Fly Ash-Based Geopolymer Clean Production, Properties and Applications

Journal of Cleaner Production 125 (2016) 253e267
Contents lists available at ScienceDirect
Journal of Cleaner Production

journal homepage: www.elsevier.com/locate/jclepro
Review
Fly ash-based geopolymer: clean production, properties and

applications
Xiao Yu Zhuang a, Liang Chen a, b, Sridhar Komarneni d, Chun Hui Zhou a, b, c, *,
Dong Shen Tong a, b, Hui Min Yang e, Wei Hua Yu a, Hao Wang c
a
Research Group for Advanced Materials & Sustainable Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology,
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
b
Engineering Research Center of Non-metallic Minerals of Zhejiang Province, Key Laboratory of Clay Minerals of Ministry of Land and Resources of the
People's Republic of China, Zhejiang Institute of Geology and Mineral Resource, Hangzhou 310007, China
c
Centre of Excellence in Engineered Fibre Composites, University of Southern Queensland, Toowoomba, Queensland 4350, Australia
d
Materials Research Laboratory, Department of Ecosystem Science and Management and Materials Research Institute, The Pennsylvania State University,
University Park, PA 16802, USA
e
Key Laboratory of High Efficient Processing of Bamboo of Zhejiang Province, China National Bamboo Research Center, Hangzhou 310012, China
a r t i c l e i n f o a b s t r a c t
Article history: Fly ash is the fine solid particulate residue driven out of the boiler with the flue gases in coal-fired power
Received 23 October 2015 plants. Now it can be used for making geopolymer which acts as a cement-like product. The geopolymer
Received in revised form technology provides an alternative good solution to the utilization of fly ash with little negative impact
2 March 2016
on environment. This review summarizes and examines the scientific advances in the preparation,
Accepted 2 March 2016
Available online 15 March 2016
properties and applications of fly ash-based geopolymer. The production of fly ash-based geopolymer is
mainly based on alkali activated geopolymerization which can occur under mild conditions and is
considered as a cleaner process due to much lower CO2 emission than that from the production of
Keywords:
Fly ash
cement. The geopolymerization can trap and fix the trace toxic metal elements from fly ash or external
Alkali activation sources. The Si/Al ratios, the type and the amount of the alkali solution, the temperature, the curing
Geopolymer conditions, and the additives are critical factors in a geopolymerization process. The mechanical per-
Waste utilization formances of the fly ash-based geopolymer, including compressive strength, flexural and splitting tensile
Cement strength, and durability such as the resistance to chloride, sulfate, acid, thermal, freeze-thaw and
Concrete efflorescence, are the primary concerns. These properties of fly ash-based geopolymer are inherently
dependent upon the chemical composition and chemical bonding and the porosity. The mechanical
properties and durability can be improved by fine tuning Si/Al ratios, alkali solutions, curing conditions,
and adding slag, fiber, rice husk-bark ash and red mud. Fly ash-based geopolymer is expected to be used
as a kind of novel green cement. Fly ash-based geopolymer can be used as a class of materials to adsorb
and immobilize toxic or radioactive metals. The factors affecting the performances of fly ash-based
geopolymer concrete, in particular aggregate, are discussed. For future studies on fly ash-based geo-
polymer, further enhancing mechanical performance, scaling up production and exploring new appli-
cations are suggested.
© 2016 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
2. Preparation and formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
3. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257
* Corresponding author. Research Group for Advanced Materials & Sustainable

Catalysis (AMSC), State Key Laboratory Breeding Base of Green Chemistry-Synthesis
Technology, College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310014, China.
E-mail addresses: clay@zjut.edu.cn, Chun.Zhou@usq.edu.au (C.H. Zhou).
http://dx.doi.org/10.1016/j.jclepro.2016.03.019
0959-6526/© 2016 Elsevier Ltd. All rights reserved.
254 X.Y. Zhuang et al. / Journal of Cleaner Production 125 (2016) 253e267
3.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

3.2. Flexural and splitting tensile strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258
3.3. Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
3.3.1. Chloride resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
3.3.2. Sulfate resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
3.3.3. Acid resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
3.3.4. Thermal resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
3.3.5. Freeze-thaw resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
3.3.6. Efflorescence resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262
4. Fly ash-based geopolymer for adsorption and immobilization of toxic metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
5. Fly ash-based geopolymer for concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
6. Conclusions and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
Abbreviations and nomenlatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
1. Introduction roughly comparable to hydrated cement in appearance, reactivity

and properties. In principle, geopolymer is a product of alkali
Fly ash is one of the solid residues composed of the fine particles activation of any aluminosilicate materials. It has a three-
that are driven out of the boiler with flue gases in coal-fired power dimensional aluminosilicate network structure with an empirical
plants. It is generally captured from flue gases by electrostatic formula of Mn[e(SiO2)zeAlO2]n$w H2O, where z is the Si/Al molar
precipitators or other particle filtration equipment before the flue ratio, M is an alkali cation, such as Naþ or Kþ, n is the polymeri-
gases reach the chimneys (Ahmaruzzaman, 2010; Gorai et al., zation degree, and w is the water content (Palomo et al., 1999a).
2006). Depending upon the source of the coal being burned, the Importantly, such a geopolymer has the similar bind performances
components of fly ash vary considerably. In general, the compo- to those of ordinary Portland cement (OPC). The alkali activation is
nents of fly ash typically include SiO2, Al2O3, CaO and Fe2O3, which conducted by adding NaOH, KOH, Na2SiO3 or K2SiO3 into fly ash
exists in the form of amorphous and crystalline oxides or various together or individually. The so-called geopolymerization can occur
minerals. According to the American Society for Testing and Ma- at room temperature or slightly elevated temperatures (usually
terials standard C 618 (ASTM C618-12a, 2012), fly ash can be clas- <100 C), and much importantly, with little CO2 emission
sified as Class C and Class F types based on their calcium oxide (Davidovits, 1991; Verdolotti et al., 2008; Zhang et al., 2016).
contents. Class C fly ash has a high calcium content, and is mainly The geopolymer technology provides a new good and green
generated from the burning of lignite coal sources. Class C fly ash solution to the utilization of fly ash, avoiding its negative impact on
has a total SiO2, Al2O3 and Fe2O3 content between 50 wt.% and environment and ecology. The alumina and silica in fly ash can be
70 wt.% and CaO content more than 20 wt.%. Class F fly ash has a activated with alkali to form geopolymer. Moreover, the toxic trace
low calcium content, and is generated from burning anthracite or metal elements can be trapped and fixed in the geopolymer
bituminous coal. Class F fly ash has a total SiO2, Al2O3 and Fe2O3 structure (Li et al., 2013a). Remarkably, the process of the produc-
content over 70 wt.% and CaO content less than 10% (Bankowski tion of fly ash-based geopolymer has lower CO2 emission compared
et al., 2004; Antiohos and Tsimas, 2007). In addition to Si, Al, Fe, with that of OPC during which much limestone (CaCO3) is calcined
and Ca, usually fly ash also contains many other trace metal ele- and decomposed at high temperatures (Davidovits, 1994; McLellan
ments, such as Ti, V, Cr, Mn, Co, As, Sr, Mo, Pb and Hg. The con- et al., 2011). Approximately 0.8 ton of CO2 is produced when one
centrations of the toxic trace elements in fly ash could be 4e10 ton of OPC is produced (Rashad and Zeedan, 2011; Yang et al.,
times higher than those in coal (Neupane and Donahoe, 2013; 2009). Fly ash-based geopolymer usually show mechanical
Nyale et al., 2014; Yao et al., 2015). It may also include small con- strength and durability nearly comparable to hydrated Portland
centrations of dioxins and polycyclic aromatic hydrocarbon com- cement and can be used as a class of green cement with natural
pounds (Shibayama et al., 2005; Nomura et al., 2010). Thus, fly ash resource efficiency (Nasvi and Gamage, 2012).
is considered as a hazardous material, and the improper disposal of In particular, recent years have witnessed many scientific ad-
fly ash will not only increase the occupation of land but also vances in the preparation technology, and the insights into the
deteriorate the environment and ecology. In last few decades, performances of the fly ash-based geopolymer. The fast-growing
increasing efforts have been made towards the utilization of fly ash, knowledge in turn results in many modification methods to
especially in an efficient and green fashion. significantly improve the production and the performances of the
Due to the cost and availability of oil and natural gas, coal-fired fly ash-based geopolymer. Meanwhile, there are increasing pilot
power plants will be still run for a long period, especially in the and commercial process. The large-scale use of fly ash-based geo-
coal-rich countries, for example, China, the USA, India and Australia polymer in construction industry seems to come true. In this review
(Lior, 2010). Under such a circumstance, the generation of fly ash article, the state-of-the-art preparation of fly ash-based geo-
remain significant and thus its economic and green utilization polymer is examined. Attention will therein be paid to under-
technology of fly ash are desired (Fig. 1a). Fly ash can be used for soil standing how the Si/Al ratios, the type and the amount of the alkali
amendment (Ukwattage et al., 2013) and nutrients (Kov a
cik et al., solution, the temperature, the curing conditions, and the additives
2011). It can also be used to make a low-cost adsorbent for waste are controllably used to reinforce properties of fly ash-based geo-
removal (Rubel et al., 2005; Yildiz, 2004). Besides, it can act as silica polymer. Discussed are the critical properties, including compres-
and alumina sources for zeolite production (Chang and Shih, 2000; sive, flexural and splitting tensile strength, and the durability such
Izidoro et al., 2012) (Fig. 1b). More recently, fly ash has been used as as chloride, sulfate, acid, thermal, freeze-thaw and efflorescence
an alternative source to make geopolymer, a new binder or cement resistance. Then, the applications of fly ash-based geopolymer are
X.Y. Zhuang et al. / Journal of Cleaner Production 125 (2016) 253e267 255
Concrete/Concrete
The usage of fly ash in 2013 products/Grout
Unreutilized Reutilized
4.34% Blended cement/Feed for
1.34% clinker
80,000,000
Flowable fills
0.02%
2%
70,000,000
Fly ash generation
0.07%
07% Structural fills/embankments
60,000,000 8.72%
72%
(shot ton)
Road base/sub-base
7.90%
0% Soil modification/stabilization
50,000,000
0.01% Blastig crit/roofing granules
40,000,000
1 15%
1.15% Mining application
30,000,000
0.58% Waste
stabilization/solidification
20,000,000
12.89% Agriculture
10,000,000 Aggregate
0 0.18% Oil field services

