STRESS-STRAIN
BEHAVIOUR
OF
ALKALI
ACTIVATED FLY-ASH CONCRETE AT ELEVATED
TEMPERATURES
M. Talha Junaid 1, Amar Khennane2, Obada Kayali3
1
2
3
Graduate student, UNSW Canberra, m.Junaid@adfa.edu.au.
Senior Lecturer, UNSW Canberra, a.khennane@adfa.edu.au.
Associate Professor, UNSW Canberra, o.kayali@adfa.edu.au.
ABSTRACT
The effects of high temperature on the strength and stress-strain relationship of alkali activated fly-ash
concrete were investigated. Stress-strain curves were obtained at the temperatures of 20, 100, 200,
300, 400 and 800oC. At ambient temperature, an analytical model for predicting the stress-strain
behaviour of such concretes is presented based on widely accepted models for OPC. High temperature
testing only covered one type of temperature-load history; that is: the samples were heated until the
test temperature is reached then loaded to failure under displacement control. All the samples tested
between 20 to 200oC underwent a decrease in strength. However, samples tested between 200oC to
400oC manifested a moderate to significant gain in strength. Above 400oC all samples underwent a
decrease in strength. The initial loss of strength may be attributed to the loss of water from the GPC
samples. This hypothesis is supported by Thermogravimetric Analysis (DTG) of fly ash based
geopolymer samples where excessive weight loss is assoCiated with this temperature range. Between
200oC and 400oC, the increase in the compressive strength of all tested concrete mixtures is attributed
to further geopolymerization occurring at these temperatures. This geopolymerization has been
proven by Differential Scanning Calorimetry (DSC) results.
Keywords: Geopolymer Concrete Stress-Strain Curves, Alkali Activated Fly Ash Concrete, Flashag,
High Temperature, Elastic Modulus
John Smith, PhD, PEng
University of British Columbia
6250 Applied Science Lane
Vancouver, BC V6T 1Z4
Canada
Email: conmat15@shaw.ca
Tel: 604-822-5853
1.
INTRODUCTION
In the last decade, or so, extensive research has been undertaken with the view to industrialize
Geopolymer Cement (GPC) concrete as an alternative to Ordinary Portland Cement (OPC) concrete
for more sustainable construction. Alkali liquids (usually a soluble metal hydro-oxide and/or alkali
silicate) are usually used to react with silica (SiO2) and alumina (Al2O3) rich natural materials, like
metakaolin or with industrial by-products such as Fly Ash (FA), Silica Fume (SF), Rice Husk Ash
(RHA) or Slag to produce binders [1-4]. Consequently, and contrary to OPC, GPC concrete does not
primarily depend on calcining limestone, which is the major source of CO2 emission in OPC concrete,
and hence can reduce emissions by 45 to 80% for each cubic meter of OPC concrete replaced [4-5].
However, if it is to be used as an alternative for OPC concrete, it needs to display similar or better
characteristics. High ranking among these are fire resistance properties. Indeed, as fire engineering
moves towards a performance based approach, many codes, in addition to the nominal (standard) fire
resistance rating of individual elements, require a full analysis of the structure, which is not possible
without prior knowledge of the evolution of the thermal and mechanical properties with temperature.
Knowledge of the mechanical and thermal properties of building materials is therefore crucial when
designing any structures where heating and cooling is a key factor, such as in a fire event.
Whilst there is evidence to suggest that GPC concrete has a better resistance to fire than OPC concrete
[5-8], most of the existing data, however, deal with the strength or spalling of small laboratory
samples [9-13]. Except for the work of Rickard et al. [7] and Provis et al. [14] on the thermal
dilatation of geopolymer cement pastes synthesized from fly ashes of variable composition, very little
is known about its deformational behavior. The aim of this study therefore is to determine some of
these properties for geopolymer concrete. For this purpose, three GPC mixes were used with nominal
strength of 40MPa, 48MPa and 60MPa. Tests were conducted at ambient temperature as well as at
100, 200, 300, 400 and 800oC.
2.
MATERIALS AND EXPERIMENTAL SET-UP
2.1 Mix design
The composition of the three different mixes containing two different types of aggregates; typical
basalt natural aggregates and non-pelletized light weight fly ash (Flashag) aggregates [15] are shown
in Table 1. The designed mixes along with the curing conditions and ambient strength and elastic
modulus values are also given. Cylindrical samples whose nominal dimensions are 75mm diameter
×150mm height were used in these tests. All tests were conducted 7 days after casting of concrete.
Mixes M7 and M14 contained natural aggregates while M4-SP contained artificial non-pelletized fly
ash aggregates. The tests were carried out using the relevant Australian Standards [16-17].
