Construction and Building Materials 85 (2015) 78–90
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Review
Geopolymer concrete: A review of some recent developments
B. Singh ⇑, Ishwarya G., M. Gupta, S.K. Bhattacharyya
CSIR-Central Building Research Institute, Roorkee 247667, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
An overview of geopolymer is
Conversion of fly ash into geopolymers/concrete.
presented alongwith its processing
parameters.
The hardened properties and
durability of geopolymer concrete are
discussed.
The design guidelines for OPC
concrete are applicable to
geopolymer concrete also.
Geopolymeric building products
developed at CSIR-CBRI are
highlighted.
Ambient cured single component
geopolymer may enhance its wider
use in the field.
a r t i c l e
i n f o
Article history:
Received 26 November 2013
Received in revised form 16 February 2015
Accepted 4 March 2015
Available online 31 March 2015
Keywords:
Geopolymer concrete
Activator
Bond strength
Compressive strength
Durability
a b s t r a c t
An overview of advances in geopolymers formed by the alkaline activation of aluminosilicates is presented alongwith opportunities for their use in building construction. The properties of mortars/concrete
made from geopolymeric binders are discussed with respect to fresh and hardened states, interfacial
transition zone between aggregate and geopolymer, bond with steel reinforcing bars and resistance to
elevated temperature. The durability of geopolymer pastes and concrete is highlighted in terms of their
deterioration in various aggressive environments. R&D works carried out on heat and ambient cured
geopolymers at CSIR-CBRI are briefly outlined alongwith the product developments. Research findings
revealed that geopolymer concrete exhibited comparative properties to that of OPC concrete which
has potential to be used in civil engineering applications.
Ó 2015 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
An overview of geopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
2.1.
Constituents effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
2.2.
C-S-H phase effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
2.3.
Effect of admixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
2.4.
Curing conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Geopolymer mortars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Geopolymer concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
⇑ Corresponding author.
E-mail address: singhb122000@yahoo.com (B. Singh).
http://dx.doi.org/10.1016/j.conbuildmat.2015.03.036
0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
5.
6.
7.
79
4.1.
Fresh and hardened properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.2.
Interfacial transition zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.3.
Bond between reinforcing bars and geopolymer concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.4.
Fire behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Durability studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.1.
Alkali-silica reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.2.
Effect of acid attack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.3.
Effect of sulphate attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.4.
Carbonation and permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.5.
Corrosion of steel reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Research and development at CSIR-CBRI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
1. Introduction
The concrete industry faces challenges to meet the growing
demand of Portland cement due to limited reserves of limestone,
slow manufacturing growth and increasing carbon taxes. It is
reported that the requirement of cement in India is likely to touch
550 million tonnes by 2020 with a shortfall of 230 million tonnes (58%) and the demand for cement has been constantly
increasing due to increased infra-structural activities of the country [1]. One effort to combat shortfall is the development of alternate binders to Portland cement aiming at to reduce the
environmental impact of construction, use of greater proportion
of waste pozzolan, and also to improve concrete performance.
Search for several alternatives such as alkali-activated cement, calcium sulphoaluminate cement, magnesium oxy carbonate cement
(carbon negative cement), supersulphated cement etc. are being
made with the advantages of Portland cement [2]. As the family
of the alkali-activated cement is growing, the alkaline cement is
classified based on a phase composition of the hydration products:
R-A-S-H (R = Na+ or K+) in the aluminosilicate based systems and RC-A-S-H in the alkali-activated slag or alkaline Portland cements
[3]. In recent years, geopolymer has attracted considerable attention among these binders because of its early compressive
strength, low permeability, good chemical resistance and excellent
fire resistance behaviour [4–9]. Because of these advantageous
properties, the geopolymer is a promising candidate as an alternative to ordinary Portland cement for developing various sustainable products in making building materials, concrete, fire
resistant coatings, fibre reinforced composites and waste
immobilization solutions for the chemical and nuclear industries.
amorphous and possess sufficient reactive glassy content, low
water demand and be able to release aluminium easily. The alkaline activators such as sodium hydroxide (NaOH), potassium
hydroxide (KOH), sodium silicate (Na2SiO3) and potassium silicate
(K2SiO3) are used to activate aluminosilicate materials. Compared
to NaOH, KOH showed a greater level of alkalinity. But in reality,
it has been found that NaOH possesses greater capacity to liberate
silicate and aluminate monomers [4]. The properties of geopolymers can be optimised by proper selection of raw materials, correct
mix and processing design to suit a particular application [4].
Viewing the importance of the subject, a collaborative project
sponsored by the European Commission – BRITE was undertaken
jointly by France, Spain and Italy on development of ‘‘Cost-effective
geopolymeric cement for innocuous stabilization of toxic elements
(GEOCISTEM)’’. The project was aimed at manufacturing geopolymeric cement by replacing potassium silicate with cheaper alkaline
volcanic tuffs [9].
Geopolymers are synthesized by the reaction of a solid
aluminosilicate powder with alkali hydroxide/alkali silicate [8]. A
schematic representation on formation of fly ash-based geopolymers/concrete is shown in Fig. 1. Under highly alkaline conditions,
polymerisation takes place when reactive aluminosilicates are
rapidly dissolved and free [SiO4] and [AlO4] tetrahedral units
are released in solution. The tetrahedral units are alternatively
linked to polymeric precursor by sharing oxygen atom, thus forming polymeric Si–O–Al–O bonds. The following reactions occur during geopolymerisation [7].