2008 2009 2010 2011 2012 2013 9.80% 52.99%
Miscellaneous/other
Year
Fig. 1. (a) The generation of fly ash during 2008 to 2013 in the USA. (b) The application fields of fly ash in 2013 in the USA (data from American Coal Ash Association, 2016).
surveyed. They involve cement-based construction, adsorption and geopolymer which is one of important parameters to govern the
immobilization of toxic metals materials. The existing challenges mechanical strength of geopolymer products (Kriven et al., 2003;
and future work are finally analyzed and proposed. Duxson et al., 2005b).
In addition to the Si/Al ratio, the microstructure of the formed
2. Preparation and formation fly ash-based geopolymer is strongly affected by the alkaline so-
lution. When fly ash contacts with alkaline (e.g. NaOH, KOH), Si4þ,
The basic and simplified principle of the formation of fly ash- Al3þ and other ions start to be released and to transfer. For instance,
based geopolymer is the alkali-facilitated decomposition of the amount of released Si4þ and Al3þ is influenced by the concen-
aluminosilicate in the fly ash and then polycondensation. The re- tration of NaOH solution. NaOH solution of high concentration
actions can proceed under mild temperatures so the production is (10 mol/L) is beneficial for decomposing aluminosilicate in the fly
considered to be energy and source efficient, namely much cleaner. ash and then release Si4þ and Al3þ (Table 1). For example, the sol-
However, the real reactions occurred in the process are very ubility of Al3þ and Si4þ in NaOH solution is higher than that in KOH
complicated and remain elusive. Apparently, there are reactions solution with the same concentration (Xu and van Deventer, 1999).
between fly ash and alkali and condensation between the resultant Moreover, the transfer of Al3þ and Si4þ species and the poly-
Si4þ and Al3þ species, followed by other complicated nucleation, condensation of aluminosilicate oligomers can also be accelerated
oligomerization, and polymerization, which finally lead to a new by an alkaline solution of high concentration (Lizcano et al., 2011).
aluminosilicate-based polymer with new amorphous three- Furthermore, different alkaline cations have different sizes and
dimensional network structure. In tests or uses, the as-prepared charge density, and they hydrate differently. These will then have a
fly ash-based geopolymer paste is further cast into a mould and certain effect on the nucleation of aluminosilicate chain, the growth
placed into an oven at a required temperature or left at room of the chain, the charge density on the chain, the rate, and the
temperature to be cured for a specific time to form construction extent of polymerization (Duxson et al., 2005a). For instance, the Kþ
(Fig. 2). cation (1.33 Å) is larger than the Naþ cation (0.97 Å) and the Kþ
The critical role in the formation geopolymerization are thought cation lead to a lower surface charge density and a higher degree of
be played by alkali activation on fly ash: in an alkaline solution polymerization of geopolymer matrix (van Jaarsveld and van
(Na2SiO3, NaOH, KOH or K2SiO3), the silica, the alumina, or the Deventer, 1999). In addition, alkaline cations can even serve as a
aluminosilicates in fly ash hydrolyze, eSieOeSie or eSieOeAle structure-directing agent in the geopolymerization. In brief, the
bonds of aluminosilicate break and release active Al3þ and Si4þ leaching rate of Si4þ and Al3þ decides the real available Si/Al ratio in
species. The active Al3þ and Si4þ species react to form nuclei and a series of reactions to form geopolymer and subsequently play a
aluminosilicate oligomers consisting of SiO4 and AlO4 tetrahedra. pivotal role in the structure of fly ash-based geopolymer. Interest-
The chains in aluminosilicate oligomers can be in the form of pol- ingly, a recent study revealed that the addition of Na2SiO3 to alkali
ysialate eAleOeSie chain, polysialate siloxo eAleOeSieSie chain, solution can increase such a Si/Al ratio, resulting in a lower porosity
and polysialate disiloxo eAleOeSieSieSie chain, depending upon and a finer pore system of geopolymer matrix (Ma et al., 2013).
the Si/Al ratio (Fig. 3). In aluminosilicate monomers, Si4þ is partially The setting time of fly ash-based geopolymer is usually
substituted by Al3þ, and the resultant negative charge in the considered for the workability of final fly ash-based geopolymer
aluminosilicate chains is balanced by alkali cations such as Naþ or products. In general, the final setting can be achieved within
Kþ (Davidovits, 2002; Dimas et al., 2009) (Fig. 2). In the context, the 1e2 h at room temperature while Class C fly ash with high CaO
Si/Al ratio significantly determines the final structure of the content and calcium-content additives, such as CaCl2, were found
resulted geopolymer materials (He et al., 2012). For example, it to shorten the setting time of fly ash-based geopolymer paste
have been found that the Si/Al ratio in the fly ash reactant has a (Rattanasak et al., 2011). In geopolymerization, Si4þ or Al3þ species
remarkable effect on the porosity (size and amount) of amorphous react with Ca2þ, either in the fly ash or from an external calcium-
Fig. 2. The schematic drawings showing the process from fly ash to fly ash-based geopolymer cement/concrete.
Fig. 3. Dependent upon the Si/Al molar ratio, different aluminosilicate chains are formed in the aluminosilicate oligomers which then further to form geopolymer (Davidovits,
2002) (reprinted and adapted by courtesy of Geopolymer Institute).
Table 1
Preparation and compressive strength of fly ash-based geopolymer with additives.
Material/alkaline activators feedstock Mixing T Curing time/ Compressive strength Refs

