Accepted Manuscript

Fly Ash-based Geopolymer: Clean Production, Properties and Applications

Xiao Yu Zhuang, Liang Chen, Sridhar Komarneni, Chun Hui Zhou, Dong Shen Tong,Hui Min Yang, Wei Hua Yu, Hao Wang

PII: S0959-6526(16)30079-8

DOI: 10.1016/j.jclepro.2016.03.019

Reference: JCLP 6868

To appear in: Journal of Cleaner Production

Received Date: 23 October 2015

Revised Date: 2 March 2016

Accepted Date: 2 March 2016

Please cite this article as: Zhuang XY, Chen L, Komarneni S, Zhou CH, Tong DS, Yang HM, Yu WH,Wang H, Fly Ash-based Geopolymer: Clean Production, Properties and Applications, Journal of CleanerProduction (2016), doi: 10.1016/j.jclepro.2016.03.019.

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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 Wangc

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 Center,

Hangzhou, Zhejiang, 310012

* C.H.ZHOU E-mail: clay@zjut.edu.cn Chun.Zhou@usq.edu.au

ABSTRACT

Fly ash is the fine solid particulate residue driven out of the boiler with the flue gases in coal-fired

power plants. Now it can be used for making geopolymer which acts as a cement-like product. The

geopolymer technology provides an alternative good solution to the utilization of fly ash with little

negative impact on environment. This review summarizes and examines the scientific advances in the

preparation, 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 cement. The geopolymerization can trap and fix the trace toxic metal elements from fly

ash or external sources. The Si/Al ratios, the type and the amount of the alkali solution, the temperature,

the curing conditions, and the additives are critical factors in a geopolymerization process. The

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mechanical performances of the fly ash-based geopolymer, including compressive strength, flexural

and splitting tensile strength, and durability such as the resistance to chloride, sulfuric, acid, thermal,

freeze-thaw and 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 geopolymer, further enhancing mechanical performance, scaling up production

and exploring new applications are suggested.

Key words: Fly ash; Alkali activation; Geopolymer; Waste utilization; Cement, Concrete

1. Introduction

Fly ash is one of the solid residues composed of the fine particles that are driven out of

the boiler with flue gases in coal-fired power plants. It is generally captured from flue gases

by electrostatic precipitators or other particle filtration equipment before the flue gases reach the

chimneys (Ahmaruzzaman, 2010; Gorai et al., 2006). Depending upon the source of the coal being

burned, the components of fly ash vary considerably. In general, the components of fly ash typically

include SiO2, Al2O3, CaO and Fe2O3, which exists in the form of amorphous and crystalline oxides or

various minerals. According to the American Society for Testing and Materials standard C 618 (ASTM

C618-12a; 2012), fly ash can be classified as Class C and Class F types based on their calcium oxide

contents. Class C fly ash has a high calcium content, and is mainly generated from the burning of

lignite coal sources. Class C fly ash has a total SiO2, Al2O3 and Fe2O3 content between 50 wt.% and 70

wt. % and CaO content more than 20 wt.%. Class F fly ash has a low calcium content, and is generated

from burning anthracite or bituminous coal. Class F fly ash has a total SiO2, Al2O3 and Fe2O3 content

over 70 wt% and CaO content less than 10% (Bankowski et al., 2004, Antiohos and Tsimas, 2007). In

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addition to Si, Al, Fe, and Ca, usually fly ash also contains many other trace metal elements, such as Ti,

V, Cr, Mn, Co, As, Sr, Mo, Pb and Hg. The concentrations of the toxic trace elements in fly ash could

be 4-10 times higher than those in coal (Neupane and Donahoe, 2013; Nyale et al., 2014; Yao et al.,

2015). It may also include small concentrations of dioxins and polycyclic aromatic

hydrocarbon compounds (Shibayama et al., 2005; Nomura et al., 2010). Thus, fly ash is considered as a

hazardous material, and the improper disposal of fly ash will not only increase the occupation of land

but also deteriorate the environment and ecology. In last few decades, increasing efforts have been

made towards the utilization of fly ash, especially in an efficient and green fashion.

Fig.1 a The generation of fly ash from 2008-2013 in the USA. b The utilization fields of fly ash in

2013 in the USA (Data from American Coal Ash Association).

Due to the cost and availability of oil and natural gas, coal-fired power plants will be still run for a

long period, especially in the coal-rich countries, for example, China, the USA, India and Australia

(Lior, 2010). Under such a circumstance, the generation of fly ash remain significant and thus its

economic and green utilization technology of fly ash are desired (Fig.1a). Fly ash can be used for soil

amendment (Ukwattage et al., 2013) and nutrients (Kováčik et al., 2011). It can also be used to make a

low-cost adsorbent for waste removal (Rubel et al., 2005; Yildiz, 2004). Besides, it can act as silica and

alumina sources for zeolite production (Chang and Shih, 2000; Izidoro et al., 2012) (Fig.1b). More

recently, fly ash has been used as an alternative source to make geopolymer, a new binder or cement

roughly comparable to hydrated cement in appearance, reactivity and properties. In principle,

geopolymer is a product of alkali activation of any aluminosilicate materials. It has a three-dimensional

0

1,00,00,000

2,00,00,000

3,00,00,000

4,00,00,000

5,00,00,000

6,00,00,000

7,00,00,000

8,00,00,000

2008 2009 2010 2011 2012 2013

Fly

ash

gen

era

tion

(s

hot

ton

)

Year

Unreutilized Reutilizeda

52.99%9.80%

0.18%

12.89%

0.58%

1.15%

0.01%

7.90%

8.72%

0.07%

0.02%

1.34%4.34%

The usage of fly ash in 2013 Concrete/Concreteproducts/GroutBlended cement/Feed forclinkerFlowable fills

Structural fills/embankments

Road base/sub-base

Soil modification/stabilization

Blastig crit/roofing granules

Mining application

Wastestabilization/solidificationAgriculture

Aggregate

Oil field services

Miscellaneous/other

b

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aluminosilicate network structure with an empirical formula of Mn [-(SiO2) z-AlO2] n·w H2O, where z is

the Si/Al molar ratio, M is an alkali cation, such as Na+ or K+, n is the polymerization degree, and w is

the water content (Palomo et al., 1999a). Importantly, such a geopolymer has the similar bind

performances to those of ordinary Portland cement (OPC). The alkali activation is conducted by adding

NaOH, KOH, Na2SiO3 or K2SiO3 into fly ash together or individually. The so-called geopolymerization

can occur at room temperature or slightly elevated temperatures (usually<100°C), and much

importantly, with little CO2 emission (Davidovits, 1991; Verdolotti et al., 2008; Zhang et al., 2016).

The geopolymer technology provides a new good and green solution to the utilization of fly ash,

avoiding its negative impact on environment and ecology. The alumina and silica in fly ash can be

activated with alkali to form geopolymer. Moreover, the toxic trace metal elements can be trapped and

fixed in the geopolymer structure (Li et al., 2013a). Remarkably, the process of the production of fly

ash-based geopolymer has lower CO2 emission compared with that of OPC during which much

limestone (CaCO3) is calcined and decomposed at high temperatures (Davidovits, 1994; McLellan et

al., 2011). Approximately 0.8 ton of CO2 is produced when one ton of OPC is produced (Rashad and

Zeedan, 2011; Yang, 2009). Fly ash-based geopolymer usually show mechanical strength and durability

nearly comparable to hydrated Portland cement and can used as a class of green cement with natural

resource efficiency (Nasvi and Gamage, 2012).

In particular, recent years have witnessed many scientific advances in the preparation technology,

and the insights into the performances of the fly ash-based geopolymer. The fast-growing knowledge in

turn results in many modification methods to significantly improve the production and the

performances of the fly ash-based geopolymer. Meanwhile, there are increasing pilot and commercial

process. The large-scale use of fly ash-based geopolymer in construction industry seems to come true.

In this review article, the state-of-the-art preparation of fly ash-based geopolymer is examined.

Attention will therein be paid to understanding how the Si/Al ratios, the type and the amount of the

alkali solution, the temperature, the curing conditions, and the additives are controllably used to

reinforce properties of fly ash–based geopolymer. Discussed are the critical properties, including

compressive, flexural and splitting tensile strength, and the durability such as chloride, sulfate, acid,

thermal, freeze-thaw and efflorescence resistance. Then, the applications of fly ash–based geopolymer

are surveyed. They involve cement-based construction, adsorption and immobilization of toxic metals

materials. The existing challenges and future work are finally analyzed and proposed.

