Effect of Curing Conditions on Strength of Fly ash-based Self-Compacting Geopolymer Concrete
Effect of Curing Conditions on Strength of
Fly ash-based Self-Compacting Geopolymer Concrete
J.Thomas
Britto
Dept
of Civil Engineering
Contact:thomasbritto3@gmail.com
Abstract
This paper reports the results of
an experimental work conducted to
investigate the effect
of curing conditions
on the compressive
strength of self-compacting geopolymer
concrete prepared by using fly
ash as base material and combination of sodium
hydroxide and sodium silicate as alkaline activator. The
experiments were conducted by varying
the curing time and curing temperature in
the range of
24-96 hours and
60-90°C respectively. The
essential workability properties
of freshly
prepared Self-compacting Geopolymer
concrete such as
filling ability, passing
ability and segregation
resistance were evaluated
by using Slump
flow, V-funnel, L-box and
J-ring test methods.
The fundamental requirements
of high flowability
and resistance to
segregation as specified
by guidelines on
Self-compacting Concrete by
EFNARC were satisfied. Test
results indicate that longer curing time and curing the
concrete specimens at
higher temperatures result
in higher compressive
strength. There was
increase in compressive
strength with the
increase in curing
time; however increase
in compressive strength after 48 hours was not significant.
Concrete specimens cured at 70°C
produced the highest
compressive strength as
compared to specimens cured at 60°C, 80°C and 90°C.
Keywords—Geopolymer Concrete,
Self-compacting Geopolymer concrete,
Compressive strength, Curing time, Curing temperature.
I. INTRODUCTION
In the growing
environmental concerns of the cement
industry, alternative cement
technologies have become
an area of
increasing interest. It
is now believed
that new binders
are indispensable for
enhanced environmental and
durability performance. In this aspect, geopolymer technology is
one of the
revolutionary developments related
to novel materials
as an alternative to
Portland cement. The
development of geopolymer
concrete is an
important step because of the potential application of
geopolymers to a wide variety of
industrial waste materials
to produce added-value
construction materials resulting in
low-cost and environmentally friendly material with similar mechanical performance and appearance properties to
those from Portland cement . Geopolymer
concrete is produced
by activating different
alumino-silicate based waste
materials with highly
alkaline solution. Curing
of freshly prepared
geopolymer concrete is the most
crucial aspect and plays an important role
in the geopolymerisation process
. Proper curing
of concrete has
a positive effect
on the final
properties of the
geopolymer concrete. Curing
of geopolymer concrete
is mostly carried
out at elevated
temperatures , however,
curing at ambient
temperatures has also
been carried out.
At ambient temperatures; the
reaction of fly
ash-based geopolymeric materials
is very slow
and usually show
a slower setting
and strength development.
It is believed
that higher temperatures
activate alumino-silicate phases in the fly
ash, therefore, they
are generally cured
at elevated temperatures between 60 0 C- 90 0 C.
Previous research
has shown that
both curing time
and curing temperature
significantly influence the
compressive strength of geopolymer
concrete. Several researchers have
investigated the effect of curing time and curing temperature on the properties of geopolymer
concrete, in their
study on fly
ash-based geopolymers have
reported that the
curing temperature and
curing time significantly affected the mechanical
strength of fly ash-based
geopolymers. They concluded
that higher curing
temperature and longer curing
time proved to result in higher compressive
strength.
we studied
the interrelationship of
certain parameters that affected the properties of fly ash-based geopolymer. They
have demonstrated that
water content, curing time and
curing temperature affected the
properties of geopolymers; specifically
the curing condition
and calcining temperature
influenced the compressive
strength. They concluded
that rapid curing
and curing at
high temperature reduced the compressive strength and caused a
negative effect on the physical
properties of the geopolymer.
In a
study done we on
compressive strength and
microstructural characteristics of class C
fly ash geopolymer, the authors
have reported that curing temperature had a
significant effect on
the compressive strength development.
Compressive strength began
to decrease after curing
for a certain period of time at higher temperature. They revealed
that prolonged curing
can break down
the granular structure of the geopolymer mixture.