( C) temperature ( C) (MPa)
Fly ash/Na2SiO3 þ NaOH (10 mol) CaCl2, CaSO4, Na2SO4, sucrose 48 h/65 26.9e32.2 Rattanasak et al.,
2011
Fly ash/NaOH (4.5, 7.0, 9.5, 12.0, 14.0, 16.5 mol/L) e/25e28 <25.5 Somna et al., 2011
Fly ash þ RHBA/Na2SiO3 þ NaOH (4,8,12 mol/L) 2/5 25 36 h-28 d/25e90 14.8e55.6 Bohlooli et al., 2012
Fly ash þ RHBA/Na2SiO3 or water glass þ NaOH (5e12 mol/L) 2/5 25 2e7 d/25e90 12.6e35.1 Riahi and Nazari,
2012
Fly ash þ wastepaper sludge/Na2SiO3 þ NaOH 1/5 91 d/23e60 31.2e60.6 Yang et al., 2012
Pulverized coal combustion fly ash þ PCC bottom ash þ flue gas desulfurization n.a. 48 h/40 25.5e55.5 Boonserm et al., 2012
gypsum/Na2SiO3þNaOH
Fly ash þ crushed granite rock þ natural river sand/Na2SiO3þNaOH AT 6e72 h/60e120 42.0e58.0 Joseph and Mathew,
2012
FFA þ N-carboxymethyl chitosan NaOH (10 mol/L) AT 6 d/60 <30.0 Li et al., 2013b
Fly ash þ RHBA/water glass þ NaOH 7,28 d/RT <58.9 Nazari et al., 2013
36 h/40e90
FFA þ crushed granite stone þ superplasticizer/Na2SiO3þNaOH 5/2 48 h; 1,3,7 d/70 40.9e53.1 Demie et al., 2013
FFA þ BFS K2SiO3/Al (85 g/L) þ NaOH (30 g/L) 7 d/RT Ogundiran et al.,
2013
FFA/NaOH 3/5 25 7 d/28 d/60 1.4e9.9 Jun and Oh 2014
Fly ash þ palm oil fuel ash/Na2SiO3þNaOH 24 h/65 <38.0 Ranjbar et al., 2014
GGBF þ palm oil fuel ash þ fly ash þ manufactured-sand/Na2SiO3þNaOH 24 h/65 9.0e66.0 Islam et al., 2014
FFA þ red mud NaOH (50wt.%) þ sodium trisilicate (2 mol/L) 28 d/AT 11.3e21.3 Zhang et al., 2014a
*T: temperature, RT: room temperature, AT: ambient temperature, BFS: blast furnace slag, FFA: Class F fly ash, RHBA: rice husk-bark ash, GGBF: ground granulated blast
furnace slag.
content additives, to form calcium silicate hydrate gel (CeSeH), danger of the release of toxic elements. By contrast, fly ash-based
calcium aluminate hydrate gel (CeAeH) or calcium aluminum sil- geopolymer can solidify and immobilize the trace of heavy metal
icate gel (CeAeSeH) (C ¼ CaO, S ¼ SiO2, A ¼ Al2O3, H ¼ H2O) in the elements. Furthermore, the simple alkaline activation of fly ash to
presence of water (Diaz et al., 2010; Chindaprasirt et al., 2011). Ca2þ produce geopolymer completely bypasses the high-temperature
is beneficial for accelerating the nuclei formation and agglomera- calcination process in OPC production. All these would justify the
tion of CeAeSeH gel and CeSeH gel (Geetha and Ramamurthy, thought that the production of fly ash-based polymer is a cleaner
2013). The rapid formation of amorphous CeAeSeH gel and process with improved natural resource efficiency.
CeSeH gel leads to a shorter setting time of the final products and
decreases the porosity, while the rapid setting time has a negative 3. Properties
on the formation of more geopolymer gel (NeAeSeH). The high
NaOH concentration can prolong the setting time by limiting the 3.1. Compressive strength
leaching of calcium and allows normal geopolymerization process
to control the setting of geopolymer paste (Hanjitsuwan et al., The improvement of mechanical properties of fly ash-based
2014). geopolymer is major concerns because its main uses are in con-
At room temperature, however, the dissolution of fly ash is not struction materials as cement and concrete. The compressive
completed (Chen et al., 2011; Xu et al., 2010). In addition, the low strength of fly ash-based geopolymer is dependent on alkali solu-
reactivity of fly ash increases the setting time of fly ash-based tions, Si/Al ratios, calcium content, curing conditions (temperature
geopolymer. As a result, curing is a necessary step, namely, geo- and time) and the various additives.
polymer paste needs to be kept within a reasonable range of The type and the concentration of the alkaline solution influ-
temperature and moisture and the extended curing time promotes ence the release of Si4þ and Al3þ from fly ash during geo-
the formation of a more cross-linked binding and denser micro- polymerization. Alkaline solution of a high concentration is
structure. It is found that when the curing temperature increases generally beneficial for obtaining high compressive strength but
from 30 to 50 C, the reactivity of fly ash becomes higher and the there is an optimal range (de Vargas et al., 2011). Go €rhan and
geopolymerization is almost complete when the curing tempera- Kürklü (2014) prepared fly ash-based geopolymer with different
ture lies between 60 and 90 C (Hardjito et al., 2004). NaOH concentrations (3 mol/L, 6 mol/L and 9 mol/L). A highest
To improve the reactivity of fly ash and the performances of compressive strength of 22 MPa was achieved when the fly ash-
geopolymer, slag, chitosan, fiber, rice husk-bark ash (RHBA) and red based geopolymer paste was activated in 6 mol/L NaOH and
mud have been added into fly ash to synthesize geopolymer cured at 85 C for 24 h.
(Table 1). Interestingly, the addition of slag, which is the waste from Na2SiO3 solution is usually used with NaOH to increase
iron extraction process from raw ore, can enhance the reactivity of compressive strength (Criado et al., 2005). This is because of
fly ash during geopolymerization (Li and Liu, 2007). Blast furnace Na2SiO3 with high viscosity can help the formation of geopolymer
slag (BFS) is the by-product of iron production industry and it gels and a compact final fly ash-based geopolymer microstructure
contains more than 70% of SiO2 and CaO (Manz, 1999). Idawati et al. is achieved. Moreover, the activation procedure also influences the
(2014) investigated the fly ash/ground blast furnace slag (GBFS)- compressive strength of fly ash-based geopolymer. For example,
based geopolymer with different fly ash/slag ratios and they found Rattanasak and Chindaprasirt (2009) first added NaOH solution to
the geopolymerization of slag-based geopolymer was dominated fly ash to leach the Si4þ and Al3þ species for 10 min, followed by
by CeAeSeH gel, while fly ash-based geopolymer was dominated using Na2SiO3 to help form a uniform geopolymer paste for another
by sodium aluminosilicate (NeAeSeH) gel. Kumar et al. (2010) 1 min. Such separate activation gave a higher strength for the fly
substituted fly ash by 5e50% GBFS to synthesize geopolymer at ash-based geopolymer. When fly ash was separately mixed and
27 C and found that the reaction was dominated by dissolution activated with NaOH (10 mol/L) and Na2SiO3 with the NaOH/
and precipitation of CeSeH gel. Moreover, Yang et al. (2012) found Na2SiO3 molar ratio of 1.0 and cured at 65 C for 48 h, the
that the initial setting time of fly ash/GBFS-based geopolymer compressive strength of fly ash-based geopolymer was 60e70 MPa.
increased and the degree of polymerization of the geopolymer As discussed in previous section, the Si/Al ratios are determined
decreased due to the high content of calcium in GBFS. RHBA is a by the source materials and alkali solution (Na2SiO3 is used). High
solid waste generated from rice husk and eucalyptus bark by Si/Al ratios increase the amount of eSieOeSie bonds to get a
biomass power plants. RHBA contains about 75% SiO2 and the higher compressive strength of fully condensed structural matrix of
addition of RHBA enriches the silica content of the geopolymer geopolymer, since the eSieOeSie bonds are stronger than
matrix and increases the amount of eSieOeSie bonds in the eSieOeAle and eAleOeAle bonds. The addition of slag, RHBA,
geopolymer gel (NeAeSeH gel). Red mud is the major residue of and red mud can alter the Si/Al ratios (Table 1). For example, Yang
Bayer process with highly alkaline of alumina refining from bauxite et al. (2014a) prepared geopolymer fly ash and high magnesium
ores (Zhang et al., 2010a). The annual generation of red mud is nickel slag (HMNS) activated by Na2SiO3 solution. It was discovered
estimated to be about 70 million tons in the world (Klauber et al., that the major phase in fly ash/HMNS geopolymer was a type of
2011). Due to the use of a highly concentrated NaOH solution in sodium magnesium aluminosilicate gel. The addition of HMNS
the bauxite processing, red mud is a highly alkaline material. Red increased the silica content. In addition, HMNS particles worked as
mud essentially consists of oxides and hydroxides of Fe, Al and Si, as microaggregates and decreased the total volume of pores in the
well as minor quantities of CaO and TiO2. Addition of red mud can geopolymer pastes. As a result, the geopolymer possessed a high
adjust the Si/Al ratio and reduce the consumption of alkali activator compressive strength of above 60 MPa when 20% HMNS was used
(Piga and Stoppa, 1993). The addition of chitosan and fiber can in- for the preparation. Though the higher content of slag increased the
crease the hydrogen bonds, bridge the micro cracks and delay the compressive strength of fly ash/HMNS-based geopolymer, it caused
development of micro cracks (Li et al., 2013b). rapid setting and crack due to autogenous shrinkage of slag (slag/
Using waste materials, fly ash and additives like slag, RHBA, red binder>70%) (Jang et al., 2014). Wang et al. (2015) prepared fly ash/
mud and fibers, to produce a construction material geopolymer can slag-based geopolymer with different fly ash/slag ratios (0 wt.%,
help reduce the consumption of mineral reserves such as limestone 20 wt.%, 40 wt.%, and 60 wt.%) and various NaOH solutions (0.5%, 1%
and the emission of greenhouse gases. Without proper utilization, and 1.5%), and then cured for 1, 3, 7 and 28 days. The increasing
fly ash is a solid waste. Improper disposal of fly ash brings the portion of slag increased the compressive strength of geopolymer
and the optimal compressive strength was 93.06 MPa. Deb et al. cured them under the 90-W microwave radiation for 5 min fol-
(2014) activated Class F fly ash/GBFS-based geopolymer (GBFS/ lowed by additional heating at 65 C for 6 h. The compressive
Class F fly ash ratio ¼ 0%, 10% and 20%) in NaOH and Na2SiO3 so- strength was comparable to that of the fly ash-based geopolymer
lution, respectively. High compressive strength increased with the cured at 65 C for 24 h. The microwave radiation quickened the
increase of the GBFS/Class F fly ash ratio. When the geopolymer dissolution of fly ash in the alkaline solution and formed a denser
concrete synthesized by 20% slag and 80% fly ash with 40% NaOH microstructure. Accordingly, the microwave radiation shortened
and Na2SiO3 solution and cured at 20 C, it had the highest the required curing time and enhanced the geopolymerization.
compressive strength of 51 MPa. Xu et al. (2010) used GBFS of grade To put together, increasing the Si/Al ratio usually enhance the
80, 100 and 120 and fly ash to synthesize geopolymer with an compressive strength of fly ash-based geopolymer. Though the
activating solution prepared from concentrated Hanford secondary increase of the Si/Al ratio can be realized by adding the external
waste (HSW) stimulant (5 mol/L NaOH mixed with solid binders). slag, RHBA and red mud, the inherent reasons are complicated.
The highest compressive strength was 52.5 MPa when the fly ash/ Advances in the preparation indicated that one of major reasons
GBFS mass ratio was 5/3. can be ascribed to the increased amount of eSieOeSie bonds
The addition of RHBA enriched the silica content of the geo- rather than eSieOeAle and eAleOeAle bonds. To this end, the