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2. Preparation and formation

The basic and simplified principle of the formation of fly ash-based geopolymer is the

alkali-facilitated decomposition of aluminoslicate in the fly ash and then polycondensation. The

reactions can proceed under mild temperatures so the production is considered to be energy and source

efficient, namely much cleaner. However, the real reactions occurred in the process are very

complicated and remain elusive. Apparently, there are reactions between fly ash and alkali and

condensation between the resultant Si4+ and Al3+ species, followed by other complicated nucleation,

oligomerization, and polymerization, which finally lead to a new aluminoslicate-based polymer with

new amorphous three-dimensional network structure. In tests or uses, the as-prepared fly ash-based

geopolymer paste is further cast into a mould and placed into an oven at a required temperature or left

at room temperature to be cured for a specific time to form construction (Fig. 2.).

Fig.2. The schematic drawings showing the process from fly ash to fly ash-based geopolymer

cement/concrete.

The critical role in the formation geopolymerization are thought be played by alkali activation on

fly ash: in an alkaline solution (Na2SiO3, NaOH, KOH or K2SiO3), the silica, the alumina, or the

aluminosilicates in fly ash hydrolyze, -Si-O-Si- or -Si-O-Al- bonds of aluminosilicate break and release

active Al3+ and Si4+ species. The active Al3+ and Si4+ species react to form nuclei and aluminosilicate

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oligomers consists of SiO4 and AlO4 tetrahedra. The chains in aluminosilicate oligomers can be in the

form of polysialate -Al-O-Si- chain, polysialate siloxo -Al-O-Si-Si- chain, and polysialate disiloxo

-Al-O-Si-Si-Si- chain, depending upon the Si/Al ratio (Fig. 3). In aluminosilicate monomers, Si4+ is

partially substituted by Al3+, and the resultant negative charge in the aluminosilicate chains is balanced

by alkali cations such as Na+ or K+ (Davidovits, 2002; Dimas et al., 2009) (Fig. 2). In the context, the

Si/Al ratio significantly determines the final structure of the resulted geopolymer materials (He et al.,

2012). For example, it have been found that the Si/Al ratio in the fly ash reactant has a remarkable

effect on the porosity (size and amount) of amorphous geopolymer which is one of important

parameters to govern the mechanical strength of geopolymer products (Kriven et al., 2003; Duxson et

al., 2005b).

Fig.3. Dependent upon the different Si/Al molar ratio, different aluminosilicate chains is formed in the

aluminosilicate oligomers which then further to form geopolymer (Davidovits, 2002) (Reprinted and

adapted by courtesy of Geopolymer Institute).

In addition to the Si/Al ratio, the microstructure of the formed fly ash-based geopolymer is

strongly affected by the alkaline solution. When fly ash contacts with alkaline (e.g. NaOH, KOH), Si4+,

Al 3+ and other ions start to be released and to transfer. For instance, the amount of released Si4+ and

Al 3+ is influenced by the concentration of NaOH solution. NaOH solution of high concentration (10

mol/L) is beneficial for decomposing aluminosilicate in the fly ash and then release Si4+ and Al3+

(Table 1). For example, the solubility of Al3+ and Si4+ in NaOH solution is higher than that in KOH

solution with the same concentration (Xu and Van Deventer, 1999). Moreover, the transfer of Al3+ and

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Si4+ species and the polycondensation of aluminosilicate oligomers can also be accelerated by an

alkaline solution of high concentration (Lizcano et al., 2011). Furthermore, different alkaline cations

have different sizes and charge density, and they hydrate differently. These will then have a certain

effect on the nucleation of aluminosilicate chain, the growth of the chain, the charge density on the

chain, the rate, and the extent of polymerization (Duxson et al., 2005a). For instance, the K+ cation

(1.33 Å) is larger than the Na+ cation (0.97 Å) and the K+ cation lead to a lower surface charge density

and a higher degree of polymerization of geopolymer matrix (van Jaarsveld and van Deventer, 1999).

In addition, alkaline cations can even serve as a structure-directing agent in the geopolymerization. In

brief, the leaching rate of Si4+ and Al3+ decide the real available Si/Al ratio in a series of reactions to

form geopolymer and subsequently play a pivotal role in the structure of fly ash-based geopolymer.

Interestingly, a recent study revealed that the addition of Na2SiO3 to alkali solution can increase such a

Si/Al ratio, resulting in a lower porosity and a finer pore system of geopolymer matrix (Ma et al.,

2013).

The setting time of fly ash-based geopolymer is usually considered for the workability of final fly

ash-based geopolymer products. In general, the final setting can be achieved within 1–2 h at room

temperature while Class C fly ash with high CaO content and calcium-content additives, such as CaCl2,

were found to shorten the setting time of fly ash-based geopolymer paste (Rattanasak et al., 2011). In

geopolymerization, Si4+ or Al3+ species react with Ca2+, either in the fly ash or from an external

calcium-content additives, to form calcium silicate hydrate gel (C-S-H), calcium aluminate hydrate gel

(C-A-H) or calcium aluminum silicate gel (C-A-S-H) (C = CaO, S = SiO2, A = Al2O3, H = H2O) in the

presence of water (Diaz et al., 2010; Chindaprasirt et al., 2011). Ca2+ is beneficial for accelerating the

nuclei formation and agglomeration of C-A-S-H gel and C-S-H gel (Geetha and Ramamurthy, 2013).

The rapid formation of amorphous C-A-S-H gel and C-S-H gel leads to a shorter setting time of the

final products and decreases the porosity, while the rapid setting time has a negative on the formation

of more geopolymer gel (N-A-S-H). The high NaOH concentration can prolong the setting time by

limiting the leaching of calcium and allows normal geopolymerization process to control the setting of

geopolymer paste (Hanjitsuwan et al., 2014).

At room temperature, however, the dissolution of fly ash is not completed (Chen et al., 2011; Xu

et al., 2010). In addition, the low reactivity of fly ash increases the setting time of fly ash-based

geopolymer. As a result, curing is a necessary step, namely, geopolymer paste needs to be kept within a

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reasonable range of temperature and moisture and the extended curing time promotes the formation of

a more cross-linked binding and denser microstructure. It is found that when the curing temperature

increases from 30 to 50°C, the reactivity of fly ash becomes higher and the geopolymerization is almost

more complete when the curing temperature lies between 60 and 90°C (Hardjito et al., 2004).

To improve the reactivity of fly ash and the performances of geopolymer, slag, chitosan, fiber, rice

husk-bark ash (RHBA) and red mud have been added into fly ash to synthesize geopolymer (Table 1).

Interestingly, the addition of slag, which is the waste from iron extraction process from raw ore, can

enhance the reactivity of fly ash during geopolymerization (Li and Liu, 2007). Blast furnace slag (BFS)

is the by-product of iron production industry and it contains more than 70% of SiO2 and CaO (Manz,

1999). Idawati et al. (2014) investigated the fly ash/ground blast furnace slag (GBFS)-based

geopolymer with different fly ash/slag ratios and they found the geopolymerization of slag-based

geopolymer was dominated by C–A–S–H type gel, while fly ash-based geopolymer was dominated by

sodium aluminosilicate (N–A–S–H) gel. Kumar et al. (2010) substituted fly ash by 5 - 50% GBFS to

synthesize geopolymer at 27°C and found that the reaction was dominated by dissolution and

precipitation of C-S-H gel. Moreover, Yang et al. (2012) found that the initial setting time of fly

ash/GBFS-based geopolymer increased and the degree of polymerization of the geopolymer decreased

due to the high content of calcium in GBFS. RHBA is a solid waste generated from rice husk and

eucalyptus bark by biomass power plants. RHBA contains about 75% SiO2 and the addition of RHBA

enriched the silica content of the geopolymer matrix and increased the amount of -Si-O-Si- bonds in the

geopolymer gel (N-A-S-H gel). Red mud is the major residue of Bayer process with highly alkaline of

alumina refining from bauxite ores (Zhang et al., 2010a). The annual generation of red mud is

estimated to be about 70 million tons in the world (Klauber et al., 2011). Due to the use of a highly

concentrated NaOH solution in the bauxite processing, red mud is a highly alkaline material. Red mud

essentially consists of oxides and hydroxides of Fe, Al and Si, as well as minor quantities of CaO and

TiO2. Addition of red mud can adjust the Si/Al ratio and reduce the consumption of alkali activator

(Piga and Stoppa, 1993). The addition of chitosan and fiber can increase the hydrogen bonds, bridge the

micro cracks and delay the development of micro cracks (Li et al., 2013b).

