REVIEW Mechanism of geopolymerization and factors influencing its development: a review Divya Khale Rubina Chaudhary Received: 22 September 2005 / Accepted: 2 May 2006 / Published online: 20 January 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Geopolymerization is a developing field of research for utilizing solid waste and by-products. It provides a mature and cost-effective solution to many problems where hazardous residue has to be treated and stored under critical environmental conditions. Geopolymer involves the silicates and aluminates of by-products to undergo process of geopolymerization. It is environmentally friendly and need moderate energy to produce. This review presents the work carried out on the chemical reaction, the source materials, and the factor affecting geopolymerization. Literature demonstrates that certain mix compositions and reaction conditions such as Al 2 O 3 /SiO 2 , alkali concentration, curing temperature with curing time, water/solid ratio and pH significantly influences the formation and properties of a geopolymer. It is utilized to manufacture precast structures and non-structural elements, concrete pavements, concrete products and immobilization of toxic metal bearing waste that are resistant to heat and aggressive environment. Geo- polymers gain 70% of the final strength in first 3–4 h of curing. Introduction Industrialization leads to the generation and release of undesirable pollutants into the environment. In order to keep pace with the rapid industrialization there is a necessity to select such process, which would cause minimum pollution in environment. In recent years, there is an increasing awareness on the quantity and diversity of hazardous solid waste generation and its impact on human health. Increasing concern about the environmental consequences of waste disposal has led to investigation of new utiliza- tion avenues [1]. The greatest problem faced by industries, as far as waste disposal is concerned is the safe and effective disposal of its effluent, sludge and by-products such as large quantities of fly ash that are produced during the combustion of coal used for electricity generation. It is estimated that by the year 2010, the amount of the fly ash produced will be about 780 million tones annually [2]. Most of this ash is disposed in landfills at suitable sites [1, 3]. Landfilling is not a desirable option because it not only causes huge financial burden to the foundries, but also makes them liable for future environmental costs and problems associated with landfilling regulations [4]. The increasing load of toxic metals in the landfill potentially increases the threat to ground water contamination. Increasing economic factors also dictate that indus- try should look forward to recycling and reuse of waste material as a better option to landfilling and discarding. Need exists for a technology that can easily and cheaply handle large quantities of waste materials and by-products containing heavy metals as an alternative to OPC (ordinary Portland cement). D. Khale R. Chaudhary (&) Hazardous Waste Management Laboratory, School of Energy And Environmental Studies, Devi Ahilya University, Takshila Campus, Khandwa Road, Indore 452001 MP, India e-mail: [email protected]J Mater Sci (2007) 42:729–746 DOI 10.1007/s10853-006-0401-4 123
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REVIEW
Mechanism of geopolymerization and factors influencingits development: a review
Divya Khale Æ Rubina Chaudhary
Received: 22 September 2005 / Accepted: 2 May 2006 / Published online: 20 January 2007� Springer Science+Business Media, LLC 2007
Abstract Geopolymerization is a developing field of
research for utilizing solid waste and by-products. It
provides a mature and cost-effective solution to many
problems where hazardous residue has to be treated
and stored under critical environmental conditions.
Geopolymer involves the silicates and aluminates of
by-products to undergo process of geopolymerization.
It is environmentally friendly and need moderate
energy to produce. This review presents the work
carried out on the chemical reaction, the source
materials, and the factor affecting geopolymerization.
Literature demonstrates that certain mix compositions
and reaction conditions such as Al2O3/SiO2, alkali
concentration, curing temperature with curing time,
water/solid ratio and pH significantly influences the
formation and properties of a geopolymer. It is utilized
to manufacture precast structures and non-structural
elements, concrete pavements, concrete products and
immobilization of toxic metal bearing waste that are
resistant to heat and aggressive environment. Geo-
polymers gain 70% of the final strength in first 3–4 h of
curing.
Introduction
Industrialization leads to the generation and release of
undesirable pollutants into the environment. In order
to keep pace with the rapid industrialization there is a
necessity to select such process, which would cause
minimum pollution in environment.