polymer matrix, increased the amount of eSieOeSie bonds in the use of Na2SiO3 or K2SiO3 with NaOH for the activation of fly ash can
geopolymer gel (NeAeSeH gel) and the compressive strength (Sata increase the Si/Al ratios, thereby leading to a more compact
et al., 2007; Tangchirapat et al., 2008). Nazari et al. (2011) added structure with higher compressive strength (Fig. 3). In addition, the
RHBA into fly ash to synthesize geopolymer paste and cured it in presence of calcium in fly ash or used as additive is beneficial for
oven at 80 C for 28 days. They found the geopolymer had the forming the amorphous CeSeH gel and CeAeSeH gel and de-
compressive strength of about 60 MPa. Songpiriyakij et al. (2010) creases the porosity and obtains geopolymer with a higher
prepared fly ash-based geopolymer using the reactants of fly ash, compressive strength. Curing had a significant effect on the
RHBA and NaOH and curing at 27 C first for 24 h and then curing at compressive strength of fly ash-based geopolymer by changing the
60 C for 24 h. It was found that the optimal SiO2/Al2O3 ratio to porosity and density of the product.
obtain the highest compressive strength (73 MPa) was 15.9. Red
mud of highly alkalinity is the major residue of Bayer process of 3.2. Flexural and splitting tensile strength
alumina refining from bauxite ores. Zhang et al. (2014a) prepared
geopolymer using fly ash and red mud at 23 C. They found Fly ash-based geopolymer would suffer from brittle failure with
compressive strengths ranging from 11.3 to 21.3 MPa and the low tensile strength and fracture toughness. A typical method is to
highest compressive strength was obtained when the Si/Al molar incorporate chitosan or fibers into the fly ash-based geopolymer
ratio was 2. matrix because this hybridization can improve the bond strength
Calcium is proved to interfere with the gelation of silica and and reinforcing bending behaviors in strain hardening and multiple
alumina in geopolymerization process and alter the microstruc- cracking procedure. In this way, the flexural and splitting tensile
tures of fly ash-based geopolymer and thus alter the compressive strength can be enhanced. The fibers for reinforcing fly ash-based
strength (Schmucker and Mackenzie, 2005). The coexistence of geopolymer composites include steel (ST) fiber (Shaikh, 2013),
CeSeH gel and the NeAeSeH gel usually improves the compres- polyvinyl alcohol (PVA) fiber (Nematollahi et al., 2015; Zhang et al.,
sive strength of final products. One of reasons is that the amor- 2006, 2008b; Sun and Wu, 2008), sweet sorghum fiber (Chen et al.,
phous CeSeH gel decreases the porosity (Temuujin and van 2014), and cotton (Alomayri et al., 2014a, 2014b).
Riessen, 2009). In order to obtain higher flexural strength, Shaikh (2013) added
Curing time and temperature also affect the compressive 2 v/v% steel fiber, 2 v/v% PVA fiber and a hybrid combination of 1 v/v
strength of fly ash-based geopolymer. Longer curing time, in the % ST þ 1 v/v% PVA fiber and investigated the deflection hardening
range of 6 he28 days, produced fly ash-based geopolymer with a behavior of hybrid fiber reinforced fly ash-based geopolymer. They
higher compressive strength (Table 1). Curing at high temperatures found a higher bond strength and flexural strength between the
increases the compressive strength by removing the water from the PVA fiber and geopolymer matrix than with cement matrix. The
fresh geopolymer, causing the collapse of the capillary pores with a alkalinity of geopolymer matrix did not affect the degradation of
denser structure (Leung and Pheerapha, 1995). Fly ash-based geo- PVA and steel fiber as seen in SEM (Fig. 4). Alomayri et al. (2014a)
polymer can be cured at room temperature, but the compressive added cotton fabric layers in fly ash-based geopolymer structure
strength develops slowly and always needs prolonged curing time to improve flexural strength. The flexural strength of the fly ash-
(Somna et al., 2011). Nasvi and Gamage (2012) found that the crack based geopolymer increased from 8.2 MPa to 31.7 MPa when the
closure and crack initiation thresholds of fly ash-based geopolymer cotton fiber content was increased from 0 to 8.3 wt.%. Moreover, the
cured at elevated temperatures (60e80 C) was higher (30e60% orientation of cotton fabric layers had effects on the flexural
peak stress) compared to those (15e30% of peak stress) cured at strength of fly ash-based geopolymer. The higher flexural strength
ambient temperature (23 C and 40 C). However, prolonged curing of the fly ash-based geopolymer reinforced with horizontally laid
at higher temperatures breaks down the granular structure of cotton fabric could be attributed to the better uniformity in load
geopolymer, resulting in the dehydration and the excessive distribution among the consecutive layers of cotton fabric, while
shrinkage, and finally decreasing the compressive strength (Palomo the geopolymer with vertical fabric orientation suffered from de-
et al., 1999b). tachments and delamination between the cotton fabric and geo-
Steam curing is the conventional heating technique, relying on polymer matrix, and had a lower flexural strength (Alomayri et al.,
the conduction of heat from the exterior to the interior of fly ash- 2014b).
based geopolymer paste by steam. The heating is non-uniform Splitting tensile strength can be increased by using N-carbox-
and required a long heating period to attain the required temper- ymethyl chitosan, and the fibers, such as PVA fiber and sweet sor-
ature. Microwave heating is based on internal energy dissipation ghum fiber as additives in fly ash-based geopolymer (Table 1).
associated with the excitation of molecular dipoles in electromag- Nematollahi et al. (2015) prepared geopolymer with short PVA fi-
netic fields, and it delivers faster and more uniform heating (Kim bers (2% v/v), Class F fly ash activated in 8.0 mol/L NaOH (28.6% w/
et al., 2015). Chindaprasirt et al. (2013b) prepared fly ash-based w) and Na2SiO3 (71.4% w/w) solution. The bond strength of the
geopolymer pastes with 10 mol/L NaOH and Na2SiO3 solution and geopolymer increased more than the cracking strength, resulting in
Fig. 4. SEM image of steel fiber in fly ash-based geopolymer matrix (left). SEM image of PVA fiber in fly ash-based geopolymer matrix (right) (reprinted from Shaikh (2013),
Copyright (2013), with permission from Elsevier).
Fig. 5. Fly ash-based geopolymer with N-carboxymethyl chitosan; the dashed lines: the hydrogen bonds formed between N-carboxymethyl chitosan macromolecules and fly ash-
based geopolymer as well as within the N-carboxymethyl chitosan macromolecules (reprinted and adapted from Li et al. (2013b), with permission of Springer).
a high fiber-bridging strength. The splitting tensile strength of PVA carboxymethyl chitosan exhibit enhanced the mechanical behavior
reinforced Class F fly ash-based geopolymer was 4.7 MPa. of the fly ash-based geopolymer. The N-carboxymethyl chitosan
Chitosan with stable crystalline structure from strong hydrogen reinforced fly ash-based geopolymer had a substantial increase of
bonds suffers poor solubility at higher pH required for geopolymer. the tensile strength from 7 MPa to 8 MPa when the N-carbox-
N-carboxymethyl chitosan, a chitosan derivative, can avoid this ymethyl chitosan content increased to 0.1 wt.%.
problem and realize the better coordination with geopolymer gel Chen et al. (2014) used alkali-pretreated sweet sorghum fiber
(Mourya et al., 2010; Pillai et al., 2009). Li et al. (2013b) mixed NaOH with fly ash to prepare geopolymer. The sweet sorghum fiber was
solution (10 mol/L), N-carboxymethyl chitosan uniform solution obtained from the bagasse waste after juice being extracted from
with fly ash under stirring under ambient conditions for 20 min. sweet sorghum stalks for ethanol production. When 2% sweet
Then, the specimens were aged at room temperature for 24 h and sorghum fibers were used, an increase about 36% of the splitting
placed in an oven for curing at 60 C for 6 days. N-carboxymethyl tensile strength of the geopolymer was obtained. But further in-
chitosan biopolymer coated on fly ash particles and N-carbox- crease of the fiber content decreased tensile strength. The flexural
ymethyl chitosan were well incorporated into geopolymer matrix. strength of the fiber-reinforced geopolymer showed a similar trend.
In particular, there were additional hydrogen bonds formed be- The addition of 2% fiber effectively carried higher tensile load and
tween N-carboxymethyl chitosan macromolecules and fly ash- thus delayed the growth of microcracks and increased the flexural
based geopolymer, in addition to the hydrogen bonds within the strength. However, further increase of the fiber content induced
N-carboxymethyl chitosan macromolecules (Fig. 5). All these in- fiber agglomeration, resulting in an increase of air bubbles
teractions led to a more condensed network structure in geo- entrapped in the composite and nonuniform fiber dispersion. As
polymer. Thus the fly ash-based geopolymer reinforced by N- consequence, flexural strength decreased. The addition of cotton to
fly ash-based geopolymer showed the similar trend in flexural deterioration of strength. In addition, the sulfate solution usually
strength. caused the breaking of eSieOeSie bonds in geopolymer gel and
According to the studies mentioned above, the increase of the leaching of silicon (Bascarevi
c et al., 2015). Bakharev (2005a)
splitting tensile strength and flexural strength in chitosan- prepared fly ash-based geopolymer using NaOH, KOH, or Na2SiO3
reinforced and fiber-reinforced fly ash-based geopolymer was pri- as activator and investigated the durability after exposed them to
marily because the micro- and macro-fibers can increase the sulfate environments (5% MgSO4 solution, 5% Na2SO4 solution and a
hydrogen bonds, bridge the micro cracks, transfer the load, and solution of 5% MgSO4 þ 5% Na2SO4) for 5 months. After immersion,
delay the development of micro cracks (Dias and Thaumaturgo, the fly ash-based geopolymer samples changed little. The geo-
2005). polymer activated by NaOH had the best sulfate resistance owing to
a more stable cross-linked structure.
3.3. Durability Sukmak et al. (2015) used a mixture of silty clay and fly ash as
the source materials to prepare geopolymer and examined the
The durability of fly ash-based geopolymer and geopolymer resistance of the geopolymer in 5 wt.% Na2SO4 and 5 wt.% MgSO4
concrete includes resistance to chloride, sulfate, acid, freeze-thaw, solutions. The decrease in the compressive strength of the clay/fly
thermal and efflorescence. Durability is closely related to the ash-based geopolymer after 240 days of exposure was 10.8% in
microstructure and the migration behavior of ions from fly ash- Na2SO4 solution and 21.6% in MgSO4 solution. Ettringite, gypsum
based geopolymer. These in turn can be adjusted by the alkali so- and brucite, were detected after the exposure to sulfate environ-
lution, curing and addition of calcium and silica fume composite ment. The CeSeH phase disappeared due to its reaction with sul-
during the preparation of fly ash-based geopolymer. fates and forming ettringite phase.
3.3.1. Chloride resistance 3.3.3. Acid resistance