ðSi2 O5 Al2 O2 Þn þ H2 O þ OH ! SiðOHÞ4 þ AlðOHÞ4
ð1Þ
ð2Þ
2. An overview of geopolymers
Geopolymer is considered as the third generation cement after
lime and ordinary Portland cement. The term ‘‘geopolymer’’ is
generically used to describe a amorphous alkali aluminosilicate
which is also commonly used for to as ‘‘inorganic polymers’’,
‘‘alkali-activated cements’’, ‘‘geocements’’, ‘‘alkali-bonded ceramics’’, ‘‘hydroceramics’’ etc. Despite this variety of nomenclature,
these terms all describe materials synthesized utilising the same
chemistry [4]. It essentially consists of a repeating unit of sialate
monomer (–Si–O–Al–O–). A variety of aluminosilicate materials
such as kaolinite, feldspar and industrial solid residues such as
fly ash, metallurgical slag, mining wastes etc. have been used as
solid raw materials in the geopolymerization technology. The
reactivity of these aluminosilicate sources depends on their chemical make-up, mineralogical composition, morphology, fineness and
glassy phase content. The main criteria for developing stable
geopolymer are that the source materials should be highly
This process releases water that is normally consumed during
dissolution. The water, expelled from geopolymer during the reaction provides workability to the mixture during handling. This is in
contrast to the chemical reaction of water in Portland cement mixture during the hydration process. It is reported that the hydration
products of metakaolin/fly ash activation are zeolite type: sodium
aluminosilicate hydrate gels with different Si/Al ratio whereas the
major phase produced in slag activation is calcium silicate hydrate
with a low Ca/Si ratio. Though many physical properties of
geopolymers prepared from various aluminosilicate sources may
appear to be similar, their microstructures and chemical properties
vary to a large extent. The metakaolin-based geopolymer has an
advantage that it can be manufactured consistently, with predictable properties both during the preparation and development.
However, its plate-shaped particles lead to rheological problems,
increasing the complexity of processing as well as the water
demand of the system [6]. Contrary to this, the fly ash-based
80
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
Fig. 1. Conversion of fly ash into geopolymers/concrete.
particles and formation of reaction products (Fig. 2). The reduced
porosity enhanced the strength of geopolymer pastes [13].
Typically, the optimum geopolymer strength was reported with
SiO2/Al2O3 ratio in the range of 3.0–3.8 and Na2O/Al2O3 ratio of
1 [14,15]. Changes in SiO2/Al2O3 ratio beyond this range have
been found to result in low strength. The setting time of geopolymer pastes increased with increasing SiO2/Al2O3 ratio of the initial
mixture.
2.2. C-S-H phase effect
Fig. 2. Pore size distribution of fly ash-based geopolymer pastes at different
activator dosages [13].
geopolymer is generally more durable and stronger than that of
metakaolin-based geopolymer [4]. The slag-based geopolymer is
considered to have high early strength and greater acid resistance
than those of metakaolin and fly ash-based systems.
2.1. Constituents effect
The most important factors affecting the properties of geopolymer pastes are: SiO2/Al2O3 ratio, R2O/Al2O3 ratio, SiO2/R2O ratio
(R = Na+ or K+) and liquid–solid ratio. The majority of research concluded that an amorphous structure of geopolymers is preferable
in order to achieve desired mechanical strength [10–15]. The
relationship between the compressive strength and SiO2/R2O ratio
showed that an increase in alkali content or decrease in silicate
content increases the compressive strength of geopolymers attributable to the formation of aluminosilicate network structures
[10,11]. Geopolymer activated with NaOH alone with Si/Na of 4/4
or less formed the crystalline zeolite (Na96Al96Sr96O384216H2O)
but at a ratio >4/4, nanosized crystals of another zeolite
(Na6[AlSiO4]64H2O) were formed [12]. The addition of even small
amount of sodium silicate to the NaOH significantly reduces crystallite formation due to templating function of silicate units. At low
activator dosage (18%), the pores developed in the fly ash-based
paste were larger and exhibited wider distributions (19.8–
2342 A) whereas at higher activator dosage (30%), the pores were
smaller and showed a narrow distribution (19.8–1155 A) mainly
due to the pore refinement as a result of more dissolution of
The effect of C-S-H phase on the geopolymerization of
aluminosilicates has been studied with a view to know its role in
early age strength [16–22]. In metakaoin/slag blends, both C-S-H
phase and aluminosilicate gel (N-A-S-H) co-exist in the paste
[16] as similar to NaOH activated high calcium fly ash-based
geopolymer [17] which are responsible for the strength increase.
The little dissolution of calcium occurs in the case of adding natural
calcium silicate minerals at lower alkalinity, resulting in less C-S-H
gel formation and subsequent strength reduction of geopolymer
pastes [18]. In the case of fly ash/slag blends, the reaction at
27 °C is dominated by the slag activation, whereas the reaction at
60 °C is due to combined activation of fly ash and slag. The
improvement in compressive strength of pastes with slag addition
is attributed to its compactness of the microstructure [19]. The
initiation of hardening in fly ash/slag geopolymer made with
potassium silicate and potassium hydroxide was due to C-S-H/CA-S-H formation and the hardening continues due to a rapid formation of a C-A-S-H, K-A-S-H and (Ca, K)-A-S-H depending on
the availability of calcium ions and pH of the system. A slower dissolution rate of calcium ions effectively increased the compressive
strength as rapid geopolymerization continues for a longer duration [20]. The low pH and limited calcium ion environment facilitate the polymerisation reaction between silicate and aluminate
species in high calcium fly ash-based geopolymers producing NA-S-H gel [21]. Guo et al. [22] reported 63.4 MPa compressive
strength of class C fly ash-based geopolymer paste showing the
role of calcium participation in the strength development.
2.3. Effect of admixtures
Kusbiantora et al. [23] reported from their studies that admixtures such as sucrose and citric acid which act as retarder in OPC
have different mechanism in fly ash-based geopolymers. Sucrose
acted as a retarder since it is absorbed by Ca, Al and Fe ions to form
insoluble metal complexes. On the other hand, citric acid acted as
an accelerator reducing the setting time by 9 and 16 min
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
respectively. Amongst the commercial superplasticizers, the naphthalene based superplasticizer was effective when single activator
was used rendering 136% increase in relative slump without any
decrease in compressive strength. Modified polycarboxylate based
superplasticizer was efficient one when multi-compound activator
was used with a decrease in compressive strength of 29% [24].
However, retarding effect of polycarboxylate based superplasticizer was also reported in fly ash/slag blended system though
the improvement in workability was significant compared to naphthalene based superplasticizer [25].
2.4. Curing conditions
Several attempts [26–31] have been made to study the effect of
different curing conditions on the properties of geopolymer pastes.
The curing temperatures were reported in the range between 40 °C
and 85 °C for complete geoplymerisation reactions. Palomo et al.
[26] studied curing of alkali activated fly ash (0.25 and 0.30 liquid/solid ratio) at 65 °C and 85 °C. They indicated that the compressive strength of geopolymers (8–12 M) cured at 85 °C for
24 h was much higher than those cured at 65 °C. The rise of
strength was much smaller when curing time was extended after
24 h. Perera et al. [27] studied the curing of metakaolin-based
geopolymers under ambient (21–23 °C) and heat conditions (40–
60 °C) with a controlled relative humidity (RH) for 24 h and found
that curing at 30% RH was preferable to that at 70% RH. Heah et al.