Table 1. Preparation and compressive strength of fly ash-based geopolymer with additives

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Material/Alkaline activators

Feedstock

Mix

T

(°C)

Curing time

/Temperature (°C)

Compressive

strength (MPa)

Refs

Fly ash

Na2SiO3 +NaOH (10 mol)

CaCl2, CaSO4, Na2SO4,

sucrose

48 h/

65

26.9-32.2 Rattanasak et al.,

2011

Fly ash

NaOH (4.5, 7.0, 9.5, 12.0, 14.0,

16.5 mol/L)

-

/25-28

<25.5 Somna et al., 2011

Fly ash+ RHBA/

Na2SiO3 +NaOH

(4,8,12 mol/L)

2/5

25 36 h-28 days/

25-90

14.8-55.6 Bohlooli et al.,

2012

Fly ash+RHBA/

Na2SiO3 or

water glass+NaOH

(5-12 mol/L)

2/5

25 2-7day/

25-90

12.6-35.1 Riahi et al.,

2012

Fly ash+

wastepaper sludge/

Na2SiO3+NaOH

1/5

91 days

23-60

31.2-60.6 Yan et al.,

2012

Pulverized coal combustion Fly

ash+

PCC Bottom ash+

flue gas desulfurization gypsum/

Na2SiO3+NaOH

n.a. 48 h

40

25.5-55.5 Boonserm et al.,

2012

Fly ash+

Crushed granite rock+

natural river sand/

Na2SiO3+NaOH

AT 6-72 h

60-120

42.0-58.0 Joseph and

Mathew, 2012

FFA+ N-carboxymethyl chitosan

NaOH (10 mol/L)

AT 6 days

60

<30.0 Li et al., 2013b

Fly ash + RHBA/

water glass+NaOH

7,28days/ RT

36h/

40–90

<58.9 Nazari et al.,

2013

FFA+crushed granite stone+

Superplasticizer/

Na2SiO3+NaOH

5/2

48 h;

1,3,7 days/

70

40.85-53.08 Demie et al., 2013

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FFA+BFS

K2SiO3/Al (85 g/L) + NaOH

(30g/L)

7 days

RT

Ogundiran et al.,

2013

FFA/NaOH

3/5

25 7 day /

28 day/

60

1.38-9.93 Jun and Oh

2014

Fly ash+Palm oil fuel ash/

Na2SiO3+NaOH

24h

65

<38 Ranjbar et al.,

2014

GGBF+Palm oil fuel ash+

Fly ash+

Manufactured-sand/

Na2SiO3+NaOH

24h

65

9-66 Islam et al.,

2014

FFA+Red mud

NaOH (50wt.%) + sodium

trisilicate (2 mol/L)

28 days

AT

11.3-21.3 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

Using waste materials, fly ash and additives like slag, RHBA, red mud and fibers, to produce a

construction material geopolymer can help reduce the consumption of mineral reserves such as

limestone and the emission of greenhouse gases. Without proper utilization, fly ash is a solid waste.

Improper disposal of fly ash brings the danger of the release of toxic elements. By contrast, fly

ash-based geopolymer can solidify and immobilize the trace of heavy metal elements. Furthermore, the

simple alkaline activation of fly ash to produce geopolymer completely bypasses the high-temperature

calcination process in OPC production. All these would justify the thought that the production of fly

ash-based polymer is a cleaner process with improved natural resource efficiency.

3. Properties

3.1 Compressive strength

The improvement of mechanical properties of fly ash-based geopolymer is major concerns

because its main uses are in construction materials as cement and concrete. The compressive strength

of fly ash-based geopolymer is dependent on alkali solutions, Si/Al ratios, calcium content, curing

conditions (temperature and time) and the various additives.

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The type and the concentration of the alkaline solution influence the release of Si4+ and Al3+ from

fly ash during geopolymerization. Alkaline solution of a high concentration is generally beneficial for

obtaining high compressive strength but there is an optimal range (de Vargas et al., 2011). Görhan and

Kürklü (2014) prepared fly ash-based geopolymer with different NaOH concentrations (3 mol/L, 6

mol/L and 9 mol/L). A highest compressive strength of 22 MPa was achieved when the fly ash-based

geopolymer paste was activated in 6 mol/L NaOH and cured at 85°C for 24 h.

Na2SiO3 solution is usually used with NaOH to increase compressive strength (Criado et al., 2005).

This is because of Na2SiO3 with high viscosity can help the formation of geopolymer gels and a

compact final fly ash-based geopolymer microstructure is achieved. Moreover, the activation procedure

also influences the compressive strength of fly ash-based geopolymer. For example, Rattanasak and

Chindaprasirt (2009) first added NaOH solution to fly ash to leach the Si4+ and Al3+ species for 10 min,

followed by using Na2SiO3 to help form a uniform geopolymer paste for another 1 min. Such separate

activation gave a higher strength for the fly ash-based geopolymer. When fly ash was separately mixed

and activated with NaOH (10 mol/L) and Na2SiO3 with the NaOH/Na2SiO3 molar ratio of 1.0 and cured

at 65°C for 48 h, the compressive strength of fly ash-based geopolymer was 60-70.0 MPa.

As discussed in previous section, the Si/Al ratios are determined by the source materials and alkali

solution (Na2SiO3 is used). High Si/Al ratios increase the amount of -Si-O-Si- bonds to get a higher

compressive strength of fully condensed structural matrix of geopolymer, since the -Si-O-Si- bonds are

stronger than -Si-O-Al- and -Al-O-Al- bonds. The addition of slag, RHBA, and red mud can alter the

Si/Al ratios (Table 1). For example, Yang et al. (2014a) prepared geopolymer fly ash and high

magnesium nickel slag (HMNS) activated by Na2SiO3 solution. It was discovered that the major phase

in fly ash/HMNS geopolymer was a type of sodium magnesium aluminosilicate gel. The addition of

HMNS increased the silica content. In addition, HMNS particles worked as micoaggregates and

decreased the total volume of pores in the geopolymer pastes. As a result, the geopolymer possessed a

high compressive strength of above 60 MPa when 20% HMNS was used for the preparation. Though

the higher content of slag increased the compressive strength of fly ash/HMNS-based geopolymer, it

caused rapid setting and crack due to autogenous shrinkage of slag (slag/binder>70%) (Jang et al.,

2014). Wang et al. (2015) prepared fly ash/slag-based geopolymer with different fly ash/slag ratios (0

wt.%, 20 wt.%, 40 wt.%, and 60 wt.%) and various NaOH solutions (0.5%, 1% and 1.5%), and then

cured for 1, 3, 7 and 28 days. The increasing portion of slag increased the compressive strength of

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geopolymer and the optimal compressive strength was 93.06 MPa. Deb et al. (2014) activated Class F

fly ash/GBFS-based geopolymer (GBFS/Class F fly ash ratio=0%, 10% and 20%) in NaOH and

Na2SiO3 solution, respectively. High compressive strength increased with the increase of the

GBFS/Class F fly ash ratio. When the geopolymer concrete synthesized by 20% slag and 80% fly ash

with 40% NaOH and Na2SiO3 solution and cured at 20°C, it had the highest compressive strength of 51

MPa. Xu et al. (2014) used GBFS of grade 80, 100 and 120 and fly ash to synthesize geopolymer with

an activating solution prepared from concentrated Hanford secondary waste (HSW) stimulant (5 mol/L

NaOH mixed with solid binders). The highest compressive strength was 52.5 MPa when the fly

ash/GBFS mass ratio was 5/3.

