Performance of fly ash based geopolymer concrete made using non-pelletized fly ash aggregates after...

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Materials and Structures ISSN 1359-5997 Mater StructDOI 10.1617/s11527-014-0404-6

Performance of fly ash based geopolymerconcrete made using non-pelletized flyash aggregates after exposure to hightemperatures

M. Talha Junaid, Amar Khennane &Obada Kayali

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ORIGINAL ARTICLE

Performance of fly ash based geopolymer concrete madeusing non-pelletized fly ash aggregates after exposureto high temperatures

M. Talha Junaid • Amar Khennane •

Obada Kayali

Received: 22 May 2014 / Accepted: 19 August 2014

� RILEM 2014

Abstract The superior performance of geopolymer

paste, mortar and concrete (GPC) when exposed to

high temperatures has been well documented. Limi-

tations still exist however due to the use of similar

aggregates, coarse and fine, in both ordinary portland

cement concrete (OPC) and GPC. The behavior of

geopolymer concrete at elevated temperatures can be

further improved by using a recently developed

lightweight non-pelletized aggregates made entirely

from fly ash (named Flashag) as replacement for

conventional natural aggregates. Tests for the ambient

and residual properties of GPC made using these new

aggregates exposed to temperatures up to 1,000 �C

were carried out. The obtained results in terms of

compressive strength, splitting strength, and modulus

of elasticity are reported and compared with those

obtained for GPC produced using ordinary natural

aggregates as well as with those reported in the

literature for OPC concretes. The effects of heat

cycling, as well as, duration of heating on the strength

and deformational behavior of GPC are also investi-

gated. At ambient temperatures all the three concretes

displayed similar properties. At elevated tempera-

tures, however, the performance of GPC made from

Flashag was found to be far superior to that of both

OPC, and GPC made from ordinary aggregates.

Additionally, for up to 4 heating cycles, GPC made

with Flashag better retained its strength and stiffness

properties as compared to GPC made with ordinary

natural aggregates. The study also found that most

changes to the strength and microstructure of GPC

occur in the first few hours of exposure after which, the

duration of heating has no significant effect.

Keywords Geopolymer concrete stress–strain

curves � Modulus of elasticity � Lightweight GPC �Cyclic heating � High temperature performance

1 Introduction

Alkali liquids (usually a soluble metal hydro-oxide

and/or alkali silicate) can be used to react with silica

(SiO2) and alumina (Al2O3) rich natural source, like

metakaolin or with industrial by-products, like fly ash

(FA), silica fume (SF), rice husk ash (RHA) or Slag to

produce binders [5, 6, 25]. Such binders mixed with

typical coarse and fine aggregates can form concrete,

usually known as alkali activated concrete or geo-

polymer concrete. Currently there is sufficient evi-

dence to suggest that geopolymer concrete has a better

resistance to fire than OPC concrete [7, 8, 10, 11, 31].

Yet the intrinsic good fire behaviour of geopolymer

paste, and to some extent mortars, tends to be reduced

by the addition of conventional aggregates. Therefore,

M. T. Junaid (&) � A. Khennane � O. Kayali

School of Engineering and Information Technology,

UNSW Canberra, Northcott Drive, Canberra, ACT 2600,

Australia

e-mail: [email protected]

Materials and Structures

DOI 10.1617/s11527-014-0404-6

Author's personal copy

to fully utilize the superior fire resistance of GPC,

aggregates more compatible with the GPC matrix are

needed. Such aggregates are available, and were

developed by Kayali [16]. They are lightweight and

entirely manufactured from fly ash and referred to as

Flashag. They are expected to perform in synergy with

the geopolymer binder, which itself contains a large

amount of fly ash.

The focus of this research therefore is to investigate

whether the use of Flashag improves the thermal

performance of GPC. Additionally the environmental

benefits of geopolymer concrete made entirely from

fly ash (binder, fine and coarse aggregates) are

enormous.

2 Nature and characteristics of geopolymer

concrete

The basic reaction mechanism for formation of the

geopolymer binder consists of three stages; namely:

dissolution of Si and Al from the source material (in

this case FA), hydrolysis or gelation, and condensation

forming a 3D network of silico-aluminates also termed

as the ‘geopolymer backbone’ [5–8, 10, 11, 31].