[28] concluded that the curing of metakaolin-based geopolymers
at ambient temperature was not feasible while increase in temperature (40 °C, 60 °C, 80 °C, 100 °C) favored the strength gain after
1–3 days. However, curing at higher temperature for a longer period of time caused failure of samples at a later age due to the thermolysis of –Si–O–Al–O– bond. Rovnanik [29] reported that curing
of metakaolin based geopolymer at elevated temperature (40–
80 °C) accelerated the strength development but in 28 days, the
mechanical properties deteriorated in comparison with results
obtained for an ambient or slightly decreased temperature.
Ebrahim and Ali [30] prepared three mixes with different formulations and cured hydrothermally at different temperatures
(45, 65, 85 °C) and time (5–20 h) after 1 and 7 days of procuring.
Longer procuring at room temperature, before the application of
heat is beneficial for higher strength development. In general, adequate curing of geopolymeric materials is required to achieve optimal mechanical and durability performance to maintain their
structural integrity [31].
81
(14 M activator solution) with 10–30 wt% aggregate exhibited an
acceptable flowability, while the mortars containing 40 & 50 wt%
aggregate were stiff and difficult to pack in the mould. Increasing
aggregate content in the mortar mixes leads to insufficient activator for complete geopolymerization of fly ash/slag. The activator
may also be utilised for wetting of aggregate leaving less availability for dissolution of these fly ash or slag particles. The compressive
strength of geopolymer mortars with high level of aggregate can be
achieved by optimising the amount of activator dosage [34].
Khandelwal et al. [35] summarised that the compressive strength,
modulus of elasticity and Poisson’s ratio of fly ash-based geopolymer mortars increased logarithmically with the increase of strain
rate. These engineering properties of geopolymer mortars compared favourably with those predicted by Standards/Codes for concrete mixtures. When bottom ash was used, the geopolymer
mortars exhibited a low compressive strength (20 MPa). With
10% replacement of sand by bottom ash, the mix exhibited a comparable compressive strength to those made with sand only. The
increase in strength (50–100%) of bottom ash mortar was also
reported when the specimens were exposed at 800 °C probably
due to activation of bottom ash [36]. When lignite bottom ash
was ground to a mean particle size of 15.7 lm (3% retained on
sieve No. 325), the compressive strength of mortars activated with
sodium hydroxide/sodium silicate was 24–58 MPa [37].
Brough and Atkinson [38] prepared geopolymer mortars using
slag, sand and activator in a ratio of 1:2.33:0.5. At water-to-total
solid ratio of 0.42, the mortar gained strength of 40 MPa. The
sodium silicate activated mortars exhibited higher compressive
strength with low levels of porosity at the interface while KOH
activated mortars were highly porous in the interfacial zone giving
low compressive strength values. Yang et al. [39,40] found that the
flow of alkali-activated mortars increased with the increase of
water-binder ratio and decrease of aggregate-binder ratio. When
the aggregate-binder ratio was larger than 2.5, the flow of mortars
decreased sharply. They also found that slag-based geopolymer
mortars exhibited much higher compressive strength but exhibited
slightly less flow than the fly ash-based geopolymer mortars for
the same mixing condition. The poor compressive strength of fly
ash-based mortars cured at low temperatures is attributed to the
presence of unreacted fly ash particles and large number of voids.
As the aggregate-binder ratio increased, the compressive strength
increased up to a ratio of 2.5 which indicated that the threshold
of aggregates in geopolymer mortars were slightly lower than
OPC mortars. The shrinkage strain of alkali-activated mortars was
also found to be lower than the OPC mortars.
3. Geopolymer mortars
4. Geopolymer concrete
Various studies [32–40] were conducted on flow and mechanical properties of geopolymer mortars because of their more relevant applications in building construction. The properties of
mortars were optimised with respect to initial flow, aggregate-binder ratio, activator-binder ratio and activator molarity.
Chindaprasirt et al. [32] reported that the compressive strength
of class C fly ash-based geopolymer mortar was 52 MPa when
cured at 70 °C for 3 days using sand-fly ash ratio of 2.75 at workable flow of 135 ± 5%. Prolonged curing at high temperatures led
to the reduction in the compressive strength because of weakening
of microstructure and increased porosity due to the loss of moisture. In another attempt [33], they produced geopolymer mortar
with a compressive strength of 86 MPa at 28 days with the help
of air classified class C fly ash (4500 cm2/g fineness) activated with
sodium silicate and NaOH (10 M) at 1:1 mass ratio. The dimensional change in terms of drying shrinkage (1–61 106 mm/
mm) was insignificant when compared with the Portland cement
mortar (700–850 106 mm/mm). The geopolymer mortars
4.1. Fresh and hardened properties
Various mix proportioning of geopolymer concrete (GPC) were
reported with target strength up to 80 MPa. The typical properties
of geopolymer concrete mixes used by the various authors (41, 45,
48, 49, 53) are summarised in Table 1. The properties of mixes
were studied with respect to water-geopolymer solid ratio, activator strength, water/Na2O ratio, curing time, curing temperature,
and age hardening. The slump of mixes varied depending on the
molarity of activator, workability aids and extra water added to
the mix [41]. The rheological parameters such as yield stress and
plastic viscosity were attempted over slump test of concrete to
assess its workability loss and flow behaviour. Yield stress gives
initial resistance to flow arose from the friction among the solid
particles while plastic viscosity governs the flow after it is initiated
resulting from viscous dissipation due to the movement of water in
the sheared material. Laskar & Bhattacharjee [42] studied the
82
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
Table 1
Typical properties of geopolymer concrete mixes.