In recent years, there is an increasing awareness on
the quantity and diversity of hazardous solid waste
generation and its impact on human health. Increasing
concern about the environmental consequences of
waste disposal has led to investigation of new utiliza-
tion avenues [1].
The greatest problem faced by industries, as far as
waste disposal is concerned is the safe and effective
disposal of its effluent, sludge and by-products such as
large quantities of fly ash that are produced during the
combustion of coal used for electricity generation. It is
estimated that by the year 2010, the amount of the fly
ash produced will be about 780 million tones annually
[2]. Most of this ash is disposed in landfills at suitable
sites [1, 3]. Landfilling is not a desirable option because
it not only causes huge financial burden to the
foundries, but also makes them liable for future
environmental costs and problems associated with
landfilling regulations [4]. The increasing load of toxic
metals in the landfill potentially increases the threat to
ground water contamination.
Increasing economic factors also dictate that indus-
try should look forward to recycling and reuse of waste
material as a better option to landfilling and discarding.
Need exists for a technology that can easily and
cheaply handle large quantities of waste materials and
by-products containing heavy metals as an alternative
to OPC (ordinary Portland cement).
D. Khale � R. Chaudhary (&)Hazardous Waste Management Laboratory, School ofEnergy And Environmental Studies, Devi AhilyaUniversity, Takshila Campus, Khandwa Road,Indore 452001 MP, Indiae-mail: [email protected]
J Mater Sci (2007) 42:729–746
DOI 10.1007/s10853-006-0401-4
123
Disposal of hazardous waste must meet at least two
conditions [5]
(1) Safe chemical encapsulation i.e. control their
release into ground water and seepage water.
(2) Structural stability with respect to adverse envi-
ronmental condition.
Production of one ton of Portland cement requires
about 2.8 ton raw materials, including fuel and other
materials and generates 5 to 10 % of dusts. Altogether
6000–14000 m3 dust-containing air-streams are gener-
ated per ton cement manufacture, which contain
between 0.7 to 800 g/m3 of dust and accounts for about
one ton of green house gas CO2 released to the
atmosphere as a result of de-carbonation of lime in the
kiln during manufacturing of cement (Eq. 1) [2, 6, 7].
5 CaCO3 þ 2 SiO2 ! 3 CaO � SiO2
þ 2 CaO � SiO2 þ 5 CO2 ð1Þ
A technology was therefore sought as an alternative
to the afore-mentioned standards and, further more, the
cost figures must not be intolerable [5]. To overcome
these problems, geopolymers emerged as a possible
solution for using the by-products and could be utilized
to manufacture precasts structure and non-structural
elements, concrete pavements, concrete products and
immobilization of toxic waste that are resistant to heat
and aggressive environment [8]. The objective of this
review is to study the work carried out on the develop-
ment of geopolymers, including the chemical reaction,
the role and effect of the source materials, and the
factors affecting mix compositions, such as curing
temperature, curing time, Al2O3/SiO2 ratio in the mix,
alkali concentration, pH and water/solid ratio.
Geopolymerization
Geopolymerization is a geosynthesis (reaction that
chemically integrates minerals) that involves naturally
occurring silico-aluminates [5]. Any pozzolanic com-
pound or source of silica and alumina, that is readily
dissolved in the alkaline solution, acts as a source of
geopolymer precursor species and thus lends itself to
geopolymerization [9]. The alkali component as an
activator is a compound from the element of first group
in the periodic table, so such material is also called as
alkali activated aluminosilicate binders or alkali acti-
vated cementitious material [10]. Silicon and aluminum
atoms react to form molecules that are chemically and
structurally comparable to those building natural rocks
[5]. The inorganic polymeric material can be considered
as an amorphous equivalent of geological feldspars, but
synthesized in a manner similar to thermosetting organic
polymers. For this reason, these materials are termed as
‘‘geopolymers’’[11].