Davidovits describes geopolymerization as an exo-

thermic reaction, and has schematised it as shown

below [6].

GPC does not require any hydraulic binder. The

role of FA in GPC is entirely different from that it

plays when used as a cement replacement material in

OPC concrete to enhance certain properties such as

workability or to reduce the heat of hydration. In such

cases FA has no pronounced effect on the strength of

concrete [27] especially early strength. However, in

GPC, Fly Ash is the sole source of aluminosilicates for

reaction with the alkaline solution to form the binder,

and is thus a critical factor in strength development.

3 OPC and GPC concrete behaviour at high

temperatures

Owing to its very different chemistry as compared to

OPC, GPC behaves very differently when exposed to

high temperatures. When OPC concrete is taken to high

temperatures, the changes in its properties are highly

irreversible [32]. It is widely accepted that OPC loses

over 80 % of strength when the temperature exceeds

800 �C. This is due to inherent physical stresses caused

by the introduction of heat energy and the chemical

breakdown of the hydrated phases in the cement paste

[12]. There have been a number of general findings

published in the literature that cover the effects of high

temperature on OPC concrete, though the extent may

depend on several factors like water-cement ratio,

curing condition, aggregate type and size, type of

admixture, and so on. These are summarised as:

1. The loss of free water and chemically bound water

in the Ca(OH)2 through dehydroxylation leads to

significant dehydration of the concrete and loss of

chemical bonding and induces spalling [12].

2. The disparity in the amount of thermal expan-

sion in the aggregate in comparison to the

shrinkage of the OPC cement means that micro

stresses are induced and lead to cracking in the

concrete.



3. Different types of aggregate have a significant

effect on the strength-temperature relationship of

concrete. For instance limestone aggregates

undergo a decarbonation at temperatures above

800 �C. This can be avoided by using siliceous

aggregates [32]. Yet quartz also undergoes a

phase change at around 570 �C (configuration a to

b), which results in very high thermal expansion.

4. The rate of heating has little effect on the sample

so long as the temperature gradient is below 10�per centimetre towards the core [32].

In previous research focusing on the residual

properties of geopolymer paste and mortar, it has

been found that fly ash based geopolymers retain most

of their residual strength after being exposed to

elevated temperatures [8, 9, 12, 13, 21–23, 29, 31].

However, the degree of strength retention is dependent

on the composition of fly ash used [8, 31]. The

relatively high degree of residual strength retention is

attributed to the fly ash based geopolymer having a

large number of small pores that act as a conduit for

water as it escapes the structure. More importantly, as

geopolymer concrete is not a hydraulic binder, it is

therefore hypothesised that moisture will have a less

pronounced effect on its thermal and mechanical

properties than it does on the properties of OPC

concrete. This means that there is minimal damage to

the geopolymer structure. It has been reported

however, that thermal testing of geopolymer using

standard fire testing profiles shows that the presence of

water plays an important role in its behaviour [30].

The high strength retention can also be due to the

gradual sintering of the un-reacted fly ash that is

present within the concrete [21–23]. Sintering occurs

when very fine powders (such as fly ash) are subject to

very high temperatures causing the molecules to

diffuse to the boundaries of the particles, which results

in their partial fusion. This means that the unused fly

ash powder, which is responsible for local weaknesses

before exposure, sinters into a strong solid material

within the concrete after exposure (Fig. 1).

4 Materials and methods

Since extensive research is available on the perfor-

mance of OPC at elevated temperatures [4, 17–20, 24,

27, 28, 32], the current research focuses on comparing

the residual properties of GPC with normal and

Flashag aggregates. Grade D sodium silicate and

12 M sodium hydroxide prepared from 98 % pure

flakes were used as alkaline solutions while ASTM

Class F Fly Ash was used as the main aluminium and

silicate source. The chemical composition of the FA

used as found out by XRF is given in Table 1, while

the particle size distribution for natural and Flashag

aggregates is given in Fig. 2.