Hardjito et al. [41]
Jimenez et al. [45]
Sofi et al. [48]
Diaz-loya et al. [49]
Pan et al. [53]
Density
(kg/m3)
Molarity
(M)
Slump
(mm)
CS
(MPa)
STS
(MPa)
FS
(MPa)
MOE
(GPa)
Poisson’s
ratio
Activator/
binder
ratio
Curing
temperature
and time
2330–2430
NR
2147–2408
1890–2371
1876–2555
10–16
8 & 12.5
NR
14
8
60–215
NR
NR
100–150
NR
30–80
29–43.5
47–56.5
10–80
65.1–77.9
3.74–6
NR
2.8–4.1
NR
2.8–5.1
5–12
6.86
4.9–6.2
2.24–6.41
NR
23–31
10.7–18.4
23–39
1.9–42
11.2–41.2
0.12–0.16
NR
0.23–0.26
0.08–0.22
0.15–0.19
0.35–0.4
0.4 & 0.55
0.45–0.59
0.4–0.94
0.4–0.65
60–80 °C for 24 h
85 °C for 20 h
23 °C till testing
60 °C for 72 h
60 °C for 24 h
CS: compressive strength; STS: splitting tensile strength; FS: flexural strength; MOE: modulus of elasticity; NR: not reported.
rheology of fly ash-based geopolymer concrete with slump varying
from 25 mm to flowing concrete with 1–20 M activator strength.
They found that the yield stress and plastic viscosity were affected
by the molar strength of the sodium hydroxide solution and the
ratio of silicate to hydroxide solution. The setting time of geopolymer concrete was reported up to 120 min. Like Portland cement
pastes and mortars, geopolymers also behave like Bingham fluid
and have a history dependent rheological profile, i.e., geopolymers
may be kept in a fluid form, if subjected to constant shearing for a
certain period of time before initial setting starts [43]. The setting
could be enhanced up to 180 min with the use of naphthalene
based admixture and extended mixing time especially in the case
of slag-based geopolymer which has the potential for a wide range
of technological applications [44].
Hardjito et al. [41] produced fly ash-based GPC with the compressive strength ranging between 30 and 80 MPa with the slump
Fig. 3. Correlations within the mechanical properties of fly ash-based geopolymer
concrete. (a) Splitting tensile strength vs compressive strength. (b) Modulus of
elasticity vs compressive strength [47].
varied from 100 to 250 mm (activator strength: 8–14 M). The optimum strength was obtained at 0.18 water-geopolymer solid ratio
cured at 90 °C. As the water-geopolymer solids ratio increased,
the compressive strength of GPC decreased analogous to the well
known relationship between compressive strength and water-cement ratio for OPC concrete. The compressive strength of GPC
remained unchanged with the age when tested after 24 h curing
at elevated temperature. Fernandez-Jimenez et al. [45] made fly
ash-based geopolymer concrete with a compressive strength of
45 MPa at 0.55 liquid/solid ratio cured at 85 °C for 20 h. The development of high early strength in GPC was explained by its compact
microstructure, formation of adequate reaction products, smaller
mean size of the pores and good aggregate-paste bond. They
observed that GPC has a much lower modulus of elasticity
(18.4 GPa) than the OPC concrete (30.3 GPa). Olivia and Nikraz
[46] proportioned fly ash-based geopolymer concrete mix with a
compressive strength of 55 MPa at 28 days and cured at different
temperatures in the range of 60–75 °C. The hardened mix had
higher tensile and flexural strengths, produced less expansion
and showed modulus of elasticity that were 15–29% lower than
that of OPC concrete mix. The drying shrinkage (0.025%) of GPC
was less than the OPC concrete (0.09%) after 12 weeks. The minimal shrinkage of GPC may also be due to the significant resistance
offered by its zeolitic microstructure towards drying loss of the
water incorporated during casting [45].
Several attempts [47–53] have also been made to establish
correlations within the mechanical properties of geopolymer concrete. It was reported that the experimental splitting tensile
strength of fly ash-based GPC was higher than the OPC concrete
(Fig. 3). The increased strength is accounted for a denser interfacial
zone established between the aggregate and geopolymer paste.
The modulus of elasticity increased as the compressive strength
of GPC increased. The modulus of elasticity of GPC was found to
be lower than the values predicted by ACI guidelines for OPC concrete. Sofi et al. [48] studied the engineering properties of fly ash/
slag-based GPC. The splitting tensile strength and flexural strength
of GPC were comparable to those models presented by the
Australian Standard (AS 3600) for OPC concrete. Although, the difference between splitting tensile and flexural strength of GPC
mixes has been found to be approximately 2 MPa, similarities
between the strength gain was apparent. Diaz-Loya et al. [49] prop
posed the equation ‘‘fr = 0.69 fc0 MPa’’ for correlation between the
flexural strength (fr) and compressive strength and the equation
p
‘‘Ec = 580 fc MPa’’ for correlation between elastic modulus (Ec)
and compressive strength of GPC (fc = compressive strength).
When compared with the typical Poisson’s ratio value of OPC concrete (0.15–0.22), the values of GPC appeared to reside toward the
low end of range (0.08–0.22). Ryu et al. [50] suggested a model for
relationship between compressive strength and splitting tensile
strength (fsp = 0.17 (f0 c)3/4) for fly ash-based GPC. Bondar et al.
[51] reported a relationship between ultrasonic pulse velocity
and compressive strength of GPC. They found that GPC showed a
lower ultrasonic pulse velocity than the OPC concrete even those
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
with the same or higher compressive strength. It was also reported
[52,53] that GPC was brittle as compared to its OPC counterpart
due to the highly cross-linked framework. The fracture energy of
GPC was also low because of its higher bond with aggregates as
compared to OPC concrete [54].
4.2. Interfacial transition zone
It is well known that the interfacial zone (ITZ) between aggregate and matrix is the weakest link in OPC concrete at which
micro-cracks usually first develop under loads [55]. Investigation
of this zone is very crucial since it is known to have different
microstructure from the bulk of the hardened paste. The high
porosity of ITZ allows the easier penetration of external agents
such as chlorides, oxygen, sulphates, etc. into concrete structure.
Contrary to this, ITZ of GPC has been identified as being dense
and much less microstructurally distinct from the bulk of binder
region [56,57]. The stronger ITZ contributes to higher splitting tensile strength, bond strength and durability of the GPC.
Lee and Deventer [56,57] discussed interface between the natural siliceous aggregates and paste in GPC using kaolin and albite as
precursors. The increase in concentration of the activating solution
increased the binding capacity of the gel with natural aggregates.