It offers attractive option for simple industrial
applications where large volume of waste materials
needs to be stabilized [5]. It is named because of the
similarities with the organic condensation polymers as
far as their hydrothermal synthesis conditions are
concerned [8]. Study of the literature and patents
demonstrated, that before 1978, the idea of using this
mineral chemistry for the development of a mineral
polymer had been totally neglected. As a function of
chemical composition of initial materials, the alkaline
cements are classified into two groups.
(i) Binders synthesized from materials rich in calcium
such as blast furnace slag that produces calcium
silicate hydrate (CSH) gel when activated with
alkaline solution.
(ii) Materials synthesized with raw materials low in
calcium and rich in SiO2 and Al2O3 such as
metakaolin. These materials when activated with
alkaline solution, formation of an amorphous
material (alkaline aluminosilicate) that develops
high mechanical strength at early ages after a soft
thermal curing [12].
These materials differ substantially from ordinary
Portland cement, as they use totally different reac-
tion pathway in order to attain structural integrity.
Pozzolanic cement depends on the presence of
calcium-silicate hydrate for matrix formation and
strength where as geopolymers utilize the polycon-
densation of silica and alumina precursors (fly ash,
kaolin, metakaolin) and a high alkali content to
attain structural strength [13].
Chemistry of geopolymer
Geopolymerization is based on chemistry of alkali
activated inorganic binders, which were accidentally
discovered by Purdon [14]. He studied the sodium
hydroxide on a variety of minerals and glasses con-
taining silicon and/or aluminum and summarized it in
two steps;(1) liberation of silica, alumina and lime and
(2) formation of hydrated calcium silicates, aluminates
as well as regeneration of caustic solution. Author
and aluminate ratio, pH and the type of activators used
has substantial effects on the final properties of
geopolymers. Certain synthesis limits existed for the
formation of strong products. Compositions lay in the
range M2O/SiO2, 0.2 to 0.48; SiO2/Al2O3, 3.3 to 4.5;
H2O/M2O, 10–25; and M2O/Al2O3, 0.8 to 1.6 but the
ratio changes while working with the waste. Research
on geopolymers conforms that curing temperature and
curing time significantly influence the compressive
strength.
Curing temperature is an important factor in the
setting of the geopolymer but curing at higher temper-
ature for more than a couple of hours seem to possibly
affect the development of compressive strength.
Increase in strength for curing periods beyond 48 h
was not very significant. This behavior is in contrast
with the behavior of OPC. Strength decreases as the
ratio of water-to-geopolymer solid by mass increases;
this trend is analogous to water-to-cement ratio in the
compressive strength in OPC. Fresh geopolymeric
material is readily workable even at a very low
liquid/solid ratio i.e. below 0.4 The sample strength is
strongly dependent on both the Si:Al and Na:Al ratios
of the material. High reactive silica content involves
the formation of high amount of alkali alumino-silicate
gel and consequently a high mechanical strength is
developed in the resulting material.
pH range in 13–14 is the most suitable for the
formation of the geopolymers with good mechanical
strength. Alkali concentration in the range of 5–10 N
plays an important role in the formation of geopolymer.
Geopolymer concrete undergo low creep and very
little drying shrinkage. Young’s modulus, Poisson’s
ratio and Tensile strength of fly ash based geopolymers
concretes possess the characteristics similar to Portland
cement concrete.
Geopolymer serves as a better alternative to OPC
for immobilizing toxic metals. About 90% of the heavy
metals get locked into the geopolymeric matrix.
To increase the cost effectiveness of geopolymeric
binders the aluminum source like kaolin has to be
replaced by some cost economic materials such as
slag, CKD, builder’s waste or little amount of cement
that will reduce the cost of the geopolymer as well as
will strengthen the matrix. It can be concluded from
the study that geopolymers are ‘‘Green materials’’
that gains 70% of its strength in first 3–4 h and
immobilizes 90% of the toxic metals within the
matrices.
Future study in the field of the application of
geopolymer is required for its commercial uses.
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