Table 1 Chemical composition of fly ash using XRF

Compound SiO2 Al2O3 Fe2O3 CaO K2O TiO2 MgO Na2O SO3 L.O.I SiO2/Al2O3

% Composition by weight 62.19 27.15 3.23 1.97 0.89 1.06 0.4 0.3 0.07 1.75 2.29

Fig. 1 SEM images of GPC a before exposure and b after temperature exposure to 1000 �C



Cylinders with nominal diameter and height of 75

and 150 mm respectively, were mixed and cast using the

procedure described in [14]. After an initial rest period

of 24 h, the samples were cured for 72 h at 80 �C. Then,

they were placed in an environmental chamber with a

relative humidity of 50 % and a temperature of 23 �C

till the time of testing. Samples containing Flashag are

identified as M4Sp, while M1 and M13 contain ordinary

natural aggregates. The composition of the mixes and

the mean compressive strength (f0

c) at ambient temper-

ature are given in Table 2.

The samples marked M13 and M4Sp were heated at

4.5 �C/min until they reached the target temperatures

(200, 400, and 1,000 �C). The target temperature was

then maintained to ensure that thermal equilibrium

was achieved. The time required for thermal equilib-

rium was determined by placing samples with embed-

ded K-type thermo-couples at the core of selected

samples. The time required to achieve consistent and

uniform core temperature was determined to be 2 h.

The furnace was then turned off and the samples were

left in the chamber to cool down to ambient temper-

ature. The samples were then removed from the

chamber, and tested for compressive strength (f0c),

modulus of elasticity (Ec) and splitting tensile strength

(fct). Strain measurements for the compressive

strength and modulus tests were done using a laser

extensometer and reflective strips stuck on the sam-

ples. Data were acquired using a data logging system.

Figure 3 shows the illustrative test setup for compres-

sive strength and elastic modulus tests while no strains

were measured during the indirect tensile tests. For

cyclic heating tests, samples were heated to 1,000 �C,

which was maintained for 2 h. The furnace was then

turned off and the samples allowed to cool within the

furnace. This was repeated to achieve the required

number of cycles, after which the samples were tested

in the test setup shown in Fig. 3.

0

10

20

30

40

50

60

70

80

90

100

1001010.10.01

Cu

m %

Pas

sin

g

Sieve Size (mm)

FlashAg Fines

Natural Fines

FlashAg 7mm

Natural 7mm

FlashAg 14mm

Natural 14mm

Fig. 2 Particle size

distribution of natural and

Flashag aggregates

Table 2 Composition and mean compressive strength (f0

c) of the GPC samples

FA

(kg/m3)

NaOH

(kg/m3)

Na2SiO3

(kg/m3)

Coarse

(kg/m3)

Fine

(kg/m3)

Water

(kg/m3)

VM and SPa

(kg/m3)f0c

(MPa)

M1 420 44 110 1,120 650 28.5 7.2 44

M13 420 44 110 1,130 570 56.2 8 38

M4Sp 440 65 162.5 660b 340b 11 8 40

a Viscosity modifier and superplasticizerb FLASHAG aggregates were used



Samples marked M1 were heated for the same

duration and at the same rate as the other samples.

They were however tested for exposure duration of 2,

4 and 6 h at temperatures of 100, 350, 600, 800 and

1,000 �C. M1 samples were not monitored for defor-

mational behaviour during loading. Methods and

loading rates for all tests comply with the AS3600

standards [1–3].

5 Results and discussion

The residual stress–strain curves for geopolymer

concrete with Flashag and natural aggregates for

different temperatures are given in Fig. 4. It is worth

noting that specimens from both mixes at all temper-

atures failed soon after reaching their peak strength,

displaying the brittle nature of GPC. However, the mix

containing Flashag (M4-SP) developed higher failure

strains at ambient temperatures when compared to

GPC containing normal aggregates. The modulus of

elasticity for M4Sp and M13 were 25 and 27 GPa

respectively at ambient temperatures, which is slightly

lower than OPC values for corresponding strengths.

No significant reduction in stiffness is observed for

M13 samples when heated to 200 �C, although there is

a marked reduction in the stiffness for temperatures of

400 �C and over. In fact the modulus of elasticity is

reduced by 60 % at 800 �C and by over 80 % when the

exposure temperature is 1,000 �C. A similar pattern is

observed for the M13 samples in compression and

tensile splitting as shown in Fig. 5a–c. After a slight

increase in both compression and splitting tensile

strength values at 200 �C, which may be attributed to

further geopolymerization at this temperature, there is

a steep decline in both properties as the temperature is

raised further.