Table 1: Mix Design Data for the Samples Tested
M7
M14
M4SP
FA
(kg/m3)
420
420
440
NaOH
(kg/m3)
60
67.6
65
Na2SiO3
(kg/m3)
150
169
162.5
Coarse
(kg/m3)
1090
1127
660*
Fine
(kg/m3)
574
575
340*
Water
(kg/m3)
32.3
0
11
VM & SP^
(kg/m3)
8
8
8
f c’
(MPa)
40.2
60.7
48.5
E
(GPa)
18.7
25.7
15.2
Density
(kg/m3)
2197
2213
1679
Rest Period
(hours)
24
24
72
72 hours
80oC
72 hours
80oC
72 hours
80oC
Curing (Dry heat)
* Flashag aggregates were used
^Viscosity Modifier and Superplasticizer
Grade D Sodium Silicate and 12M 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.
2.2. Testing regimes
The behavior of GPC was studied at 20, 100, 200, 300, 400 and 800oC. It must be noted that the tests
conducted in this study covered only one type of temperature-load histories; that is: the samples were
heated until the test temperature is reached and thermal equilibrium achieved then loaded to failure
under displacement control. Heating rates of 5oC/min [18] was used for all tests as shown in Figure 1.
Standard tests were conducted to determine the compressive strength of the samples, while the elastic
modulus was calculated using the stress strain curves obtained at each temperature.
Temp (C)
2hr – Time to achieve
Thermal Equilibrium
800C
=5
C/m
in
400C
300C
rate
ting
Hea
Test
Performed
Here
200C
100C
Time (hr)
Figure 1: Heating Regime used in this Study
3.
3.1.
RESULTS AND DISCUSSIONS
Stress-Strain Curves at Ambient Temperature
Stress strain curves for the three mixes at ambient temperatures are shown in Figure 2-a. As M4SP is
lightweight GPC, the elastic moduli as well as the strain at maximum stress are significantly different
from the other two concrete types. Several existing analytical stress-strain models were studied and
compared with the GPC experimental data; among them those proposed by Collins et al. [19], and
Sargin and Handa [20]. These models closely predicted the stress-strain behaviour of GPC. A
comparison of the representative stress strain relationship of GPC with the above models for the three
types of mixes tested at ambient temperatures are presented in Figure 2-b to 2-d.
a)
b)
c)
d)
Figure 2: Stress-Strain Curves of GPC Samples at Ambient Temperatures
Collins et al., [19] model for stress strain curves is given by
 c  f c'
c
n
 cm n  1  ( c  cm ) nk
while Sargin and Handa [20] proposed the following model to express the stress strain relationship of
OPC concretes.
c 
A   2
f c'
1  ( A  2)
with
n = 0.8+ (fc’/17), A=Eco/Ec ,  = c/cm ,
k = 0.67+ (fc’/62) when c/cm >1
= 1.0 when c /cm ≤1
where fc’ represents the peak compressive stress, cm the strain at peak stress, Ec the initial tangent
modulus or elastic modulus, and Eco the secant modulus from origin to peak compressive stress.
3.2.
Compressive Strength Stress-Strain Curves at Elevated Temperatures
Figure 3-a shows the evolution of the compressive strength as a function of the test temperature, while
Figures 3-b to 3-c show respectively the stress strain curves for the mixes M7, M14 and M4SP.. All
the samples underwent a decrease in strength between ambient and 200oC. This initial decline was
followed by a moderate to significant gain in strength between 200oC and 400oC. Above 400oC all
samples underwent again a decrease in strength.
a)
b)
c)
d)
Figure 3: Compressive strength and stress strain curves at elevated temperatures
The initial loss of strength may be attributed to the loss of water, from the GPC samples. This
hypothesis is supported by Thermogravimetric Analysis (DTG) of fly ash based geopolymer samples
[21-27] where excessive weight loss is associated with this temperature range. The escaping water
leaves behind numerous voids, which compromise the structural integrity of the system [28-30]. This
is supported by the fact that the mix with the largest water content (M4SP) experienced the greatest
strength loss during this exposure range. Moreover, the temperatures are low enough and the exposure
time is relatively short, thus limiting the amount of geopolymerization, which may positively impact
the strength of GPC.
Between 200oC and 400oC, the increase in the compressive strength of all mixes is attributed mainly
to two phenomena. Firstly, between 200oC and 300oC further geopolymerization occurs. The
Differential Scanning Calorimetry (DSC) results presented by Rahier et al. [27] and the ΔT
thermograms by Pan et al. [24] confirm exothermic peaks. These peaks have been proven to be
characteristic of geopolymerization. Secondly, the less pronounced strength gain at temperatures over
300oC have been attributed to the general stiffening of the geopolymer gel and the removal of
moisture from between the gel particles. This stiffening of the gel results in slight strength gains in
GPC samples [24].