The presence of chloride salts decreased the interfacial bonding
strength between the paste and aggregate probably by causing
gel crystallisation near the aggregate surfaces which resulted in
debonding. In another attempt, they found that the addition of
0.5 M soluble silicate into an activating solution (10 M NaOH and
2.5 M sodium silicate) facilitates the formation of an aluminiumenriched aluminosilicate surface onto the aggregates through
accelerated Si-preferential dissolution of kaolin and albite. The surface formed during albite leaching was found to possess a similar
Si/Al ratio to the real interface between a silicious aggregate and
fly ash/metakaolin geopolymer paste activated with 10 M NaOH
solution. Without soluble silicates, no deposited aluminosilicate
interface was observed. This suggested that both high concentration of alkali and soluble silicate are essential for the formation
of a strong interface between silicious aggregates and geopolymer
pastes. Zhang et al. [58] reported that at the beginning, there were
many large voids in the fresh ITZ in potassium poly(sialate)
geopolymer concrete. As hydration proceeded, these voids were
completely filled with the hydration products. At this stage, the
difference in the microstructure between the ITZ and matrix was
hardly distinguishable. The contents of K/Al and Si/Al in the ITZ
were higher than those in the matrix. Demei et al. [59] presented
Fig. 4. Bond strength of fly ash-based geopolymer concrete as a function of steel bar
diameter [47].
83
FESEM analysis of ITZ in the fly ash-based self compacting geopolymer concrete with varying superplasticizer dosages. They reported
that relatively a loose and porous ITZ was found at low superplasticizer dosages (3%) whereas a dense ITZ was found between
the aggregate and geopolymer paste at higher dosage (7%). They
also found that the compressive strength increased with decrease
in the thickness of ITZ and this relationship depends on the superplasticizer dosage.
4.3. Bond between reinforcing bars and geopolymer concrete
The transfer of forces across the interface between concrete and
reinforcing steel bar is of fundamental importance in the structural
design [60]. Bond stresses in the reinforced concrete arise from two
distinct situations. The first is anchorage or development where
bars are terminated. The second is flexural bond or the change of
force along a bar due to a change in bending moment along the
member. The bond strength of reinforcing bars with concrete is
governed by several factors such as the strength of the concrete,
the thickness of the concrete surrounding the reinforcing bar, the
confinement of the concrete due to transverse reinforcement and
the bar geometry. Generally, the bond strength between the
reinforcing bar and matrix increases with increasing steel bar
diameter and compressive strength of GPC (Fig. 4). There is a
greater amount of slip for larger size rebars in GPC.
Sarker [61] found that the bond strength of fly ash-based GPC
increased with the increase of concrete cover-bar diameter ratio
(1.71–3.62) and the concrete compressive strength (25–29 MPa).
He also observed that GPC has higher bond strength than the
OPC concrete because of higher splitting tensile strength and dense
interfacial transition zone between the aggregate and geopolymer
paste. Bond-slip behaviour [45] of GPC showed that the embedded
steel bar of 8 mm dia broke before slipping and concrete cracking
whereas the bar embedded in OPC concrete slipped. For
16 mm bar, GPC failed by matrix cracking while the bars in OPC
concrete were again observed to slip. Sofi et al. [62] reported that
the values of bond strength of steel bars in fly ash-based GPC were
comparable in both beam-end as well as direct pullout specimen
tests. The normalised bond strength increased with a reduction
in rebar size. The bond strength tested according to AS 3600, ACI
318-02 and EC2 recommendations showed that these Codes are
applicable and also safe to predict the developmental length for
GPC.
Attempts were also made to study behaviour of reinforced fly
ash-based GPC beams and columns with respect to longitudinal
tensile reinforcement ratio and concrete compressive strength as
test variables [63–66]. Sumajouw et al. [63] reported that the
flexural capacity of beams increased with the increase in tensile
reinforcement (0.64–2.69%) but the effect of concrete compressive
strength was marginal. The ductility index increased significantly
for beams having longitudinal reinforcement ratio less than 2%.
They also studied the strength of reinforced GPC slender columns
with respect to the compressive strength of concrete, longitudinal
reinforcement ratio and load eccentricity. The design provisions
mentioned in the Standards for OPC concrete can be used for
designing geopolymer concrete columns also. Dattatreya et al.
[64] found that the load carrying capacity of reinforced slag-based
GPC beams was 17.7% more than the Portland pozzolana cement
concrete beams at 2.68% tension reinforcement. Yost et al. [65]
indicated that load–deflection behaviour of GPC beam was identical to OPC beam. The maximum strain obtained for under-reinforced beam was less than 3000 microstrains which is generally
assumed for design work. The predicted neutral axis depth was
15% less than the experimentally achieved value for GPC. The
Whitney’s stress block for strength calculation was found applicable for GPC also. Ng et al. [66] investigated potential use of steel
84
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
fibres (up to 1.5 wt%) to replace conventional shear reinforcement
in GPC beams of 2250 mm span length. They found that the
increase in fibre volume led to an increase in the cracking load
and the ultimate shear strength. A good correlation of test data
was observed with the predictive fib Model Code 2010.
carbonation, alkali-silica reaction and freeze–thaw attack. In view of
this, several studies are being carried out to understand the behaviour of geopolymers exposed to these conditions.
4.4. Fire behaviour
Alkali-silica reaction (ASR) causes gradual but severe deterioration of hardened Portland cement concrete in terms of its
strength loss, cracking, volume expansion etc. It involves the reaction between the hydroxyl ion in the pore solution within the concrete matrix and reactive silica of the aggregate. In general terms,
the reactions will proceed in stages, with the first stage being the
hydrolysis of reactive silica by hydroxyl ions to form alkali-silica
gel and a later secondary overlapping stage being the absorption
of water by the gel, which will result in increase of volume [72].
In general, concrete has good property with respect to fire resistance. However, it is known that the residual strength of OPC concrete after firing between 800 °C and 1000 °C does not exceed 20–
30% normally because of dehydration and destruction of C-S-H &
other crystalline hydrates, aggregate types, permeability etc. Fire
introduces high temperature gradient and as a result, the hot layer
tends to separate and spall from the cooler interior layer of the
body [67]. Contrary to this, geopolymers possess good fire resistance at elevated temperature because of the existence of highly
distributed nano-pores in the ceramic like microstructure that
allows physically and chemically bonded water to migrate and
evaporate without damaging the aluminosilicate network [4].
During fire, several events such as evaporation of water adsorbed
by N-A-S-H gel, formation of anhydrous products, crystallization
of stable anhydrous phases and melting (sintering) leading to
destruction generally occurred. The phase transformation of
geopolymers during fire is depicted below.