The addition of RHBA enriched the silica content of the geopolymer matrix, increased the amount

of -Si-O-Si- bonds in the geopolymer gel (N-A-S-H gel) and the compressive strength (Sata et al.,

2007; Tangchirapat et al., 2008). Nazari et al. (2011) added RHBA into fly ash to synthesize

geopolymer paste and cured it in oven at 80°C for 28 days. They found the geopolymer had the

compressive strength of about 60 MPa. Songpiriyakij et al. (2010) prepared fly ash-based geopolymer

using the reactants of fly ash, RHBA and NaOH and curing at 27°C first for 24h and then curing at

60°C for 24h. It was found that the optimal SiO2/Al 2O3 ratio to obtain the highest compressive strength

(73 MPa) was 15.9. Red mud of highly alkalinity is the major residue of Bayer process of alumina

refining from bauxite ores. Zhang et al. (2014a) prepared geopolymer using fly ash and red mud at

23°C. They found compressive strengths ranging from 11.3 to 21.3 MPa and the highest compressive

strength was obtained when the Si/Al molar ratio was 2.

Calcium is proved to interfere with the gelation of silica and alumina in geopolymerization

process and alter the microstructures of fly ash-based geopolymer and thus alter the compressive

strength (Schmucker and Mackenzie, 2005). The coexistence of C-S-H gel and the N-A-S-H gel

usually improves the compressive strength of final products. One of reasons is that the amorphous

C-S-H gel decreases the porosity (Temuujin and van Riessen, 2009).

Curing time and temperature also affect the compressive strength of fly ash-based geopolymer.

Longer curing time, in the range of 6 hours to 28 days, produced fly ash-based geopolymer with a

higher compressive strength (Table 1). Curing at high temperatures increases the compressive strength

by removing the water from the fresh geopolymer, causing the collapse of the capillary pores with a

denser structure (Leung and Pheerapha, 1995). Fly ash-based geopolymer can be cured at room

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temperature, but the compressive strength develops slowly and always needs prolonged curing time

(Somna et al., 2011). Nasvi et al. (2012; 2014) found that the crack closure and crack initiation

thresholds of fly ash-based geopolymer cured at elevated temperatures (60-80°C) was higher (30-60%

peak stress) compared to those (15-30% of peak stress) cured at ambient temperature (23°C and 40°C).

However, prolonged curing at higher temperatures breaks down the granular structure of geopolymer,

resulting in the dehydration and the excessive shrinkage, and finally decreasing the compressive

strength (Palomo et al., 1999b).

Steam curing is the conventional heating technique, relying on the conduction of heat from the

exterior to the interior of fly ash-based geopolymer paste by steam. The heating is non-uniform and

required a long heating period to attain the required temperature. Microwave heating is based on

internal energy dissipation associated with the excitation of molecular dipoles in electromagnetic fields,

and it delivers faster and more uniform heating (Kim et al., 2015). Chindaprasirt et al., (2013b)

prepared fly ash-based geopolymer pastes with 10 mol/L NaOH and Na2SiO3 solution and cured them

under the 90-W microwave radiation for 5 min followed by additional heating at 65°C for 6 h. The

compressive strength was comparable to that of the fly ash-based geopolymer cured at 65°C for 24 h.

The microwave radiation quickened the dissolution of fly ash in the alkaline solution and formed a

denser microstructure. Accordingly, the microwave radiation shortened the required curing time and

enhanced the geopolymerization.

To put together, increasing the Si/Al ratios usually enhance the compressive strength of fly

ash-based geopolymer. Though the increase of the Si/Al ratios can be realized by adding the external

slag, RHBA and red mud, the inherent reasons are complicated. Advances in the preparation indicated

that one of major reasons can be ascribed to the increased amount of -Si-O-Si- bonds rather than

-Si-O-Al- and -Al-O-Al- bonds. To this end, the use of Na2SiO3 or K2SiO3 with NaOH for the

activation of fly ash can increase the Si/Al ratios, thereby leading to a more compact structure with

higher compressive strength (Fig. 3.). In addition, the presence of calcium in fly ash or used as additive

is beneficial for forming the amorphous C-S-H gel and C-A-S-H gel and decreases the porosity and

obtain geopolymer with a higher compressive strength. Curing had a significant effect on the

compressive strength of fly ash-based geopolymer by changing the porosity and density of the product.

3.2 Flexural and splitting tensile strength

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Fly ash-based geopolymer would suffer from brittle failure with low tensile strength and fracture

toughness. A typical method is to incorporate chitosan or fibers into the fly ash-based geopolymer

matrix because this hybridization can improve the bond strength and reinforcing bending behaviors in

strain hardening and multiple cracking procedure. In this way, the flexural and splitting tensile strength

can be enhanced. The fibers for reinforcing fly ash-based geopolymer composites include steel (ST)

fiber (Shaikh, 2013), polyvinyl alcohol (PVA) fiber (Nematollahi, 2015; Zhang et al., 2006; Zhang et

al., 2008b; Sun and Wu, 2008), sweet sorghum fiber (Chen et al., 2014), and cotton (Alomayri et al.,

2014a; 2014b).

In order to obtain higher flexural strength, Shaikh (2013) added 2 v/v% steel fiber, 2 v/v% PVA

fiber and a hybrid combination of 1 v/v% ST + 1 v/v% PVA fiber and investigated the deflection

hardening behavior of hybrid fiber reinforced fly ash-based geopolymer. They found a higher bond

strength and flexural strength between the PVA fiber and geopolymer matrix than with cement matrix.

The alkalinity of geopolymer matrix did not affect the degradation of PVA and steel fiber as seen in

SEM (Fig.4.). Alomayri et al. (2014a) added cotton fabric layers in fly ash-based geopolymer structure

to improve flexural strength. The flexural strength of the fly ash-based geopolymer increased from 8.2

MPa to 31.7 MPa when the cotton fiber content was increased from 0 to 8.3 wt%. Moreover, the

orientation of cotton fabric layers had effects on the flexural strength of fly ash-based geopolymer. The

higher flexural strength of the fly ash-based geopolymer reinforced with horizontally laid cotton fabric

could be attributed to the better uniformity in load distribution among the consecutive layers of cotton

fabric, while the geopolymer with vertical fabric orientation suffered from detachments and

delamination between the cotton fabric and geopolymer matrix, and had a lower flexural strength

(Alomayri et al., 2014b).

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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).

Splitting tensile strength can be increased by using N-carboxymethyl chitosan, and the fibers, such

as PVA fiber and sweet sorghum fiber as additives in fly ash-based geopolymer (Table 1). Nematollahi

et al., (2015) prepared geopolymer with short PVA fibers (2% v/v), Class F fly ash activated in 8.0

mol/L NaOH (28.6% w/w) and Na2SiO3 (71.4% w/w) solution. The bond strength of the geopolymer

increased more than the cracking strength, resulting in a high fiber-bridging strength. The splitting

tensile strength of PVA reinforced Class F fly ash-based geopolymer was 4.7 MPa.

Chitosan with stable crystalline structure from strong hydrogen bonds suffers poor solubility at

higher pH required for geopolymer. N-carboxymethyl chitosan, a chitosan derivative, can avoid this

problem and realize the better coordination with geopolymer gel (Mourya et al., 2010; Pillai et al.,

2009). Li et al., (2013b) mixed NaOH solution (10 mol/L), N-carboxymethyl chitosan uniform solution

with fly ash under stirring under ambient conditions for 20 min. Then, the specimens were aged at

room temperature for 24 h and placed in an oven for curing at 60°C for 6 days. N-carboxymethyl

chitosan biopolymer coated on fly ash particles and N-carboxymethyl chitosan were well incorporated

into geopolymer matrix. In particular, there were additional hydrogen bonds formed between

N-carboxymethyl chitosan macromolecules and fly ash-based geopolymer, in addition to the hydrogen

bonds within the N-carboxymethyl chitosan macromolecules (Fig. 5). All these interactions led to a

more condensed network structure in geopolymer. Thus the fly ash-based geopolymer reinforced by

N-carboxymethyl chitosan exhibit enhanced the mechanical behavior of the fly ash-based geopolymer.

The N-carboxymethyl chitosan reinforced fly ash-based geopolymer had a substantial increase of the

tensile strength from 7 MPa to 8 MPa when the N-carboxymethyl chitosan content increased to 0.1

wt.%.

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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).