One possible explanation for this change in behav-

iour may be the ‘glass transition’ of GP matrix, which

is characterised by abrupt loss of stiffness and

strength. Although this occurs between 520 and

680 �C in GP pastes [29], the presence of aggregates

and use of varying source material may affect the

exact value of this temperature. It is also observed that

normal GPC retains almost 50 % of splitting tensile

and compressive strength values after 1,000 �C expo-

sure. This is much higher than OPC, which retains

only about 5 % of its compressive strength and less

than 20 % of its splitting tensile strength at 800 �C.

The loss of stiffness in OPC is more accentuated as the

temperature is increased over 200 �C [26, 33, 34].

A rather unique behaviour is observed for the

Flashag (M4-SP) samples. At temperatures of 400 �C

and below, the samples steadily lose their stiffness and

strength. This may be attributed to the fact that Flashag

are lightweight aggregates with numerous voids, and

carry a significant amount of water to attain saturated

surface dry (SSD) condition. During heating this free

water escapes leaving behind voids in the matrix.

These voids contribute to the loss of stiffness and

strength at relatively low temperatures. Nevertheless

at higher temperatures M4-SP samples exhibit far

superior strength and stiffness properties. At 800 �C

the samples regain all but 20 % of their compressive

strength and modulus of elasticity, while the splitting

tensile strength is also increased from 50 % at 400 �C

to over 60 % at 800 �C of ambient values. This trend

continues as the temperate reaches 1,000 �C. The

residual values of compressive strength, modulus of

elasticity and splitting tensile strength after exposure

to 1,000 �C was recorded as 85, 95 and 62 % of

ambient, respectively. The initial loss of strength and

stiffness, attributed to the voids left behind by

escaping free water, is offset by the gain in these

properties due to accelerated geopolymerization and

changes in the material at higher temperatures [15]. A

more significant factor explaining why the strength of

Flashag-GPC concrete surpassed normal aggregate-

GPC concrete is the tight aggregate-matrix bond

accomplished with Flashag [16], where the matrix is

essentially thermally compatible with the aggregates.

A plausible consequence of this observation may be

that after cooling down from high temperatures, GPC

Fig. 3 Test setup



with Flashag aggregates does not develop cracking at

the aggregate-matrix interface, while GPC with nor-

mal aggregates develops cracks at the aggregate-

matrix interface (Fig. 6a). The latter is attributed to

incompatibility between the expansion and contrac-

tion of aggregates and matrix, and hence results in loss

of strength and stiffness. On the other hand, GPC with

Flashag is far more thermally compatible internally

resulting in fewer cracks and near perfect matrix-

aggregate interface, contributing to higher strength

and stiffness (Fig. 6b). Also Flashag, contains numer-

ous voids which act more or less like tiny springs

distributed throughout the samples, reducing the

stresses developed due to the cooling period, hence

resulting in reduced cracking. Moreover, as the

temperature is increased over and above 400 �C, the

fly ash particles present in both the matrix and the

aggregates fuse together to form a near-monolithic

phase, which may contribute towards the increase in

strength and stiffness of M4-SP samples after 400 �C.

The effect of time of exposure to high temperatures

on the residual strength of M1 samples was studied

and it was found that changes to GPC occur in the first

2 h of exposure (Fig. 7) after which the duration of

exposure has an insignificant effect on the residual

compressive strength of GPC. All samples experi-

enced a loss in strength when tested after cooling.

Depending on the exposure temperature, the percent-

age of strength lost varies between 25 and 50 %. At

higher temperatures of 600, 800 and 1,000 �C the

strength loss is between 40-50 % while at lower

temperatures the strength loss is 25–30 %. Although

M1 mix is different from M13 mix, their loss of

strength percentage values are comparable at these

temperatures, advocating the hypothesis that the loss

in strength may not be a result of changes to the

material itself but a result of the incompatibilities

between the aggregates and the matrix.