Kong et al. [68] found that the residual strength of fly ash-based
geopolymer pastes increased by 6% after exposure to 800 °C,
whereas the strength of metakaolin-based geopolymer pastes was
reduced by 34%. During heating, the high permeability of fly ashbased geopolymer provides the escape route for moisture in the
matrix, thereby decreasing the damage. The strength increase is
also partly attributed to the sintering reaction of unreacted fly ash
particles. Geopolymer pastes made with metakaolin and potassium
based activator showed an enhanced post-elevated temperature
performance compared to sodium based activator system. The
strength deterioration reduced with increasing Si/Al ratio (>1.5)
[69]. Aggregate size larger than 10 mm resulted in good strength
performance in both ambient and elevated temperature (800 °C).
The strength loss in fly ash-based geopolymer concrete at elevated
temperatures is attributed to thermal mismatch between the
geopolymer paste and aggregate [70]. No spalling was reported in
the samples by Zhao and Sanjayan [71] when fly ash-based GPC
with compressive strength ranging from 40 to 100 MPa was
exposed to 850 °C. They also found that at the same strength level,
GPC possessed higher spalling resistance under fire than the OPC
concrete due to its increased porosity.
5.1. Alkali-silica reaction
(i) Acid-based reaction
H0:38 SiO2:19 þ 0:38NaOH ! Na0:38 SiO2:19 þ 0:38H2 O
ð3Þ
(ii) Attack of the siloxane bridges and disintegration of the silica
2
Na0:38 SiO2:19 þ 1:62NaOH ! 2Na2þ þ H2 SiO4
ð4Þ
In geopolymer concrete, the un-utilised alkali after geopolymerization of aluminosilicates is expected to react with the silica of the
aggregates causing disruption of their siloxane bridges. It is
reported that geopolymer mortars using aggregates of different
reactivities expanded less than the corresponding Portland cement
mortars [73]. The geopolymer mortars appeared to be sound without any surface cracking. The cause of expansion in slag-based
geopolymer mortars is the formation of sodium calcium silicate
hydrate reaction product with rosette-type morphology [74].
Contrary to this, there was no significant expansion in fly ashbased geopolymer mortars. The formation of crystalline zeolites
was very slow and since these minerals are usually found in the
gaps of the matrix, the existence of stress that might generate
cracking is unlikely [75]. Geopolymer mortar bars made with fly
ash/slag blends expanded less than 0.1% limit prescribed in ASTM
C1260-07 after 16 days (Fig. 5). At 90 days exposure, these mortars
failed to meet the specified criteria. Increasing slag content in fly
ash/slag mix increased the expansion of resulting systems [76].
ASR has also been claimed to be helpful in providing a strong bond
at the paste-aggregate interface, thus enhancing the tensile
strength of GPC [8]. Patil et al. [73] indicated that sandstone, quartz
and limestone aggregates in geopolymer concrete were not prone
5. Durability studies
One of the major problems associated with OPC concrete is its
long term durability which had always been an issue against
aggressive environments. The deterioration of concrete is usually
assessed for sulphate attack, chloride induced corrosion, atmospheric
Fig. 5. Alkali-silica reaction in various geopolymer and OPC mortars under an
accelerated condition (1 M NaOH) at 80 °C [76].
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
85
to ASR. During accelerated mortar bar test, a slight expansion was
noticed because of re-initiation of the geopolymerization process
of unreacted fly ash particles leading to lower porosity and higher
strength. The lower sensitivity of reactive aggregates in GPC provides economic advantages in areas where high quality deposits
of aggregates have been depleted.
5.2. Effect of acid attack
The acid resistance of geopolymer pastes/concrete was studied
by several authors [77–84]. The extent of degradation depends on
the concentration of acid solution and period of exposure.
Davidovits et al. [8] indicated that metakaolin-based geopolymer
pastes showed only 7% mass loss when sample was immersed in
5% H2SO4 for 30 days. It was also reported that fly ash-based
geopolymer pastes retained a dense microstructure after 3 months
exposure in HNO3. Temuujin et al. [77] concluded that acid and
alkaline resistance of fly ash-based geopolymer strongly depend
on its mineralogical composition. High solubility of Al, Si and Fe
ions was obtained in both strong alkali and acid solutions. The performance of fly ash-based geopolymer pastes when exposed to 5%
acetic acid and 5% H2SO4 solutions was superior to ordinary
Portland cement pastes. The deterioration in pastes was connected
to depolymerisation of the aluminosilicate network and formation
of zeolites [78].
ð5Þ
Wallah and Rangan [41] found that the reduction in compressive strength of fly ash-based GPC in 0.5% H2SO4 solution was
20% after 12 months exposure. This value was 52% and 65%
respectively when samples exposed to 1% and 2% H2SO4 solution.
Pitting and erosion on the surface of the concrete were also
observed. The loss in strength of concrete is mainly due to the
degradation in the geopolymer matrix rather than the aggregate.
They concluded that the acid resistance of GPC was superior to
OPC concrete. Ariffin et al. [79] exposed GPC made with a blend
of pulverized fuel ash and palm oil fuel ash in 2% solution of sulphuric acid for 18 months. The weight loss in GPC was 8% while
OPC concrete exhibited 20% weight loss. The strength reduction
in GPC was 35% in 18 months as against 68% strength loss in OPC
concrete after 30 days and was severely deteriorated after
18 months. The C-S-H could have severe deleterious effect on
OPC concrete while N-A-S-H gel appeared to have little effect on
the structure of GPC. Sathia et al. [80] reported the weight loss in
concrete samples was less than 5% after 3 months exposure in 3%
H2SO4 solution. Bakharev et al. [81] found that slag-based GPC
(40 MPa) exhibited 33% reduction in strength compared to 47%
in OPC concrete when exposed in acetic acid solution (pH 4) for
12 months. The slag particles and low calcium C-S-H with average
Ca/Si ratio of 1 were more stable in the acid solution than the constituents of the OPC pastes. During immersion in 2% H2SO4 solution, the strength loss was 11% compared to 36.2% for OPC
concrete.
Fig. 6. Atomic force microscope images of fly ash-based geopolymer exposed under
sulphate after 4 months [86].
5.3. Effect of sulphate attack
Fly ash-based geopolymer pastes did not deteriorate significantly, under the influence of water, sodium sulphate (4.4%)
and ASTM sea water [82]. Only some fluctuations in flexural
strength were observed between 7 days and 3 months exposures.