Chen et al., (2014) used alkali-pretreated sweet sorghum fiber with fly ash to prepare

geopolymer. The sweet sorghum fiber was obtained from the bagasse waste after juice being extracted

from sweet sorghum stalks for ethanol production. When 2 % sweet sorghum fibers were used, an

increase about 36 % of the splitting tensile strength of the geopolymer was obtained. But further

increase of the fiber content decreased tensile strength. The flexural strength of the fiber-reinforced

geopolymer showed a similar trend. The addition of 2 % fiber effectively carried higher tensile load

and thus delayed the growth of microcracks and increased the flexural strength. However, further

increase of the fiber content induced fiber agglomeration, resulting in an increase of air bubbles

entrapped in the composite and nonuniform fiber dispersion. As consequence, flexural strength

decreased. The addition of cotton to fly ash-based geopolymer showed the similar trend in flexural

strength.

According the studies mentioned above, the increase of splitting tensile strength and flexural

strength in chitosan-reinforced and fiber-reinforced fly ash-based geopolymer was primarily because

the micro- and macro-fibers can increase the hydrogen bonds, bridge the micro cracks, transfer the load,

and delay the development of micro cracks (Dias and Thaumaturgo, 2005).

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3.3 Durability

The durability of fly ash-based geopolymer and geopolymer concrete includes resistance to

chloride, sulfate, acid, freeze-thaw cycles, heat, fire and efflorescence. Durability is closely related to

the microstructure and the migration behavior of ions from fly ash-based geopolymer. These in turn can

be adjusted by the alkali solution, curing and addition of calcium and silica fume composite during the

preparation of fly ash-based geopolymer.

3.3.1 Chloride resistance

Resistance to chloride is one of the main areas in durability of cement and concrete. Chloride

penetration promotes the corrosion of the embedded steel bars when steel bars are used as

reinforcements for geopolymer concrete. Chloride penetrates into fly ash-based geopolymer through

capillary absorption, hydrostatic pressure and diffusion of ions.

The relatively high concentration of NaOH enabled the leaching of more Si and Al from fly ash

and produced a better degree of polycondensation and resulted in a decrease the porosity of fly

ash-based geopolymer. The porosity of the material affects the chloride penetration. For example,

Chindaprasirt and Chalee (2014) prepared fly ash-based geopolymer using a Na2SiO3/NaOH mixture

solution as the activator. The concentrations of NaOH were 8, 10, 12, 14, 16 and 18 mol/L with a

constant SiO2/Al 2O3 molar ratio. The chloride penetration decreased with the increase of NaOH

concentration used in geopolymerization process owing to the refinement of the pore structures as a

result of polycondensation reaction.

Ismail et al. (2013) examined the permeability of chlorides in Class F fly ash/slag-based

geopolymer, slag-based geopolymer, and OPC by the chloride accelerated method (NordTest NT Build

492) and the ponding method (ASTM C1543). AgNO3 was used to reveal chloride penetration depths.

The result showed that a little formation of silver chloride for the slag-based geopolymer (Fig. 6A), and

fly ash/slag-based geopolymer (Fig. 6B, Fig. 6C), while OPC-based concrete (Fig. 6D) had the deepest

chloride penetration depth. They found the slag promote the formation of denser C–A–S–H gel

contributing to a higher mechanical strength, and durability under chloride exposure and fly ash

promote the formation of more porous N–A–S–H gels, reducing the resistance to chloride transport.

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Although a higher porosity was observed in the geopolymer compared to the OPC specimens,

geopolymer still showed a higher chloride resistance.

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 (Reprinted from Ismail et al. (2013), Copyright (2013), with

permission from Elsevier).

Yang et al., (2014b) prepared fly ash/slag-based geopolymer (Slag/Fly ash ratio= 0, 0.25 and 0.50)

and exposed the materials to a 3% NaCl solution for 72 h. The C-A-S-H gel formed in the fly

ash/slag-based geopolymer resulted in lower chloride diffusion compared with the N–A–S–H gel. In

addition, the incorporation of slag in fly ash-based geopolymers led to the refinement of the pore

structure.

3.3.2 Sulfate resistance

The migration of Na+ from fly ash-based geopolymer into the sulfate solution results in vertical

cracks and causes the deterioration of strength. In addition, the sulfate solution usually caused the

breaking of -Si-O-Si- bonds in geopolymer gel and the leaching of silicon (Baščarević et al., 2015).

Bakharev (2005a) prepared fly ash-based geopolymer using NaOH, KOH, or Na2SiO3 as activator and

investigated the durability after exposed them to sulfate environments (5% MgSO4 solution, 5%

Na2SO4 solution and a solution of 5% MgSO4 +5% Na2SO4) for 5 months. After immersion, the fly

ash-based geopolymer samples changed little. The geopolymer activated by NaOH had the best sulfate

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resistance owing to a more stable cross-linked structure.

Sukmak et al., (2015) used a mixture of silty clay and fly ash as the source materials to prepare

geopolymer and examined the resistance of the geopolymer in 5 wt% Na2SO4 and 5 wt% MgSO4

solutions. The decrease in the compressive strength of the clay/fly ash-based geopolymer after 240

days of exposure was 10.8% in Na2SO4 solution and 21.6% in MgSO4 solution. Ettringite, gypsum and

brucite, were detected after the exposure to sulfate environment. The C-S-H phase disappeared due to

its reaction with sulfates and forming ettringite phase.

3.3.3 Acid resistance

The lifetime of cement and concrete can be severely shortened in the acidic environment. For fly

ash-based geopolymer, the acid attack is associated with the depolymerization of aluminosilicate

network structure and the liberation of silicic acid (Si(OH)4). When immersed in a strong acid solution,

Na+ and K+ from fly ash-based geopolymer could be substituted by H+ or H3O+, breaking -Si-O-Al-

bond and -Si-O-Si- bond, and releasing silicic acid (Breck, 1974).

Different alkali solutions (NaOH, KOH and Na2SiO3) have been used by Bakharev (2005b) to

prepare fly ash-based geopolymer followed by immersing the geopolymers in 5% CH3COOH and

H2SO4 solutions for 5 months. The fly ash-based geopolymer activated by 8% NaOH had more stable

structure and showed high resistance to both acid solutions, while the fly ash-based geopolymer

activated by KOH showed an increase in the average pore diameter and the number of active sites in

the geopolymer gels on the surface, leading to a lower durability.

The effect of calcium and silica fume on the acid resistance of fly ash-based geopolymer is also

noteworthy. Lloyd et al. (2012) investigated the corrosion rates of Class C fly ash-based and slag-based

geopolymer in HNO3 and H2SO4 acids (pH=1.0-3.0). It was found that the calcium content introduced

from the Class C fly ash or slag reduced the mass transport rates by forming fine and tortuous pore

networks in geopolymer. In addition, as the increase of alkali content (Na2O), the nature of the

corroded layer became harder, more brittle and more prone to the development of cracks (Fig. 7.). The

high content of alkali is beneficial for releasing more calcium and aluminium and for forming

geopolymer gels while when removed upon exposure to acid, the geopolymer structure was more

vulnerable under acidic conditions. Chindaprasirt et al. (2014) added silica fume (1.5%, 3.75% and

5.0%) to fly ash to prepare geopolymer composite mortars. The acid resistance of composite mortars

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was tested in 3 vol% H2SO4 acid. The optimum silica fume addition of 3.75% increased the strength of

fly ash-based geopolymer and showed the best acid durability owing to the increase of C-S-H gels and

a more dense structure.

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 sulphuric acid (Reprint from Lloyd

et al. (2012), with permission from Springer).

Curing at a high temperature is beneficial to the resistance of geopolymer to acid. Nguyen et al.

(2013) cured the fly ash-based geopolymer at 80°C for 10 h, and then immersed the geopolymer 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 of about 20 MPa which was much higher than OPC in HCl solution

(1 mol/L, 2 mol/L and 4 mol/L). In addition, Chindaprasirt et al. (2013a) cured fly ash-based

geopolymer with 90 W microwaves for 5 min. They found the microwave radiation accelerated the

geopolymerization and gave enhanced densification comparable to the conventional curing. The

microwave cured fly ash-based geopolymer was immersed in 3 vol% of H2SO4 and found that it only

had a small loss of strength under the acid attack.

3.3.4 Thermal resistance

When fly ash-based geopolymer is exposed to elevated temperatures, shrinkage of geopolymer

occurs in proportion to the amount of water being vaporized from the structure (Rickard et al., 2011).