Stress–strain relationship of GPC with normal and

lightweight fly ash aggregates (Flashag) after 1, 2 and

4 cycles of heating from ambient to 1,000 �C are

presented in Fig. 8. As with the duration of heating,

the number of exposure cycles has an insignificant

effect on the stiffness and strength properties of GPC

irrespective of the type of aggregates used. This

further strengthens the argument that all changes

within the GPC occur within the first few hours of

exposure after which the system becomes very stable.

0

5

10

15

20

25

30

35

40

45

Str

ess

(MP

a)

Strain ( µµm/µm)

M13 M4-SP

0

5

10

15

20

25

30

35

40

45

Str

ess

(MP

a)

Strain ( µm/µm)

M13 M4-SP

0

5

10

15

20

25

30

35

40

45

Str

ess

(MP

a)

Strain (µm/µm)

M13 M4-SP0

5

10

15

20

25

30

35

40

45

0 0.002 0.004 0.006 0 0.002 0.004 0.006

0 0.002 0.004 0.006 0 0.002 0.004 0.006 0.008

Str

ess

(MP

a)Strain (µm/µm)

M13 M4-SP

(a) (b)

(c) (d)

Fig. 4 Residual stress–

strain curves for GPC with

Flashag (M4-SP) and

ordinary natural aggregates

(M13) after exposure to

different temperatures. a at

23 �C, b at 200 �C, c at

400 �C and d at 1,000 �C



Nevertheless, the residual compressive strength after

the first cycle for both sample types returned margin-

ally higher values. GPC with Flashag also retained

higher strength and elastic modulus values when

compared to GPC with normal aggregates. However,

GPC with normal aggregates exhibited an increase in

post-peak ductile behaviour, while GPC with Flashag

still failed in a brittle mode after peak strength (Fig. 8).

6 Conclusions

A detailed study was conducted to investigate the

behaviour of GPC with normal and lightweight non-

pelletized fly ash aggregates after exposure to extreme

temperatures, and to compare the results with those for

OPC. It was found that when exposed to extreme

temperatures:

0

20

40

60

80

100

120

f cT/f c

' (%

)

Temp (oC)

M13M4SPOPC after [26]

0

20

40

60

80

100

120

EcT

/Ec(%

)

Temp (oC)

M13M4SPOPC after [27]

0

20

40

60

80

100

120

0 500 1000 0 500 1000

0 500 1000

f ct

T/f c

t(%

)

Temp (oC)

M13

M4SP

OPC after [28]

(b)(a)

(c)

Fig. 5 Variation in GPC

properties as a function of

temperature. a compressive

strength as function of temp,

b modulus of elasticity as

function of temp and

c splitting strength as

function of temp

(a) (b)

Matrix

Matrix

FlashAgg

Matrix-Agg Interface

Fig. 6 SEM Images of GPC with a view of GPC matrix containing normal aggregates (1000 �C) and b view of GPC matrix containing

flashag aggregates (1000 �C)



1. GPC, both with normal or non-pelletized fly ash

aggregates, outperforms OPC. Reported data

indicate that OPC loses substantial structural

integrity at temperatures over 400 �C. GPC on

the other hand, retains over 60 % of its strength

and stiffness at similar temperatures.

2. At even higher temperatures, GPC containing

aggregates which were manufactured from fly ash

as their basic component, has outmatched the

performance of normal aggregate GPC with

properties almost equivalent to those at ambient

temperatures.

3. Most changes in strength and microstructure of

GPC samples occur in the first 2 h of exposure to

high temperatures after thermal equilibrium is

reached. After the initial changes have taken

place, the duration of exposure has an insignifi-

cant effect on the strength of GPC samples.

4. At high temperatures of over 600 �C, GPC under-

goes material changes and the fly ash particles start

to collapse and fuse to form a homogeneous melt,

which contributes to higher residual strength

compared to OPC. However, cracking is observed

in these homogeneous regions in samples contain-

ing normal aggregates. Samples containing Fla-

shag experience far less cracking and retain higher

residual strength and stiffness values.

5. Cyclic exposure to 1,000 �C has an insignificant

effect on the strength and stiffness properties of

both GPC samples with normal and Flashag

aggregates. This further strengthens the argument

that all changes within the GPC occur within the

first few hours of exposure after which the system

becomes very stable.

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http://dx.doi.org/10.1051/matecconf/20141101003

http://dx.doi.org/10.1051/matecconf/20141101003

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