The least strength change was observed in the pastes exposed in
the 5% Na2SO4 and 5% MgSO4 solutions while most significant
deterioration was observed in the 5% mixed sulphate solution
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B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
(Na2SO4 + MgSO4) after 5 months exposure [83]. In fly ash/slag
system, the extensive physical deterioration of pastes was
observed during immersion in MgSO4 solution after 3 months
exposure but not in Na2SO4 solution. The calcium sulphate dihydrate formed in paste was identified as being particularly damaging to the materials in MgSO4 [84]. Atomic force microscopic
images of fly ash-based geopolymer pastes exposed to sulphate
environment are shown in Fig. 6. In the case of Na2SO4 solution,
only exposition of grains was clearly visible while in MgSO4 solution, both exposition of grains and dissolved aluminosilicate
matrix were observed showing severity of MgSO4 attack [85].
The deterioration is considered mainly due to the destruction of
aluminosilicate skeleton, liberation of silicic acid, leaching of
sodium ion etc. [86]. These reactions seem to have significant
effect on the mechanical strength. The geopolymer prepared with
NaOH activator had the best performance over those made with a
synergistically used sodium silicate and NaOH/KOH activators,
which is attributed to its stable cross-linked aluminosilicate polymer structure.
Several attempts [41,87] have been made to study sulphate
resistance of GPC. The deterioration in concrete was evaluated in
terms of its visual appearance, weight loss and change in compressive strength. Hardjito et al. [41] observed that there was no significant effect of 5% Na2SO4 solution in the compressive strength,
the weight loss and the dimension of fly ash-based GPC after
3 months exposure. Rajamane et al. [87] reported sulphate resistance of fly ash-based GPC for 3 months in 5% Na2SO4 and 5%
MgSO4 solutions. The weight loss in samples was 2.4% only.
There was 2–29% loss of compressive strength as compared to 9–
38% in the OPC concrete. The deterioration of OPC concrete can
be attributed to the formation of expansive gypsum and ettringite
which can cause expansion, cracking and spalling in the concrete.
Contrary to this, GPC in general do not contain Ca(OH)2 and monosulphoaluminate in the matrix to cause expansion.
5.4. Carbonation and permeability
Bernal et al. [88] studied slag/metakaolin-based GPC (w/b ratio
0.47) under an accelerated carbonation test using CO2 concentration of 3.0 ± 0.2% at 20 °C for 28 days. They found that the compressive strength decreased monotonically as the carbonation
proceeds. The relationship between the pore volume and extent
of carbonation was much more similar with samples with different percentages of metakaolin contrary to the slag-based samples.
This suggested that porosity is not the only parameter controlling
the strength loss of the carbonated binder. There must be a
convoluting effect due to the binder gel chemistry, which determines the residual level of strength after an accelerated carbonation. Olivia and Nikraz [46] reported lower water permeability
(2.46–4.67 1011 m/s) of GPC (activator-fly ash ratio, 0.30–0.40
cured at 60 °C for 24 h) than the OPC concrete due to its denser
paste and smaller pore inter-connectivity. They also reported that
the water-geopolymer solids ratio was the most influential
parameter that affects the properties of GPC. Bondar et al. [51]
studied the oxygen and chloride permeability of alkali-activated
concrete made with the Iranian natural pozzolan (Taftan andesite
and Shahindej dacite). They concluded that alkali-activated natural pozzalona concrete has 10–35% lower oxygen permeability at
normal curing conditions for 90 days compared with the OPC concrete. The rapid chloride permeability test gave high values for
the alkali-activated concrete. This is probably due to the very
high alkali ion concentration in the pore solution promoting
higher electrical conductivity in the GPC. This effect seems to
reduce with age due to a change in the porosity of the GPC
microstructure.
5.5. Corrosion of steel reinforcement
Corrosion potential is a technique used to detect the state of
reinforcement without disturbing the structures. This is important
because the intensity of corrosion of steel in concrete is generally
known only after the concrete has cracked or disrupted. Various
studies [46,80,89] were reported to estimate the corrosion potential of steel within the GPC as per ASTM C876. Olivia and Nikraz
[46] reported that the half cell potential of GPC was lower than
the specified value of 404 mV mentioned in the Standard for severe corrosion after 91 days. Sathia et al. [80] also reported corrosion potential up to 300 mV which showed a probable
corrosion indication due to the lower pH of concrete during the
half-cell potential measurement. Accelerated corrosion results
showed that GPC mixes exhibited low level corrosion activity
and time to failure that were 3.86–5.70 times longer than those
of the OPC concrete. Under impressed voltage, a crack appeared
suddenly in the concrete when time to failure was reached and this
was followed immediately by high current reading. The large
amounts of fly ash and alkaline activators in the GPC mix increased
the availability of ions that can produce high electrical resistance at
high impressed voltage. This enhanced the cathodic reaction and
reduces the rate of corrosion, which in turn, reduces the tensile
stress of the specimens, thus decreasing the risk of cracking and
clearly extending the time to failure [46]. Reddy et al. [89] compared the durability of GPC with that of OPC concrete exposed to
marine environment for a period of 21 days. The initial corrosion
current measured for GPC (71–91 mA) was much lower than that
of OPC concrete (772 mA). The OPC specimens initially recorded
decrease in the current but later started increasing while the GPC
current never showed significant increase.
6. Research and development at CSIR-CBRI
A systematic R&D work is initiated at CSIR-Central Building
Research Institute, Roorkee on the development of heat and ambient cured geopolymers using fly ash, slag and other aluminosilicates as precursors. In view of variability in the constituents of
fly ash, the property optimisation of geopolymeric pastes was carried out as a function of activator concentration and its dosage,
water-geopolymer solid ratio, curing time and curing temperature
[13]. Geopolymerisation reaction, thermal stability, identification
of bond linkages and microstructural features were analysed by
various techniques such as quasi isothermal DSC, TGA, FTIR and
FESEM. The durability of geopolymer pastes/mortars was also
studied in terms of alkali-silica reaction and also in acidic and sulphate environments for 4 months [76,86]. The suitability of these
geopolymer pastes was assessed in making various geopolymeric
products such as mortars & concrete, bricks, solid & hollow blocks,
insulation concrete, foam, sandwich composites and temperature
resistant coatings (Fig. 7(a–c)). Attempt was also made to utilise
lime sludge, a waste from paper industry with the geopolymeric
binders for making paving blocks.