Some of the fly ash-based geopolymers exhibit a strength increase after exposure to high temperature,

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and have been used as concrete, thermal barriers, refractories and fire resistant structures. Heat

travelled faster in geopolymer concrete than in OPC concrete when exposed to fire, resulted in less

temperature gradient inside geopolymer concrete (Sarker et al., 2014). OPC concrete showed an

average residual strength of 90%, 52% and 11-16% respectively after exposed to fire at 400°C, 650°C

and 800-1,000°C, whereas the average residual strengths of geopolymer concretes were 93%, 82%, and

21-29% after the same treatments. Moreover, the OPC concrete suffered severe spalling and extensive

surface cracking after exposure at 800-1,000°C, while there was no spalling and only minor surface

crackings in the geopolymer concrete. Guerrieri and Sanjayan (2010) exposed fly ash/slag-based

geopolymer (fly ash/slag ratio=100, 65/35, 50/50, 35/65) to 800°C. They found the geopolymer

specimens with very low initial strengths (< 7.6MPa) experienced an increase in residual strength up to

90% gain after exposure to 800°C while specimens with initial strengths of 28 MPa and specimens with

initial strengths of 83 MPa had residual strength losses of approximately 70% and 90% after exposure

to 800°C, respectively. The different residual strengths of fly ash/slag-based geopolymer after exposure

to 800°C were caused by the further hydration and sintering of the unreacted fly ash or slag.

Geopolymer with higher initial strengths have a smaller capacity to allow for thermal incompatibilities

between the inner and outer parts of the specimen due to uneven temperatures arising during heating.

Rickard et al. (2010) investigated the thermal characteristics of Class F fly ash-based geopolymer at the

temperature>500°C. The maximum shrinkage was approximately 3% when the geopolymer was heated

to 900°C. The iron oxides (15 wt.%) in fly ash directly affected the thermal properties of the

geopolymer by influencing the thermal expansion, altering the phase composition and changing the

morphology after heating.

The pore structure and density of fly ash-based geopolymer facilitates the escape of moisture and

help avoid damaging during heating. Kong et al., (2007) prepared fly ash-based geopolymer and

exposed it to 800°C. They found a small amount of moisture escaped from fly ash matrix and the pore

spaces provided escape routes for moisture in the matrix thereby decreasing the likelihood of the

damage to the matrix at elevated temperatures. Bakharev (2006) compared the effects of heating at

800-1200°C on the geopolymer made form Class F fly ash activated by NaOH and KOH, respectively.

The geopolymer activated by NaOH had a rapid strength deterioration and a high shrinkage at 800°C

due to a dramatic increase of average pore size, while the other activated by KOH showed a significant

increase of compressive strength upon heating and the strength deterioration only started at 1000°C.

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The addition of foaming agent of a low concentration led to a low density with cellular structure.

Rickard and van Riessen (2014) prepared foamed Class F fly ash-based geopolymer with a Si/Al ratio

of 2.5 (ρ 0.9 g/cm3, k 0.3 W/m·K). Under a simulated fire for 60 to 90 min, the foamed

geopolymer lost its strength significantly. Foaming significantly reduced the strength of Class F fly

ash-based geopolymer, while the thermal insulating property improved as their thermal conductivities

were reduced by approximately half.

Using Ca(OH)2 as additive to synthesize fly ash-based geopolymer formed the C–S–H gel and

helped the different mineral formation and transformation processes at elevated temperatures.

Dombrowski et al. (2007) prepared fly ash-based geopolymer and investigated the influence of the

calcium content on thermal resistance. When the geopolymer exposed to temperatures higher than

600°C, sodalite and amorphous aluminosilicates were formed and then they were transformed into

nepheline, resulted in a denser structure. As the temperature was further increased, the nepheline was

converted into albite. The fly ash-based geopolymer with 8% Ca(OH)2 addition for the preparation had

the highest strength and the lowest shrinkage with the highest amount of nepheline at 800°C and of

feldspar at 1,000°C.

Geopolymer exposed to fire has the tendency to shrinkage and cracking as well. According to the

International Standards Organization standard (ISO 834), Sarker et al. (2014) test the fire resistance of

the fly ash-based geopolymer and OPC by exposing the samples to fire heating at 400°C, 650°C, 800°C

and 1000°C. When exposed to fire at 1,000°C, fly ash-based geopolymer had only minor surface

cracking and the average mass loss of only 4.8%, while OPC had the average mass loss of 90%.

3.3.5 Freeze-thaw resistance

Freeze-thaw attack usually causes cement and concrete expansion, internal cracking, and scaling

mass loss (Hobbs, 2001). It is a major attack to cement and concrete second to chloride attack. Fly ash

based-geopolymer has been reported to show no sign of damage even after 150 freezing cycles.

Temuujin et al. (2014) prepared geopolymer from Class F fly ash activated by a NaOH/Na2SiO3

solution and cured it at 70°C for 22 h. The freeze-thaw cycles of geopolymer were higher than 40.

The addition of Na2SiO3 solution did not improve the freeze-thaw durability of fly ash-based

geopolymer. The presence of crystalline, anhydrite and amorphous calcium compounds (such as lime)

in fly ash negatively influenced the freeze-thaw resistance. Na2SiO3 reacted with calcium compounds

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(CaO, CaSO4 and amorphous calcium aluminosilicate) in the fly ash source and formed C–A–S–H gel

or Ca(OH)2. The formation of Ca(OH)2 weakened internal structure and increased water permeability

and had a negative effect on the freeze-thaw resistance.

Air entrainment has been proved as an effective method to increase freeze-thaw resistance of OPC

(Mindess and Young, 1981). Sun and Wu (2013) compared fly ash-based geopolymer and OPC with or

without air entrainment for 300 freeze–thaw cycles according to ASTM C666. OPC without air

entrainment deteriorated most seriously, showing a strength loss of about 20% after 300 cycles, while

OPC with air entrainment lost only 5%. Fly ash-based geopolymer without air entrainment had a final

loss of 8.4% after 300 cycle. With the air entainment, fly ash-based geopolymer showed a strength loss

of 6.8%. Clearly, the air entrainment had no much positive influence on fly ash-based geopolymer.

Brooks et al. (2010) studies the effects of air entrainment on the scaling rate of fly ash-based

geopolymer. Air-entrained fly ash-based geopolymer showed slight scaling after 40 freeze-thaw cycles

while non-air entrained fly ash-based geopolymer showed no scaling. Air entrainment significantly

reduced the compressive strength of fly ash-based geopolymer 10-30% than the non-air entrained fly

ash-based geopolymer and the uniform and stable pore structure believed to increase freeze-thaw

durability was not formed in non-air entrained fly ash-based geopolymer.

3.3.6 Efflorescence resistance

Geopolymer, especially with a high alkali content and a low calcium content, tends to have a

porous and open microstructure. The aqueous solution remains entrapped in the pores or bonded to the

network when the three-dimensional geopolymer structure has finally formed (Davidovits, 2008).

Within the pore network, the excess sodium oxide is mobile, and is prone to form white crystal (i.e.

efflorescence) when in contact with atmospheric CO2 and results in the degradation of geopolymer

(Najafi and Allahverdi, 2009; Pacheco-Torgal and Jalali, 2010).

The efflorescence behavior of fly ash-based geopolymers is strongly dependent on the alkali

activator solution type, curing temperature and calcium content. Using KOH instead of NaOH as

activator reduces the efflorescence of geopolymer (Duxson et al., 2006). The high alkali concentration

in the pore solution and the weak binding of Na+ in the geopolymer structure are the reasons for more

mobile Na+ in pore solution and cause the severe efflorescence of geopolymer (Bortnovsky et al., 2008).

K+ is more strongly bound to the aluminosilicate gel framework, and also potassium carbonate crystals

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are usually less visually evident than their sodium counterparts (Szklorzová and Bílek, 2008; Škvára et

al., 2008). Zhang et al., (2014b) activated fly ash by NaOH to synthesize geopolymer and cured (23°C,

80°C). They found geopolymer at high temperature exhibiting much lower efflorescence than those

synthesized at low temperature. The curing was beneficial for reorganization and crystallization of

N-A-S-H gels and decreasing the efflorescence rate.

