03-05-2012, 01:44 PM
GEOPOLYMER CONCRETE : A REVIEW OF DEVELOPMENT AND
OPPORTUNITIES
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Introduction
Use of concrete and environment impact
Utilization of concrete as a major construction material is a worldwide phenomenon and the
concrete industry is the largest user of natural resources in the world (1). This use of concrete is driving
the massive global production of cement, estimated at over 2.8 billion tonnes according to recent industry
data (2). Associated with this is the inevitable carbon dioxide emissions estimated to be responsible for 5
to 7% of the total global production of carbon dioxide (3). Significant increases in cement production have
been observed and were anticipated to increase due to the massive increase in infrastructure and
industrialization in India, China and South America (4).
Geopolymer Concrete Development
Geopolymer concrete is concrete which does not utilize any Portland cement in its production.
Rather, the binder is produced by the reaction of an alkaline liquid with a source material that is rich in
silica and alumina. Geopolymers were developed as a result of research into heat resistant materials
after a series of catastrophic fires (5). The research yielded non-flammable and non-combustible
geopolymer resins and binders.
Geopolymer is being studied extensively and shows promise as a greener alternative to Portland
cement concrete. Research is shifting from the chemistry domain to engineering applications and
commercial production of geopolymer. It has been found that geopolymer concrete has good engineering
properties (6,7).
The use of fly ash has additional environment advantages. The annual production of fly ash in
Australia in 2007 was approximately 14.5 million tonnes of which only
million tonnes were utilized in
beneficial ways; principally for the partial replacement of Portland cement (8). Development of
geopolymer technology and applications would see a further increase in the beneficial use of fly ash,
similar to what has been observed in the last 14 years with the use of fly ash in concrete and other
building materials.
Geopolymer Concrete Properties
High-early strength gain is a characteristic of geopolymer concrete when dry-heat or steam
cured, although ambient temperature curing is possible for geopolymer concrete (9). It has been used to
produce precast railway sleepers and other pre-stressed concrete building components. The early-age
strength gain is a characteristic that can best be exploited in the precast industry where steam curing or
heated bed curing is common practice and is used to maximize the rate of production of elements.
Recently geopolymer concrete has been tried in the production of precast box culverts with successful
production in a commercial precast yard with steam curing.
Geopolymer concrete has excellent resistance to chemical attack and shows promise in the use
of aggressive environments where the durability of Portland cement concrete may be of concern. This is
particularly applicable in aggressive marine environments, environments with high carbon dioxide or
sulphate rich soils. Similarly in highly acidic conditions, geopolymer concrete has shown to have superior
acid resistance and may be suitable for applications such as mining, some manufacturing industries and
sewer systems. Commercial geopolymer sewer pipes are in use today. Current research at Curtin
University of Technology is examining the durability of precast box culverts manufactured from
geopolymer concrete which are exposed to a highly aggressive environment with wet-dry cycling in
sulphate rich soils.
The bond characteristics of reinforcing bar in geopolymer concrete have been researched and
determined to be comparable or superior to Portland cement concrete (10,11). The mechanical properties
offered by geopolymer suggest its use in structural applications is beneficial.
Geopolymer Concrete Materials
Fly Ash
The fly ash used in the production of geopolymer concrete at Curtin University is Class F fly ash
sourced from the coal fired power station approximately 200 km south of Perth, Western Australia. The
results of X-ray fluorescence testing (XRF) are shown in Table 1 for the fly ash used in the research
program. The class F fly ash is characterized by high silicon and aluminum contents and low calcium
content, and a loss on ignition of 0.46.
Alkaline solutions
Sodium based alkaline solutions were used to react with the fly ash to produce the binder.
Sodium-silicate solution type A53 was used for the concrete production. The chemical composition is
shown in Table 2. Sodium hydroxide solution was prepared by dissolving sodium hydroxide pellets in
water. The pellets are commercial grade with 97% purity thus 14 molar solutions were made by
dissolving 404 grams of sodium hydroxide pellets in 596 g of water.
The sodium hydroxide solution was prepared one to two days prior to the concrete batching to
allow the exothermically heated liquid to cool to room temperature. The sodium silicate solution and the
sodium hydroxide solution were mixed just prior to the concrete batching. This is a different process to
that which had been employed previously at Curtin University where the two alkaline solutions were
mixed 24 hour prior to casting.
Basic mixture proportions
The basic mixture proportions used for the majority of the trial mixtures was based upon previous
research on the geopolymer mixture proportions and is detailed in Table 3 (6,12). These mixture
proportions are characterized by an alkaline liquid to fly ash by mass of 0.35 and aggregate to total mass
proportion of approximately 75% with the nominal strengths, as shown in Table 3, and elevated
temperature curing in a steam room at 60oC for 24 hours. Modifications to the basic mixture proportions
were used to assess the impact of different variables, especially aggregate grading and type as detailed
in later sections of this paper.
Aggregates
Coarse aggregates with nominal sizes of 7mm, 10mm and 20mm granite and dolerite, were
sourced from two local quarries. The aggregates had a particle density of 2.6 tonnes/cubic metre for the
granite and 2.63 tonnes/cubic metre for the dolerite. The dolerite aggregate was used in one series of
trial mixtures to assess the impact of aggregate type on workability and strength gain of the geopolymer
concrete. Fine sand was sourced from a local supplier. The sand has a low clay content (less than 4%)
and fineness modulus of 1.99.