Fly ash-based GPC mixes were made with the compressive
strength of 25–55 MPa using absolute volume method adopted
for OPC concrete mixes. The strength of GPC increased with
decreasing water-geopolymer solid ratio as it is said analogous to
the water-cement ratio of the OPC concrete. The compressive
strength increased with increasing molarity of the activator (10–
16 M) probably due to the formation of stable aluminosilicate networks following the dissolution of silica and alumina in the solution from the fly ash. It was found that the splitting tensile
strength of GPC was more than those of predicted values as per
ACI 318 guideline and other existing empirical equations. A trend
line curve between the compressive strength and modulus of
B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
87
Fig. 7a. Light weight fly ash-based geopolymer concrete sheets using EPS beads and in-situ foaming [85].
Fig. 7b. Fly ash-based geopolymer bricks [85].
Fig. 7c. Fly ash-based geopolymer solid and hollow blocks [85].
elasticity showed that the elastic modulus was lower (17%) than
the one predicted by Ivan Diaz-Loya et al. for GPC and also the values obtained with ACI guidelines. As expected, the bond strength of
steel bar embedded in GPC increased with increasing steel bar
diameter and compressive strength of concrete. It was noted that
the bond strength between geopolymer paste and reinforcing bars
was found to be higher than the OPC concrete [47].
Light weight geopolymer concrete was proportioned with the
help of fly ash, activators, expanded polystyrene beads (EPS – up
to 3 wt% or 91 vol%), admixtures and fine & coarse aggregates
(Fig. 7(a)). It was noted that a decrease in the strength was more
when larger size of EPS beads (<4.75 mm) were added in the mortars probably due to their less surface area/volume ratio. By adding
coarse aggregate, the compressive strength (18 MPa) and density
(1500 and 1840 kg/m3) of EPS geopolymer concrete, comply the
minimum specified criteria of ACI 213R-03 guidelines for structural light weight concrete (compressive strength, 17 MPa; density
1120–1920 kg/m3). To meet the requirement of insulation concrete, the addition of 20% coarse aggregate (10 mm maximum size)
into EPS/geopolymer mix exceeds its compressive strength
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B. Singh et al. / Construction and Building Materials 85 (2015) 78–90
Fig. 8. Fire test of fly ash-based geopolymers as per BS 476 showing surface of very low spread of flame [90].
(15 MPa) as specified (13.1 MPa) in ASTM C 90. Regarding fire
performance, the samples were non-ignitable and exhibited Class
I-very low spread of flame as per BS EN-476 part 7 (Fig. 8). It
was noted that the fire propagation index of the samples was <3
exhibiting no support to fire growth. Flammability data obtained
from a cone calorimeter showed that the insulation concrete had
insignificant heat release rate (9.63 kW/m2) and effective heat of
combustion (3.75 MJ/kg). The thermal conductivity of insulation
concrete was found in the range of 0.427–0.852 W/mK. It was concluded that light weight concrete can be engineered by proper
selection of variables in making insulating materials for use in
buildings [90].
A geopolymer foam composition has been developed using fly
ash, activator, filler, surfactant, buffer and strengthening agent. It
sets at room temperature within 2 h and completely cured after
24 h. The density of foam was lying in the range of 600–800 kg/
m3. It can be easily prepared by a simple mixing followed by pouring into mould. SEM examination revealed that pores in the samples were uniformly distributed. The flammability test carried
out by a cone calorimeter showed that the total heat released, mass
loss, smoke release and CO/CO2 yield were insignificant. It resists
against fire between 700 °C and 800 °C. The foam can be used as
a core material in the sandwich and insulation panels [85].
Geopolymer bricks of size 230 115 75 mm were produced
using fly ash-based pastes, coarse fly ash and natural sand
(Fig. 7(b)). The bricks were cured at 80 °C for 2 h. The bricks were
obtained with density ranged between 1920 and 2100 kg/m3,
water absorption, 10–15% and dry compressive strength, 12–
25 MPa depending on activator concentration. These bricks can
be easily jointed with ordinary cement mortars [85].
The solid geopolymer blocks of size 300 200 150 mm were
produced on a machine using coarse aggregate (1180 kg/m3), fine
aggregate (296 kg/m3), flyash (494 kg/m3), activator (111 kg/m3)
and water (55.55 kg/m3). The properties of blocks are: density,
2100 kg/m3; compressive strength, 9.87 MPa; water absorption
(24 h), <10%; drying shrinkage, <1%. The hollow blocks of size
400 300 200 mm were also produced on a block making
machine. The properties of blocks are: density, 1200 kg/m3; compressive strength, 5 MPa; net weight, 20 kg. It was noted that the
cost of solid and hollow geopolymer blocks was about 15% and
10% higher than the OPC concrete blocks (Fig. 7(c)).
7. Conclusions
Based on the discussions, it is concluded that geopolymer concrete has considerable potential to be used as a construction
material in several applications. A number of key properties have
been investigated and very high strengths have been attained.
The design provisions mentioned in ACI guidelines and other
National Code for OPC concrete are reported to be applicable for
geopolymer concrete also. The production of ready mixed geopolymer concrete can be achieved which represents the successful
implementation of a technically very challenging product.
However, it presents significant scientific challenges associated
with the need for a better understanding of the setting reactions
involved, the relationship between mix design characteristics, the
short and long term mechanical properties and overall durability.
Although, significant progress was made, there is an immense need
to work out generalisation of water-geopolymer solids ratio, bond
between reinforcement and geopolymer paste, structural behaviour of reinforced GPC members, corrosion of reinforcement in
geopolymer concrete etc. An appropriate code of practice for
geopolymers and their products need to be formulated based on
research data and field data for mass adaptation by the users. It
is felt that the widespread uptake of geopolymer technology is hindered by a number of factors, in particular issues to do with a lack
of long term-durability data. In this relatively new research field,
there are also difficulties in compliance with regulatory standard,
specifically those defining chemical composition in cement.
Conventionally, geopolymer binders require heat curing, high
pH and also have difficulty in field handling. Therefore, efforts
are needed to develop a room temperature cured one component
geopolymer system using solid activators instead of alkaline solutions in view of its wider acceptance in the field.
Acknowledgements
This paper forms part of a Supra Institutional Project of CSIR
R&D program (Govt. of India) and is published with the permission
of Director, CSIR-Central Building Research Institute, Roorkee.
Authors are also thankful to Ms. Sarika Sharma, Mr. Ankur Singh,
Mr. Rakesh Paswan and Mr. Md. Reyazur Rahman for their help
during the work.
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