Resistance to chloride is one of the main areas in durability of The lifetime of cement and concrete can be severely shortened
cement and concrete. Chloride penetration promotes the corrosion in the acidic environment. For fly ash-based geopolymer, the acid
of the embedded steel bars when steel bars are used as re- attack is associated with the depolymerization of aluminosilicate
inforcements for geopolymer concrete. Chloride penetrates into fly network structure and the liberation of silicic acid (Si(OH)4). When
ash-based geopolymer through capillary absorption, hydrostatic immersed in a strong acid solution, Naþ and Kþ from fly ash-based
pressure and diffusion of ions. geopolymer could be substituted by Hþ or H3Oþ, breaking
The relatively high concentration of NaOH enabled the leaching eSieOeAle bond and eSieOeSie bond, and releasing silicic acid
of more Si4þ and Al from fly ash and produced a better degree of (Breck, 1974).
polycondensation and resulted in a decrease the porosity of fly ash- Different alkali solutions (NaOH, KOH and Na2SiO3) have been
based geopolymer. The porosity of the material affects the chloride used by Bakharev (2005b) to prepare fly ash-based geopolymer
penetration. For example, Chindaprasirt and Chalee (2014) pre- followed by immersing the geopolymer in 5% CH3COOH and H2SO4
pared fly ash-based geopolymer using a Na2SiO3/NaOH mixture solutions for 5 months. The fly ash-based geopolymer activated by
solution as the activator. The concentrations of NaOH were 8, 10, 12, 8% NaOH had more stable structure and showed high resistance to
14, 16 and 18 mol/L with a constant SiO2/Al2O3 molar ratio. The both acid solutions, while the fly ash-based geopolymer activated
chloride penetration decreased with the increase of NaOH con- by KOH showed an increase in the average pore diameter and the
centration used in geopolymerization process owing to the number of active sites in the geopolymer gels on the surface,
refinement of the pore structures as a result of polycondensation leading to a lower durability.
reaction. The effect of calcium and silica fume on the acid resistance of fly
Ismail et al. (2013) examined the permeability of chlorides in ash-based geopolymer is also noteworthy. Lloyd et al. (2012)
Class F fly ash/slag-based geopolymer, slag-based geopolymer, and investigated the corrosion rates of Class C fly ash-based and slag-
OPC by the chloride accelerated method (NordTest NT Build 492) based geopolymer in HNO3 and H2SO4 acids (pH ¼ 1.0e3.0). It
and the ponding method (ASTM C1543). AgNO3 was used to reveal was found that the calcium content introduced from the Class C fly
chloride penetration depths. The result showed that a little for- ash or slag reduced the mass transport rates by forming fine and
mation of silver chloride for the slag-based geopolymer (Fig. 6A), tortuous pore networks in geopolymer. In addition, as the increase
and fly ash/slag-based geopolymer (Fig. 6B, C), while OPC-based of alkali content (Na2O), the nature of the corroded layer became
concrete (Fig. 6D) had the deepest chloride penetration depth. harder, more brittle and more prone to the development of cracks
They found the slag promote the formation of denser CeAeSeH gel (Fig. 7). The high content of alkali is beneficial for releasing more
contributing to a higher mechanical strength, and durability under calcium and aluminum and for forming geopolymer gels while
chloride exposure and fly ash promote the formation of more when removed upon exposure to acid, the geopolymer structure
porous NeAeSeH gels, reducing the resistance to chloride trans- was more vulnerable under acidic conditions. Chindaprasirt et al.
port. Although a higher porosity was observed in the geopolymer (2014) added silica fume (1.5%, 3.75% and 5.0%) to fly ash to pre-
compared to the OPC specimens, geopolymer still showed a higher pare geopolymer composite mortars. The acid resistance of com-
chloride resistance. posite mortars was tested in 3 vol% H2SO4 acid. The optimum silica
Yang et al. (2014b) prepared fly ash/slag-based geopolymer fume addition of 3.75% increased the strength of fly ash-based
(Slag/Fly ash ratio ¼ 0, 0.25 and 0.50) and exposed the materials to a geopolymer and showed the best acid durability owing to the in-
3% NaCl solution for 72 h. The CeAeSeH gel formed in the fly ash/ crease of CeSeH gels and a more dense structure.
slag-based geopolymer resulted in lower chloride diffusion Curing at a high temperature is beneficial to the resistance of
compared with the NeAeSeH gel. In addition, the incorporation of geopolymer to acid. Nguyen et al. (2013) cured the fly ash-based
slag in fly ash-based geopolymers led to the refinement of the pore geopolymer at 80 C for 10 h, and then immersed the geopolymer
structure. in HCl solution (1 mol/L, 2 mol/L and 4 mol/L). It was found that the
fly ash-based geopolymer still maintained a compressive strength
3.3.2. Sulfate resistance of about 20 MPa which was much higher than OPC in HCl solution
The migration of Naþ from fly ash-based geopolymer into the (1 mol/L, 2 mol/L and 4 mol/L). In addition, Chindaprasirt et al.
sulfate solution results in vertical cracks and causes the (2013a) cured fly ash-based geopolymer with 90 W microwaves
Fig. 6. Boundary of chloride penetration in 28-day cured concretes at the end of the Nord Test procedure, as a function of slag/fly ash ratio: (A) 100 wt.% slag, (B) 75 wt.% slag/
25 wt.% fly ash, (C) 50 wt.% slag/50 wt.% fly ash, (D) OPC (reprinteded from Ismail et al. (2013), Copyright (2013), with permission from Elsevier).
Fig. 7. The appearance of the corroded layers on the fly ash-based geopolymer samples, containing 7% Na2O (left) and 15% Na2O (right) after 28 days exposure to pH 1.0 sulfuric acid
(reprinted from Lloyd et al. (2012), with permission from Springer).
for 5 min. They found the microwave radiation accelerated the average residual strength of 90%, 52% and 11e16% respectively after
geopolymerization and gave enhanced densification comparable to exposed to fire at 400 C, 650 C and 800e1000 C, whereas the
the conventional curing. The microwave cured fly ash-based geo- average residual strengths of geopolymer concretes were 93%, 82%,
polymer was immersed in 3 vol% of H2SO4 and found that it only and 21e29% after the same treatments. Moreover, the OPC concrete
had a small loss of strength under the acid attack. suffered severe spalling and extensive surface cracking after
exposure at 800e1000 C, while there was no spalling and only
3.3.4. Thermal resistance minor surface crackings in the geopolymer concrete. Guerrieri and
When fly ash-based geopolymer is exposed to elevated tem- Sanjayan (2010) exposed fly ash/slag-based geopolymer (fly ash/
peratures, shrinkage of geopolymer occurs in proportion to the slag ratio ¼ 100, 65/35, 50/50, 35/65) to 800 C. They found the
amount of water being vaporized from the structure (Rickard et al., geopolymer specimens with very low initial strengths (<7.6 MPa)
2011). Some of the fly ash-based geopolymers exhibit a strength experienced an increase in residual strength up to 90% gain after
increase after exposure to high temperature, and have been used as exposure to 800 C while specimens with initial strengths of
concrete, thermal barriers, refractories and fire resistant structures. 28 MPa and specimens with initial strengths of 83 MPa had residual
Heat traveled faster in geopolymer concrete than in OPC concrete strength losses of approximately 70% and 90% after exposure to
when exposed to fire, resulted in less temperature gradient inside 800 C, respectively. The different residual strengths of fly ash/slag-
geopolymer concrete (Sarker et al., 2014). OPC concrete showed an based geopolymer after exposure to 800 C were caused by the
further hydration and sintering of the unreacted fly ash or slag. geopolymer were higher than 40. The addition of Na2SiO3 solution
Geopolymer with higher initial strengths have a smaller capacity to did not improve the freeze-thaw durability of fly ash-based geo-
allow for thermal incompatibilities between the inner and outer polymer. The presence of crystalline, anhydrite and amorphous
parts of the specimen due to uneven temperatures arising during calcium compounds (such as lime) in fly ash negatively influenced
heating. Rickard et al. (2010) investigated the thermal character- the freeze-thaw resistance. Na2SiO3 reacted with calcium com-
istics of Class F fly ash-based geopolymer at the temperature pounds (CaO, CaSO4 and amorphous calcium aluminosilicate) in the
>500 C. The maximum shrinkage was approximately 3% when the fly ash source and formed CeAeSeH gel or Ca(OH)2. The formation
geopolymer was heated to 900 C. The iron oxides (15 wt.%) in fly of Ca(OH)2 weakened internal structure and increased water