The presence of calcium helps formed C–S–H in fly ash-based geopolymer and a smaller pore size

with low-permeability, which prevents alkali diffusion (Lloyd et al., 2010). Conversely, for foamed fly

ash-based geopolymer, the foaming caused large pore size and high porosity of geopolymer and it led

to fast alkali leaching and caused faste efflorescence consequently (Zhang et al., 2014b).

4. Fly ash-based geopolymer for adsorption and immobilization of toxic metals

Geopolymerization offers an alternative technique to immobilize soluble heavy metals from fly

ash, slag or other industrial and residential wastes (Malviya and Chaudhary, 2006; Ojovan and Lee,

2005; Li and Wang, 2005). The hazardous elements, such as Ba, Cd, Co, Cr, Cu, Nb, Ni, Pb, Sn, and U

can be tightly fixed in the three-dimension structure of fly ash-based geopolymer matrix. The main

mechanisms of metal immobilization in fly ash-based geopolymer are physical encapsulation and

chemical stabilization. The high pH values enhance the oxyanionic mobility, such as As, B, Mo, Se, V

and W (Izquierdo et al., 2009). Temuujin et al. (2014) prepare geopolymer from high-calcium

Mongolian fly ash (CaO =14–30 wt.%) with a high radioactivity (314–343 Bq/kg) and used NaOH

solution or mixtures of NaOH and Na2SiO3 as activator and cured it at 70°C for 22 h. The radioactivity

of the radioactive high-calcium Mongolian fly ash-based geopolymer product was then became 130–

152 Bq/kg, within the standard safe limits for construction of dwellings.

Fly ash-based geopolymer showed higher immobilization of metal ions than OPC and fly ash itself.

Li et al., (2013a) compared the immobilization of 133Cs+ in fly ash-based geopolymer and OPC.

Leaching tests was carried out in deionized water, sulfuric acid and magnesium sulfate solutions,

respectively. It was revealed that neither the microstructure nor the geopolymeric phases of fly

ash-based geopolymer were significantly affected by the incorporation of 133Cs+. The cumulative

fraction leaching concentration (CFLC) of fly ash-based geopolymer in deionized water was only 5.4 %

of that of OPC for 25°C, and 6.1 % for 70°C. The leaching of 133Cs+ from the geopolymer in 5% (w/w)

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magnesium sulfate solution was only 9.1 % that from OPC. The difference of leaching of 133Cs+

between the geopolymer and OPC in sulfate acid solution was small. Al-Zboon et al., (2011) used fly

ash-based geopolymer as an adsorbent for lead Pb(II) removal from wastewater. They found the lead

removal efficiency of fly ash-based geopolymer was 90.6%, which was much higher than that of raw

fly ash (39.87%) (pH = 5; C0 = 100 ppm; contact time = 120 min). The removal efficiency was 95%

when the dose of fly ash-based geopolymer was 0.07 g at 40°C, pH=5, and 120 min contact time in

1,000 ppm standard lead (Pb) solution.

The fly ash-based geopolymer showed different metal immobilization behaviors in acid and

sulfate solutions. Zhang et al., (2008a) prepared geopolymer with fly ash, sand and some heavy metal

additives, such as Cr(VI), Cd(II) and Pb(II), to investigate the immobilization effect of heavy metal

ions and the leaching behavior in H2SO4 and Na2SO4 solutions. The low level of heavy metal salts had

little effect on the compressive strength of the geopolymer. The immobilization of these species was

based on the chemical binding into the geopolymer gel or into aluminosilicate phases in geopolymer.

They found Pb was immobilized effectively by a chemical binding mechanism in geopolymer, Cr(VI)

was ineffectively immobilized, and Cd immobilization was depended on the solubility of a hydroxide

phase with effectively at high pH but poor at low pH.

5. Fly ash-based geopolymer for concrete

The growing environmental awareness and the demand for high efficiency of natural resource

encourage the construction industry to look for alternative materials (McKelvey et al., 2002; Schneider

et al., 2011). Fly ash has already been used as supplementary cementitious material in concrete industry

for over 50 years in the world (Uysal and Akyuncu, 2012). In the hydration of traditional cement, fly

ash with a high content of Al2O3 and SiO2 can be activated by Ca(OH)2 and thus produce more C-S-H

gel, C-A-H gel and C-A-S-H gel to fill the capillary of concrete effectively, thereby increasing concrete

strength (Sahmaran and Li, 2009). Fly ash can reduce the hydration heat and thermal cracking of

concrete at early stage, and improve the mechanical and durability properties.

Fly ash-based geopolymer concrete completely moves away from OPC and the high CO2 emission

associated with OPC production (Cakir and Akoz, 2008; Aldea et al., 2000; Habert et al., 2011). Fly

ash-based geopolymer itself is a good binder that can be used as cement to mix with aggregates to

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produce the geopolymer concrete. Fly ash-based geopolymer cement has shown a better performance

than OPC in some areas aspects or conditions. For example, fly ash-based geopolymer concrete has a

denser microstructure with lower chloride diffusion and lower porosity compared with OPC concrete.

Reddy et al. (2013) used Class F fly ash as source material and NaOH and Na2SiO3 solution as

activator to synthesize geopolymer cement. They found that the geopolymer concrete had excellent

resistance to chloride attack and less corrosion cracking when exposed to simulated seawater and

induced current.

Alkali silica reaction (ASR) is a common chemical reaction between the OH- in the pores within

the concrete matrix and the reactive aggregate compounds in concrete (Swamy and Al-Asali, 1988;

Diamond, 1975). ASR causes the strength loss, cracking, and expansion of the concrete structure. Fly

ash-based geopolymer concrete is significantly less vulnerable to ASR than OPC concrete.

Kupwade-Patil and Allouche (2013) prepared steel reinforced fly ash-based geopolymer concrete with

quartz, sandstone and siliceous limestone as aggregates, and compared the chloride diffusion in a cyclic

wet-dry chloride environment over a period of 12 months with OPC concrete. It was found that fly

ash-based geopolymer concrete specimens did not exceed the ASTM threshold for expansion while

OPC concrete exceeded the permissible threshold. In addition, the leaching and visual cracks were

observed in the OPC concrete but not in the fly ash-based geopolymer concrete.

In concrete, aggregate takes up as high as 85% of the material. Interactions among aluminosilicate

framework, alkali cations, additives and aggregate in fly ash-based geopolymer concrete are important

factors that influence the overall mechanical performance. The strong interfacial interactions between

the aggregate and the fly ash-based geopolymer matrix in a large zone contributes to the high splitting

tensile strengths between geopolymer and steel reinforcements (Topark-Ngarm et al., 2015).

The size of the aggregates affects the fly ash-based geopolymer concrete performance. Kong and

Sanjayan (2010) studied the effect of aggregate size on the compressive strength at high temperatures

of the fly ash-based geopolymer concrete containing crushed old basalt aggregates, river sand and slag

aggregates. It was found the thermal incompatibility between the geopolymer matrix and the aggregates

caused the strength loss of geopolymer concrete specimens at elevated temperatures. Large aggregates (>

10 mm) resulted in good strength performances at both ambient and elevated temperatures while

smaller sized aggregates (< 10 mm) promoted spalling and extensive cracking at elevated temperature.

Recycled coarse aggregate has been used with fly ash geopolymer to synthesize concrete with

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acceptable properties. Sata et al., (2013) used crushed structural concrete beams and crushed clay

bricks as recycled coarse aggregates for making fly ash-based geopolymer concrete. The geopolymer

concrete with recycled aggregate showed lower compressive strengths (2.9–10.3 MPa) than those

containing natural aggregate, however, it was within the typical strength distribution reported

(American Concrete Institute (ACI) committee 522, 2010). In addition, the total void ratio of the

geopolymer concrete with recycled aggregate (21.7-26.9%) was similar to those with natural aggregate

(24.2-27.4%), and the water permeability was 0.71-1.47 cm/s versus 1.18-1.71 cm/s. Nuaklong et al.,

(2016) used recycled aggregates from old concrete with compressive strength of 30-40 MPa and high

calcium fly ash to make fly ash-based geopolymer. They found fly ash-based geopolymer with recycled

aggregate showed compressive strengths of 30.6-38.4 MPa, which were slightly lower than those of fly

ash-based geopolymer concretes with crushed limestone.