The strength values at 800oC are rather interesting. For samples M14 and M4SP a sharp decrease in
strength is reported ( 45% of the ambient strength). This may be due to the disintegration of the
geopolymer gel and the formation of new phases within the system. Junaid et al. [30] while studying
the aspects of the deformational behaviour of GPC concrete at elevated temperatures reported that
within the temperature period from 600oC to 700oC, there was a sudden significant slowdown in the
rate of expansion followed by an immediate resumption of expansion at the previous fast rate. This is
most likely to be indicative of a material transformation either in the aggregates, the unreacted fly ash
or the geopolymer matrix. As the temperature is kept constant at 800oC, GPC concrete experienced a
very rapid development of contraction strains falling almost 35% in 90 mins. This simply cannot be
explained by drying shrinkage as there is no moisture left in the sample at 800oC. This could be due to
the phenomenon known as ‘viscous sintering’ [31], which occurs at temperatures between 550 and
850oC [7]. According to Skvara et al. [32] who used Radioisotope Thermoelectric Generators (RTG)
diffraction, this process is accompanied by the formation of new crystalline phase (probably albite,
sillimanite, nepheline, labradorite), which may result in densification and contraction. Since M7 has
the least amount of alkaline solution it can be hypothesised that, relatively larger amounts of unreacted fly ash is present in these mixes. When exposed to temperature of 800oC, these particles sinter
and form a homogeneous mass, which contributes to higher strength.
Although desirable, this healing phenomenon taht takes place between 200 and 400 oC makes it
impossible at this stage to develop generalized analytical equations for GPC at high temperatures. To
develop such a model, the energy of the geopolymerization needs to be quantified at all temperatures.
Additionally, the samples tested during this research were virgin samples undergoing their first
loading cycles without a sustained loading as would not be the case in a practical structure where
sustained stresses are present. By comparison to OPC concrete, for which a sustained stress during
heating affects the compressive strength [33], it is hypothesised that a sustained stress during heating
would also affect the compressive strength of GPC. To illustrate this point let us hypothesise that a
sustained stress of 37 MPa is applied to the M7 sample as shown in Figure 4. According to the
strength envelope obtained without stress during heating, the sample would fail at about 100 oC before
“healing” at around 250oC. This, of course, is not physically possible as it violates the second
principle of thermodynamics [34-36]. Unless such testing is conducted, development of a generalized
analytical model for the stress-strain relationship of GPC at higher temperatures is not possible.
Figure 4: Illustration of a physically impossible situation
3.3. Elastic modulus
Figure 5 shows the evolution of the elastic modulus as a function of the test temperature. After a
drastic loss of stiffness between 20 and 200oC, the stiffness loss is rather slight for M7 and M14
samples till 400oC, with M7 ‘recovering’ about 10% of its lost stiffness. This minor increase in
stiffness may be a result of further geopolymerization in the matrix resulting in ‘healing’. M7 retains
approximately 55% of its elastic modulus at 800oC while both M14 and M4SP retained only 35% of
their ambient modulus value. The higher retention of stiffness in M7 samples can be attributed to the
healing phenomenon experienced by these M7 samples.
Figure 5: Variation of the elastic modulus as a function of the test temperature
3.4.
Strain at peak stress
The strain at peak stress was also recorded for all the tests. Its variation with the test
temperature is shown in Figure 6. At high temperatures, all the samples displayed a ductile
failure as is evidenced from the relatively large post peak strains developed. Strains at peak
stress are in the range of 0.00337 mm/mm (minimum) to 0.006159 mm/mm (maximum). The
samples containing Flashag aggregates (M4SP) consistently developed the highest strains at
all temperatures. This is due to the fact that Flashag are lightweight aggregates with many
voids; hence more compliant.
Figure 6: Variation of the strain at peak stress as a function of the test temperature.
4.
CONCLUSIONS
The study presents the results of an experimental program to characterize the behavior of alkali
activated fly ash concrete at elevated temperatures. It was found that when exposed to temperatures,
all samples experienced an initial loss in strength and stiffness properties up to 200oC. This is believed
to be attributed to the damage caused by the escaping water. This hypothesis is supported by
Thermogravimetric Analysis (DTG) of fly ash based geopolymer samples published earlier where
excessive weight loss is associated with this temperature range. The escaping water leaves behind
numerous voids, thus compromising the structural integrity of the system. All samples, irrespective of
the aggregate type experience a slight to significant increase in strength properties between 200 and
400oC. The increase in compressive strength of all mixes is believed to be the result of further
geopolymerization and to a lesser extent to the general stiffening of the geopolymer gel and the
removal of moisture from between the gel particles. At 800oC, all samples experienced a loss of
strength properties. This may be due to the disintegration of the geopolymer gel and/or the formation
of new phases within the system either in the aggregates, the un-reacted fly ash or the geopolymer
matrix.
At high temperatures, all the samples displayed a ductile failure as is evidenced from the relatively
large post peak strains developed. Samples containing Flashag aggregates consistently developed the
highest strains at all temperatures. This is due to the fact that Flashag are lightweight aggregates with
numerous air voids and hence more compliant. The air voids enabled the samples to undergo larger
strains while strains developed in those containing natural aggregates are comparable to OPC.
It was also found that alkali activated concrete retains more strength and stiffness at high temperatures
as compared to OPC. Samples with natural aggregates retained more strength properties as compared
to samples with Flashag. Most importantly, it was found that unless tests under sustained stress during
heating are conducted, development of a generalized analytical model for the stress-strain relationship
of GPC at higher temperatures is not yet possible.
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