ash directly affected the thermal properties of the geopolymer by permeability and had a negative effect on the freeze-thaw
influencing the thermal expansion, altering the phase composition resistance.
and changing the morphology after heating. Air entrainment has been proved as an effective method to in-
The pore structure and density of fly ash-based geopolymer crease freeze-thaw resistance of OPC (Mindess et al., 1981). Sun and
facilitates the escape of moisture and help avoid damaging during Wu (2013) compared fly ash-based geopolymer and OPC with or
heating. Kong et al. (2007) prepared fly ash-based geopolymer and without air entrainment for 300 freeze-thaw cycles according to
exposed it to 800 C. They found a small amount of moisture ASTM C666. OPC without air entrainment deteriorated most seri-
escaped from fly ash matrix and the pore spaces provided escape ously, showing a strength loss of about 20% after 300 cycles, while
routes for moisture in the matrix thereby decreasing the likelihood OPC with air entrainment lost only 5%. Fly ash-based geopolymer
of the damage to the matrix at elevated temperatures. Bakharev without air entrainment had a final loss of 8.4% after 300 cycles.
(2006) compared the effects of heating at 800e1200 C on the With the air entrainment, fly ash-based geopolymer showed a
geopolymer made form Class F fly ash activated by NaOH and KOH, strength loss of 6.8%. Clearly, the air entrainment had no much
respectively. The geopolymer activated by NaOH had a rapid positive influence on fly ash-based geopolymer. Brooks et al. (2010)
strength deterioration and a high shrinkage at 800 C due to a studies the effects of air entrainment on the scaling rate of fly ash-
dramatic increase of average pore size, while the other geopolymer based geopolymer. Air-entrained fly ash-based geopolymer showed
activated by KOH showed a significant increase of compressive slight scaling after 40 freeze-thaw cycles while non-air entrained
strength upon heating and the strength deterioration only started fly ash-based geopolymer showed no scaling. Air entrainment
at 1000 C. The addition of foaming agent of a low concentration led significantly reduced the compressive strength of fly ash-based
to a low density with cellular structure. Rickard and van Riessen geopolymer 10e30% than the non-air entrained fly ash-based
(2014) prepared foamed Class F fly ash-based geopolymer with a geopolymer and the uniform and stable pore structure believed
Si/Al ratio of 2.5 (r z 0.9 g/cm3, k z 0.3 W/m$K). Under a simulated to increase freeze-thaw durability was not formed in non-air
fire for 60e90 min, the foamed geopolymer lost its strength entrained fly ash-based geopolymer.
significantly. Foaming significantly reduced the strength of Class F
fly ash-based geopolymer, while the thermal insulating property 3.3.6. Efflorescence resistance
improved as their thermal conductivities were reduced by Geopolymer, especially with a high alkali content and a low
approximately half. calcium content, tends to have a porous and open microstructure.
Using Ca(OH)2 as additive to synthesize fly ash-based geo- The aqueous solution remains entrapped in the pores or bonded to
polymer formed the CeSeH gel and helped the different mineral the network when the three-dimensional geopolymer structure
formations and transformation processes at elevated tempera- has finally formed (Davidovits, 2008). Within the pore network, the
tures. Dombrowski et al. (2007) prepared fly ash-based geo- excess sodium oxide is mobile, and is prone to form white crystal
polymer and investigated the influence of the calcium content on (i.e. efflorescence) when in contact with atmospheric CO2 and re-
thermal resistance. When the geopolymer exposed to tempera- sults in the degradation of geopolymer (Najafi and Allahverdi,
tures higher than 600 C, sodalite and amorphous aluminosili- 2009; Pacheco-Torgal and Jalali, 2010).
cates were formed and then they were transformed into The efflorescence behavior of fly ash-based geopolymers is
nepheline, resulted in a denser structure. As the temperature was strongly dependent on the type of alkali activator solution, curing
further increased, the nepheline was converted into albite. The fly temperature and calcium content. Using KOH instead of NaOH as
ash-based geopolymer with 8% Ca(OH)2 addition for the prepa- activator reduces the efflorescence of geopolymer (Duxson et al.,
ration had the highest strength and the lowest shrinkage with 2006). The high alkali concentration in the pore solution and the
the highest amount of nepheline at 800 C and of feldspar at weak binding of Naþ in the geopolymer structure are the reasons
1000 C. for more mobile Naþ in pore solution and cause the severe efflo-
Geopolymer exposed to fire has the tendency to shrinkage and rescence of geopolymer (Bortnovsky et al., 2008). Kþ is more
cracking as well. According to the International Standards Organi- strongly bound to the aluminosilicate gel framework, and also
zation standard (ISO 834), Sarker et al. (2014) test the fire resistance potassium carbonate crystals are usually less visually evident than
of the fly ash-based geopolymer and OPC by exposing the samples their sodium counterparts (Szklorzova and Bílek, 2008; Skv ara
to fire heating at 400 C, 650 C, 800 C and 1000 C. When exposed et al., 2008). Zhang et al. (2014b) activated fly ash by NaOH to
to fire at 1000 C, fly ash-based geopolymer had only minor surface synthesize geopolymer and cured at 23 C and 80 C. They found
cracking and the average mass loss of only 4.8%, while OPC had the geopolymer at high temperature exhibiting much lower efflores-
average mass loss of 90%. cence than those synthesized at low temperature. The curing was
beneficial for reorganization and crystallization of NeAeSeH gels
3.3.5. Freeze-thaw resistance and decreasing the efflorescence rate.
Freeze-thaw attack usually causes cement and concrete expan- The presence of calcium helps form CeSeH gel in fly ash-based
sion, internal cracking, and scaling mass loss (Hobbs, 2001). It is a geopolymer and a smaller pore size with low-permeability, which
major attack to cement and concrete second to chloride attack. Fly prevents alkali diffusion (Lloyd et al., 2010). Conversely, for foamed
ash based-geopolymer has been reported to show no sign of fly ash-based geopolymer, the foaming caused large pore size and
damage even after 150 freezing cycles. Temuujin et al. (2014) pre- high porosity of fly ash-based geopolymer and it led to fast alkali
pared geopolymer from Class F fly ash activated by a NaOH/Na2SiO3 leaching and caused fast efflorescence consequently (Zhang et al.,
solution and cured it at 70 C for 22 h. The freeze-thaw cycles of 2014b).
4. Fly ash-based geopolymer for adsorption and thereby increasing concrete strength (Sahmaran and Li, 2009). Fly
immobilization of toxic metals ash can reduce the hydration heat and thermal cracking of concrete
at early stage, and improve the mechanical and durability
Geopolymerization offers an alternative technique to immobi- properties.
lize soluble heavy metals from fly ash, slag or other industrial and Fly ash-based geopolymer concrete completely moves away
residential wastes (Malviya and Chaudhary, 2006; Ojovan et al., from OPC and the high CO2 emission associated with OPC pro-
2005). The hazardous elements, such as Ba, Cd, Co, Cr, Cu, Nb, Ni, duction (Cakir and Akoz, 2008; Aldea et al., 2000; Habert et al.,
Pb, Sn, and U can be tightly fixed in the three-dimension structure 2011). Fly ash-based geopolymer itself is a good binder that can
of fly ash-based geopolymer matrix. The main mechanisms of metal be used as cement to mix with aggregates to produce the geo-
immobilization in fly ash-based geopolymer are physical encap- polymer concrete. Fly ash-based geopolymer cement has shown a
sulation and chemical stabilization. The high pH values enhance the better performance than OPC in some areas aspects or conditions.
oxyanionic mobility, such as As, B, Mo, Se, V and W (Izquierdo et al., For example, fly ash-based geopolymer concrete has a denser
2009). Temuujin et al. (2014) prepare geopolymer from high- microstructure with lower chloride diffusion and lower porosity
calcium Mongolian fly ash (CaO ¼ 14e30 wt.%) with a high radio- compared with OPC concrete. Reddy et al. (2013) used Class F fly
activity (314e343 Bq/kg) and used NaOH solution or mixtures of ash as source material and NaOH and Na2SiO3 solution as activator
NaOH and Na2SiO3 as activator and cured it at 70 C for 22 h. The to synthesize geopolymer cement. They found that the geopolymer
radioactivity of the radioactive high-calcium Mongolian fly ash- concrete had excellent resistance to chloride attack and less
