agricultural waste and
used as fuel, as mentioned earlier, where its pozzolanic value lies unutilized.
Thus, the incorporation of RHA in concrete as a mineral admixture adds value,
both from the economical and ecological point of view.
Rice,
produced from paddy, is a cereal grain and the most important staple food for a
large part of the world’s human population, especially in tropical Latin
America, the West Indies, and east, south, and southeast Asia.
According to one estimate, the world paddy production is
expected to touch 847–915 × 106 ton by the year 2030, from the current (2008)
level of 683 × 106 ton; out
of which around 600–774 × 106 ton paddy and from that around 120–155 × 106
ton RH shall be produced in the Asian
countries. The abundant availability of RH in the rice producing countries provides us a huge scope to recover its heat
value to generate power and to use the RHA produced in cement and concrete on a
large scale.
The production of RHA with cogeneration of power as well
as its application in cement and concrete, both contribute toward the reduction
of green house gas (GHG) emissions. It is found that the generation of power
through the combustion of RH reduces carbon emissions, in comparison to coal,
oil, and natural gas.
RHA is added to cement and concrete as a partial
replacement of cement to the extent of 30%. Thus, it reduces the consumption of
PC and to that extent contributes toward the reduction of CO2 emission, which
is a GHG, in the manufacture of PC.
The reduction of GHG through such practices has been provided
with incentives under the United Nations (UN) framework. The Kyoto Protocol is
part of the United Nations Framework Convention on Climate Change (UNFCCC) and
has set an agenda for reducing global GHG emissions. If CO2 emissions can be
shown and verified to be reduced due to different practices, then Certified
Emission Reductions (CERs) are issued under the Clean Development Mechanism
(CDM) of UNFCCC. These CERs are tradable in the primary and secondary market
and generate revenue for the CERs holding party. The readers are advised to go
through the UNFCCC documents to obtain more information on the subject. When RHA is used in cement and concrete
manufacture as a cement substitute, there is potential to earn CERs.
There are other environmental benefits of substituting Portland
cement with RHA. The need for quarrying and mining primary raw materials and
fuel is reduced, namely, limestone, clay, and coal, and thus overall reduction
in emissions of dust, CO2, and acid gases is attained. As the cement and
concrete industry uses RHA with amorphous silica, the health issues, mainly
associated with the fine crystalline silica, are minimal.
The large-scale application of RHA in the construction
industry requires industrial and economic policy planning and efforts in the
following areas:
a) Creation of general awareness about the benefits of
using RH in power generation and RHA in cement and concrete. In India, the government
took lead promoting the utilization of pulverized fuel ash (PFA) in cement and
concrete, through the Fly Ash Mission. It is time that similar missions are
taken up to create awareness about
the less known mineral admixtures, such as RHA.
b) RH is produced by the farmers in their paddy fields.
The RHA producing unit will require continuous supply and adequate storage of
RH. Thus, a viable method of collection and transportation of RH from the paddy
fields to the RHA producing unit will have to be put in practice.
c) Identification of a techno-economically feasible method
to produce and process RHA along with the cogeneration of power to suit the local
conditions.
d) Formulation of national Standards on the quality
assessment and the use of RHA in cement and concrete.
In
summary, the application of RHA in the construction industry shows tremendous potential
for the rice-producing countries, both from the point of view of promoting the
sustainable development of construction industry and as a valuable input for
the economic growth of these countries.
Ref: “Mineral
Admixtures in Cement and Concrete”, CRC Press (http://www.crcpress.com/product/isbn/9781439817926). Author: Dr J D Bapat (http://www.drjdbapat.com)
Written for engineers, book focuses on making more
workable and durable concrete using mineral admixtures. For each mineral
admixture, book looks at manufacturing and processing, physical
characteristics, chemical and mineralogical composition, quality control, and
reported experiences. It also examines the provisions of national standards.It
encourages engineers to more effectively use these and other wastes in cement
and concrete to support more sustainable growth of industry. Buy this book
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