Different superplasticizers (naphthalene (N) and polycarboxylates (PC)) as the water reduction

admixture have been used to improve the workability of the geopolymer concrete by Nematollahi and

Sanjayan (2014). PC superplasticizer with a dosage of 3.3 wt.% caused significant reduction (54%) in

strength with reference to concrete without superplasticizer. By contrast, N superplasticizer with a

dosage of 1.19% caused a 22% reduction in strength. The use of superplasticizers was not beneficial to

geopolymer concrete used at elevated temperatures.

Injecting and storing CO2 into underground has been proposed as one of the long-term strategies

for handling greenhouse gases (Laudet et al., 2011; Bachu and Bennion, 2009). OPC-based sealant has

been used in injection wells and it has been found experiencing cement degradation under CO2-rich

down-hole conditions. The low permeability of fly ash-based geopolymer can effectively prevent CO2

leakage. Nasvi et al. (2013) found that the apparent CO2 permeability of fly ash-based geopolymer

cement is in the range of 2×10-21 - 6×10-20 m2, which is lower than the typical oil well cements (10-20 -

10-11 m2). Moreover, Nasvi et al. (2014) prepared fly ash-based geopolymer cement from fly ash

activated by a mixed Na2SiO3/NaOH solution and tested the CO2 permeability under three conditions:

(1) temperatures of 23–70°C; (2) CO2 injection pressures of 6–17 MPa; and (3) confining pressures of

12–20 MPa. The results showed that the apparent CO2 permeability of geopolymer increased with

curing temperature and the increasing rates were as high as 200–1,000 %. The maximum permeability

(0.04 lD) value obtained was approximately 5,000 times lower than the permeability value (200 lD)

recommended by the American petroleum industry (API) for a typical well sealant. The findings

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indicated that there is a great potential in using fly ash-based geopolymer cement for carbon storage.

6. Conclusions and future work

Geopolymer technology offers a facile approach for fly ash utilization. In principle, the

geopolymerization includes the dissolution of alumina, silica, aluminosilicate in the fly ash feedstock

by alkali, the recombination of Al3+ and Si4+ species and the generation of three-dimensional

amorphous aluminosilicate polymers. The preparation and formation, and properties of the fly

ash-based geopolymer products depend heavily upon the chemical and physical characteristics of fly

ash, alkali activators, curing conditions and the addition of slag, fiber and red mud.

NaOH and Na2SiO3 are used as alkali activators; CaCl2 is used as chemical additives to form

C-A-S-H gel, C-A-H gel and C-S-H gel and change the setting time; and slag, fiber, RHBA and red

mud are used as additives to change the Si/Al ratios and introduce the hydrogen bonds between

geopolymer gel (N-A-S-H gel) and the additive molecules.

For the practical applications of fly ash-based geopolymer, mechanical properties (such as

compressive strength, splitting tensile strength and flexural strength) and durability (chloride, sulfate,

acid, thermal, freeze-thaw cycles and efflorescence resistances) should be comprehensively considered.

The Si/Al ratios of reactants, alkali solutions, curing conditions, and the addition of slag, fibers, RHBA,

red mud and calcium can be fine tuned to improve those properties. The changes of Si/Al ratios, alkali

solutions, and adding slag, RHBA, red mud and calcium can lead to the different gels, including

N-A-S-H gel, C-A-S-H gel, C-A-H gel and C-S-H gel. The gels influence the final structure of

geopolymer and control the ionic transport. Alkali solutions influence the hydrolyzation of fly ash and

the porosity of geopolymer structure. The porosity influences the migration of alkali from fly ash-based

geopolymer into the ion solutions, the moisture and then has an effect on the mechanical strength and

durability. Fly ash-based geopolymer with compact and denser structure shows high mechanical

strength and good resistance to chloride, sulfate and acid solutions and good efflorescence.

Fly ash-based geopolymer can be used as cement to mix with aggregates to form concrete. In this

context, considering the low cost, low CO2 emission and low energy usage in the production of fly

ash-based geopolymer, fly ash-based geopolymer cement and concrete are regarded as possible

alternative green materials to OPC. Fly ash-based geopolymer has also been used to adsorb and

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immobilize toxic metals and it shows better adsorption and immobilization performance than those of

OPC. Besides, fly ash-based geopolymer cement has been used as a well sealant to store CO2 in the

underground. However, the CO2 permeability value of fly ash-based geopolymer is still lower than the

permeability value for a typical well sealant recommended by the API.

Though last decades or so have witness remarkable advance in the science and technology of fly

ash-based geopolymer as discussed above, there exists several issues and some issues are proposed

below for future work.

(1) Inherently, to control the production and to improve the performances of fly ash-based

geopolymer, the reaction mechanisms in each step should be uncovered in details. To this end, involved

are many aspects such as thermodynamics, kinetics, identifications of intermediates and insights into

their structures, and the degrees to which the -Si-O-Al- are oligomerized and polymerized. All these

will become more sophisticated once the additional elements or additives are included. But further

studies must be made in order to ensure that the correlation of production-structure-performances is

clear-out.

(2) Most of fly ash-based geopolymer concretes are brittle and sensitive to cracking. Such

behavior not only imposes constraints in applications, but also affects the long-term durability of

geopolymer concretes (Zhao et al., 2007; Zhang et al., 2010b; Pernica et al., 2010). Therefore,

innovation in the preparation to create improved fly ash-based geopolymer composites is still needed.

(3) At present, in most cases, fly ash-based geopolymer are merely produced at laboratory scale

with empirical formulations. It could be exciting to see a report by Wagners (Wagners Earth Friendly

Concrete product) that the production of fly ash-based geopolymer concrete is being realized on a

large-scale. To put producing and using fly ash-based geopolymer on a large-scale are encouraging and

need further input and endeavor,

(4) As for the uses of fly ash-based geopolymer for toxic metals adsorption and immobilization

and sealing CO2, the performances are still unsatisfactory. Changing of the recipes for preparation is

worth for further investigation.

(5) Instead of using fly ash-based geopolymer as alternative cement, it is also possible to endow

fly ash-based geopolymer with more functionalities or unique properties. Consequently, new

applications of fly ash-based geopolymer are worth exploring and can be found. For instance, fly

ash-based geopolymer with biomass can be developed as a class of novel lightweight fireproof

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materials.

Abbreviations

ACI: American Concrete Institute

API: American Petroleum Industry

ASR: Alkali silica reaction

ASTM: American Society for Testing and Materials

BFS: Blast furnace slag

C-A-H gel: Calcium aluminate hydrate gel

C-A-S-H gel: Calcium aluminum silicate gel

CCS: Carbon capture and storage

C-S-H gel: Calcium silicate hydrate gel

CFA: Class C fly ash

CFLC: Cumulative fraction leaching concentration

FFA: Class F fly ash

GGBF: Ground granulated blast furnace slag

HHW: Harford secondary waste

HMNS: High magnesium nickel slag

ISO: International Standards Organization

N-A-S-H gel: Sodium aluminosilicate gel

OPC: ordinary Portland cement

PC: Polycarboxylates

PVA: Polyvinyl alcohol

RHBA: Rice husk-bark ash

Acknowledgement

The authors wish to acknowledge the financial support from the National Natural Scientific Foundation of China (21373185; 21506188),

the Distinguished Young Scholar Grants from the Natural Scientific Foundation of Zhejiang Province (ZJNSF, R4100436), ZJNSF

(LQ12B03004), Zhejiang “151 Talents Project’’, the projects (2010C14013 and 2009R50020-12) from Science and Technology

Department of Zhejiang Provincial Government and the financial support by the open fund from Key Laboratory of Clay Minerals of

Ministry of Land and Resources of The People's Republic of China(2014-K02), and Engineering Research Center of Non-metallic

Minerals of Zhejiang Province (ZD2015k07), Zhejiang Institute of Geology and Mineral Resource, China, and the State Key Laboratory

Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology (GCTKF2014006). The authors also feel

grateful to four anonymous reviewers for their comments and suggestions, which are much helpful for us to improve the manuscript.

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Highlights

� Fly ash is activated by alkali to form geopolymer and such a process is cleaner.

� Slag, rice husk-bark ash, fiber and red mud are added to improve the performance of fly ash-based

geopolymer.

� Mechanical properties and durability of fly ash-based geopolymer are main concerns.

� Fly ash-based geopolymer is used as cement and as fixation materials for toxic metals.

� Improving performance, scaling-up production and finding new applications are proposed for

future.