based geopolymer product was then became 130e152 Bq/kg, corrosion cracking when exposed to simulated seawater and
within the standard safe limits for construction of dwellings. induced current.
Fly ash-based geopolymer showed higher immobilization of Alkali silica reaction (ASR) is a common chemical reaction be-
metal ions than OPC and fly ash itself. Li et al. (2013a) compared the tween the OH in the pores within the concrete matrix and the
immobilization of 133Csþ in fly ash-based geopolymer and OPC. reactive aggregate compounds in concrete (Swamy and Al-Asali,
Leaching tests was carried out in deionized water, sulfuric acid and 1988; Diamond, 1975). ASR causes the strength loss, cracking, and
magnesium sulfate solutions, respectively. It was revealed that expansion of the concrete structure. Fly ash-based geopolymer
neither the microstructure nor the geopolymeric phases of fly ash- concrete is significantly less vulnerable to ASR than OPC concrete.
based geopolymer were significantly affected by the incorporation Kupwade-Patil and Allouche (2013) prepared steel reinforced fly
of 133Csþ. The cumulative fraction leaching concentration (CFLC) of ash-based geopolymer concrete with quartz, sandstone and sili-
fly ash-based geopolymer in deionized water was only 5.4% of that ceous limestone as aggregates, and compared the chloride diffusion
of OPC for 25 C, and 6.1% for 70 C. The leaching of 133Csþ from the in a cyclic wet-dry chloride environment over a period of 12
geopolymer in 5% (w/w) magnesium sulfate solution was only 9.1% months with OPC concrete. It was found that fly ash-based geo-
that from OPC. The difference of leaching of 133Csþ between the polymer concrete specimens did not exceed the ASTM threshold for
geopolymer and OPC in sulfate acid solution was small. Al-Zboon expansion while OPC concrete exceeded the permissible threshold.
et al. (2011) used fly ash-based geopolymer as an adsorbent for In addition, the leaching and visual cracks were observed in the
lead Pb(II) removal from wastewater. They found the lead removal OPC concrete but not in the fly ash-based geopolymer concrete.
efficiency of fly ash-based geopolymer was 90.6%, which was much In concrete, aggregate takes up as high as 85% of the material.
higher than that of raw fly ash (39.87%) (pH ¼ 5; C0 ¼ 100 ppm; Interactions among aluminosilicate framework, alkali cations, ad-
contact time ¼ 120 min). The removal efficiency was 95% when the ditives and aggregate in fly ash-based geopolymer concrete are
dose of fly ash-based geopolymer was 0.07 g at 40 C, pH ¼ 5, and important factors that influence the overall mechanical perfor-
120 min contact time in 1000 ppm standard lead (Pb) solution. mance. The strong interfacial interactions between the aggregate
The fly ash-based geopolymer showed different metal immo- and the fly ash-based geopolymer matrix in a large zone contrib-
bilization behaviors in acid and sulfate solutions. Zhang et al. utes to the high splitting tensile strengths between geopolymer and
(2008a) prepared geopolymer with fly ash, sand and some heavy steel reinforcements (Topark-Ngarm et al., 2015).
metal additives, such as Cr(VI), Cd(II) and Pb(II), to investigate the The size of the aggregates affects the fly ash-based geopolymer
immobilization effect of heavy metal ions and the leaching concrete performance. Kong and Sanjayan (2010) studied the effect
behavior in H2SO4 and Na2SO4 solutions. The low level of heavy of aggregate size on the compressive strength at high temperatures
metal salts had little effect on the compressive strength of the of the fly ash-based geopolymer concrete containing crushed old
geopolymer. The immobilization of these species was based on the basalt aggregates, river sand and slag aggregates. It was found the
chemical binding into the geopolymer gel or into aluminosilicate thermal incompatibility between the geopolymer matrix and the
phases in geopolymer. They found Pb was immobilized effectively aggregates caused the strength loss of geopolymer concrete spec-
by a chemical binding mechanism in geopolymer, Cr(VI) was inef- imens at elevated temperatures. Large aggregates (>10 mm)
fectively immobilized, and Cd immobilization was depended on the resulted in good strength performances at both ambient and
solubility of a hydroxide phase with effectively at high pH but poor elevated temperatures while smaller sized aggregates (<10 mm)
at low pH. promoted spalling and extensive cracking at elevated temperature.
Recycled coarse aggregate has been used with fly ash geo-
5. Fly ash-based geopolymer for concrete polymer to synthesize concrete with acceptable properties. Sata
et al. (2013) used crushed structural concrete beams and crushed
The growing environmental awareness and the demand for high clay bricks as recycled coarse aggregates for making fly ash-based
efficiency of natural resource encourage the construction industry geopolymer concrete. The geopolymer concrete with recycled
to look for alternative materials (McKelvey et al., 2002; Schneider aggregate showed lower compressive strengths (2.9e10.3 MPa)
et al., 2011). Fly ash has already been used as supplementary than those containing natural aggregate, however, it was within the
cementitious material in concrete industry for over 50 years in the typical strength distribution reported (American Concrete Institute
world (Uysal and Akyuncu, 2012). In the hydration of traditional (ACI) committee 522, 2010). In addition, the total void ratio of the
cement, fly ash with a high content of Al2O3 and SiO2 can be acti- geopolymer concrete with recycled aggregate (21.7e26.9%) was
vated by Ca(OH)2 and thus produce more CeSeH gel, CeAeH gel similar to those with natural aggregate (24.2e27.4%), and the water
and CeAeSeH gel to fill the capillary of concrete effectively, permeability was 0.71e1.47 cm/s versus 1.18e1.71 cm/s. Nuaklong
et al. (2016) used recycled aggregates from old concrete with gels, including NeAeSeH gel, CeAeSeH gel, CeAeH gel and
compressive strength of 30e40 MPa and high calcium fly ash to CeSeH gel. The gels influence the final structure of geopolymer and
make fly ash-based geopolymer. They found fly ash-based geo- control the ionic transport. Alkali solutions influence the hydro-
polymer with recycled aggregate showed compressive strengths of lyzation of fly ash and the porosity of geopolymer structure. The
30.6e38.4 MPa, which were slightly lower than those of fly ash- porosity influences the migration of alkali from fly ash-based
based geopolymer concretes with crushed limestone. geopolymer into the ion solutions, the moisture and then has an
Different superplasticizers (naphthalene (N) and poly- effect on the mechanical strength and durability. Fly ash-based
carboxylates (PC)) as the water reduction admixture have been geopolymer with compact and denser structure shows high me-
used to improve the workability of the geopolymer concrete by chanical strength and good resistance to chloride, sulfate and acid
Nematollahi and Sanjayan (2014). PC superplasticizer with a dosage solutions and good efflorescence.
of 3.3 wt.% caused significant reduction (54%) in strength with Fly ash-based geopolymer can be used as cement to mix with
reference to concrete without superplasticizer. By contrast, N aggregates to form concrete. In this context, considering the low
superplasticizer with a dosage of 1.19% caused a 22% reduction in cost, low CO2 emission and low energy usage in the production of
strength. The use of superplasticizers was not beneficial to geo- fly ash-based geopolymer, fly ash-based geopolymer cement and
polymer concrete used at elevated temperatures. concrete are regarded as possible alternative green materials to
Injecting and storing CO2 into underground has been proposed OPC. Fly ash-based geopolymer has also been used to adsorb and
as one of the long-term strategies for handling greenhouse gases immobilize toxic metals and it shows better adsorption and
(Laudet et al., 2011; Bachu and Bennion, 2009). OPC-based sealant immobilization performance than those of OPC. Besides, fly ash-
has been used in injection wells and it has been found experi- based geopolymer cement has been used as a well sealant to
encing cement degradation under CO2-rich down-hole conditions. store CO2 in the underground. However, the CO2 permeability
The low permeability of fly ash-based geopolymer can effectively value of fly ash-based geopolymer is still lower than the
prevent CO2 leakage. Nasvi et al. (2013) found that the apparent permeability value for a typical well sealant recommended by the
CO2 permeability of fly ash-based geopolymer cement is in the API.
range of 2 1021e6 1020 m2, which is lower than the typical Though last decades or so have witness remarkable advance in
oil well cements (1020e1011 m2). Moreover, Nasvi et al. (2014) the science and technology of fly ash-based geopolymer as dis-
prepared fly ash-based geopolymer cement from fly ash acti- cussed above, there exists several issues and some issues are pro-
vated by a mixed Na2SiO3/NaOH solution and tested the CO2 posed below for future work.
permeability under three conditions: (1) temperatures of (1) Inherently, to control the production and to improve the
23e70 C; (2) CO2 injection pressures of 6e17 MPa; and (3) performances of fly ash-based geopolymer, the reaction mecha-
confining pressures of 12e20 MPa. The results showed that the nisms in each step should be uncovered in details. To this end,
apparent CO2 permeability of geopolymer increased with curing involved are many aspects such as thermodynamics, kinetics,
temperature and the increasing rates were as high as 200e1000%. identifications of intermediates and insights into their structures,
The maximum permeability (0.04 lD) value obtained was and the degrees to which the eSieOeAle are oligomerized and
approximately 5000 times lower than the permeability value (200 polymerized. All these will become more sophisticated once the
lD) recommended by the American petroleum industry (API) for a additional elements or additives are included. But further studies
typical well sealant. The findings indicated that there is a great must be made in order to ensure that the correlation of production-
potential in using fly ash-based geopolymer cement for carbon structure-performance is clear-out.
storage. (2) Most of fly ash-based geopolymer concretes are brittle and
sensitive to cracking. Such behavior not only imposes constraints in
6. Conclusions and future work applications, but also affects the long-term durability of geo-
polymer concretes (Zhao et al., 2007; Zhang et al., 2010b; Pernica
Geopolymer technology offers a facile approach for fly ash uti- et al., 2010). Therefore, innovation in the preparation to create
lization. In principle, the geopolymerization includes the dissolu- improved fly ash-based geopolymer composites is still needed.
tion of alumina, silica, aluminosilicate in the fly ash feedstock by (3) At present, in most cases, fly ash-based geopolymer are
alkali, the recombination of Al3þ and Si4þ species and the genera- merely produced at laboratory scale with empirical formulations. It
tion of three-dimensional amorphous aluminosilicate polymers. could be exciting to see a report by Wagners (Wagners Earth
The preparation and formation, and properties of the fly ash-based Friendly Concrete product, 2016) that the production of fly ash-
geopolymer products depend heavily upon the chemical and based geopolymer concrete is being realized on a large-scale. To
physical characteristics of fly ash, alkali activators, curing condi- put producing and using fly ash-based geopolymer on a large-scale
tions and the addition of slag, fiber, RHBA and red mud. are encouraging and need further input and endeavor.
NaOH and Na2SiO3 are used as alkali activators; CaCl2 is used as (4) As for the uses of fly ash-based geopolymer for toxic metals
chemical additives to form CeAeSeH gel, CeAeH gel and CeSeH adsorption and immobilization and sealing CO2, the performances
gel and change the setting time; and slag, fiber, RHBA and red mud are still unsatisfactory. Changing of the recipes for preparation is
are used as additives to change the Si/Al ratios and introduce the worth for further investigation.
hydrogen bonds between geopolymer gel (NeAeSeH gel) and the (5) Instead of using fly ash-based geopolymer as alternative
additive molecules. cement, it is also possible to endow fly ash-based geopolymer with
For the practical applications of fly ash-based geopolymer, me- more functionalities or unique properties. Consequently, new ap-
chanical properties (such as compressive strength, splitting tensile plications of fly ash-based geopolymer are worth exploring and can
strength and flexural strength) and durability (chloride, sulfate, be found. For instance, fly ash-based geopolymer with biomass can
acid, thermal, freeze-thaw cycles and efflorescence resistances) be developed as a class of novel lightweight fireproof materials.
should be comprehensively considered. The Si/Al ratios of re-
actants, alkali solutions, curing conditions, and the addition of slag, Acknowledgment
fibers, RHBA, red mud and calcium can be fine tuned to improve
those properties. The changes of Si/Al ratios, alkali solutions, and The authors wish to acknowledge the financial support from the
adding slag, RHBA, red mud and calcium can lead to the different National Natural Scientific Foundation of China (21373185;
21506188), the Distinguished Young Scholar Grants from the Nat- Bakharev, T., 2006. Thermal behaviour of geopolymers prepared using class F fly ash
and elevated temperature curing. Cem. Concr. Res. 36 (6), 1134e1147.
ural Scientific Foundation of Zhejiang Province (ZJNSF, R4100436),
Bankowski, P., Zou, L., Hodges, R., 2004. Reduction of metal leaching in brown coal
ZJNSF (LQ12B03004), Zhejiang “151 Talents Project”, the projects fly ash using geopolymers. J. Hazard. Mater. B114, 59e67.
(2010C14013 and 2009R50020-12) from Science and Technology Bas
carevi c, Z., Komljenovi c, M., Miladinovi c, Z., Nikoli c, V., Marjanovi c, N.,
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and structure of fly ash based geopolymers. Mater. Struct. 48, 683e697.
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Ministry of Land and Resources of The People's Republic of China investigations and fuzzy logic modeling of compressive strength of geo-
(2014-K02), and Engineering Research Center of Non-metallic polymers with seeded fly ash and rice husk bark ash. Compos. Part B Eng. 43,
1293e1301.
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Geology and Mineral Resource, China, and the State Key Laboratory polymerization of bottom ash by incorporating fly ash and using waste gypsum
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Abbreviations and nomenlatures 4185e4191.
Chen, C., Gong, W., Lutze, W., Pegg, I.L., Zhai, J., 2011. Kinetics of fly ash leaching in
ACI American Concrete Institute strongly alkaline solutions. J. Mater. Sci. 46, 590e597.
Chen, R., Ahmari, S., Zhang, L.Y., 2014. Utilization of sweet sorghum fiber to rein-
API American Petroleum Industry force fly ash-based geopolymer. J. Mater. Sci. 49, 2548e2558.
ASR Alkali silica reaction Chindaprasirt, P., Chalee, W., 2014. Effect of sodium hydroxide concentration on
ASTM American Society for Testing and Materials chloride penetration and steel corrosion of fly ash-based geopolymer concrete
under marine site. Constr. Build. Mater. 63, 303e310.
BFS Blast furnace slag
Chindaprasirt, P., Rattanasak, U., Jaturapitakkul, C., 2011. Utilization of fly ash blends
CeAeH gel Calcium aluminate hydrate gel from pulverized coal and fluidized bed combustion in geopolymeric materials.
CeAeSeH gel Calcium aluminum silicate gel Cem. Concr. Compos. 33, 55e60.
CCS Carbon capture and storage Chindaprasirt, P., Rattanasak, U., Taebuanhuad, S., 2013a. Resistance to acid and
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CFA Class C fly ash Struct. 46, 375e381.
CeSeH gel Calcium silicate hydrate gel Chindaprasirt, P., Rattanasak, U., Taebuanhuad, S., 2013b. Role of microwave
CFLC Cumulative fraction leaching concentration radiation in curing the fly ash geopolymer. Adv. Powder Technol. 24,
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FFA Class F fly ash Chindaprasirt, P., Paisitsrisawat, P., Rattanasak, U., 2014. Strength and resistance to
GGBF Ground granulated blast furnace slag sulfate and sulfuric acid of ground fluidized bed combustion fly ash-silica fume
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ISO International Standards Organization Fuel 84, 2048e2054.
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OPC ordinary Portland cement Davidovits, J., 1994. Global warming impact on cement and aggregates industries.
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Fly Ash-Based Geopolymer Clean Production, Properties and Applications

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Journal of Cleaner Production 125 (2016) 253e267

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Journal of Cleaner Production

Fly ash-based geopolymer: clean production, properties and

* Corresponding author. Research Group for Advanced Materials & Sustainable

3.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

1. Introduction roughly comparable to hydrated cement in appearance, reactivity

0 0.18% Oil field services

Material/alkaline activators feedstock Mixing T Curing time/ Compressive strength Refs

3.3.1. Chloride resistance 3.3.3. Acid resistance

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