Iin his
research study on
effect of curing
temperature on the
development of hard
structure of metakaolin-based geopolymer,
has reported that
curing temperature had significant effect on the setting and
hardening of metakaolin-based geopolymer. The
author has demonstrated
that higher curing
temperatures and longer
curing time increase
the early age
compressive and flexural
strengths. However, elevated temperature during early stage of hardening
process results to the formation
of larger pores
consequently increases cumulative
pore volume, which
has a negative
effect on the
final mechanical properties
of geopolymeric material.
This study
aimed to analyze
the effect of
curing time and
curing temperature on
the compressive strength
of fly ash-based
self-compacting geopolymer concrete.
Test results indicate
that longer curing
time and curing
the concrete specimens at elevated temperature up to 70 0
C result in higher compressive strength.
II. EXPERIMENTAL DETAILS
A.
Materials
In the present study, Low-calcium
(ASTM Class F) Fly ash was used as a
source material for the synthesis of SCGC. Fly
ash was obtained from Manjung power station, Lumut, Perak, Malaysia.The
chemical composition of Fly ash as determined
by X-Ray Fluorescence (XRF) analysis is shown in Table I.
Locally available
crushed coarse aggregate
of maximum size
14 mm having
specific gravity of
2.66 was used
in the preparation
of all test
specimens. The coarse
aggregate was used in saturated surface dry (SSD)
condition. Natural Malaysian sand having specific gravity of 2.61 and the fineness
modulus of 2.76
was used as
fine aggregate. Fine
aggregate was sieved for the size less than 5mm and
used in dry condition.
For the
alkaline activator, a
combination of sodium
hydroxide and sodium
silicate solution was
used. Sodium hydroxide
in pellets form
with 99% purity,
supplied by QuickLab Sdn
Bhd, Malaysia and
Sodium silicate solution (Grade A53 with Na2O = 14.26%, SiO2
= 29.43% and water = 56.31%) obtained from Malay-Sino Chemical Industries
Sdn Bhd,
Malaysia were used.
To prepare sodium
hydroxide solution, sodium
hydroxide pellets were dissolved in ordinary
drinking water. Both the liquid solutions were mixed together and alkaline solution was prepared.
To achieve
higher workability and
required flowability of
the fresh concrete,
a commercially available
superplasticizer (Sika Viscocrete-3430) supplied
by Sika Kimia
Sdn Bhd, Malaysia,
and a specified
amount of extra
water (other than
the water used
for the preparation
of sodium hydroxide
solution) was also used.
TABLE
I
CHEMICAL
COMPOSITION OF FLY ASH AS DETERMINED BY XRF
Oxide (%) by mass
Silicon dioxide (SiO 2 ) 51.3
Aluminum oxide (Al 2 O 3 ) 30.1
Ferric oxide (Fe 2 O 3 ) 4.57
Total SiO 2 + Al 2 O 3
+ Fe 2 O 3 85.97
Calcium oxide (CaO) 8.73
Phosphorus pentoxide (P 2 O 5 ) 1.6
Sulphur trioxide (SO 3 ) 1.4
Potassium oxide (K 2 O) 1.56
Titanium dioxide (TiO 2 ) 0.698
B.
Design
of Mix Proportion
In this
experimental work, a mix proportion
with the content
of 400 kg/m 3
of fly ash
was designed to
study the effect
of curing time
and curing temperature
on the compressive strength of
self-compacting geopolymer concrete. Four levels of curing time i.e. 24, 48, 72
and 96 hours and four ranges of curing
temperature i.e. 60°C, 70°C, 80°C and 90°C
were used. The
details of the
mix proportion are
given in Table II. In order to attain required
workability characteristics of self-compacting geopolymer
concrete, a water
content of 12% and superplasticizer dosage of 7% by mass
of the fly ash were also used. The
alkaline solution-to-Fly ash ratio was kept
0.5 whereas the ratio of
sodium silicate to sodium hydroxide
and concentration of
sodium hydroxide were
2.5 and 12
M respectively.
TABLE
II
DETAILS
OF MIX PROPORTION
Fly Ash (Kg/m 3 ) 400
Fine Aggregate (Kg/m 3 ) 850
Coarse Aggregate (Kg/m 3 ) 950
Sodium Hydroxide (Kg/m 3 ) 57
Concentration of NaOH Solution (Molarity) 12
Sodium Silicate (Kg/m 3 ) 143
Super plasticizer (%) 7
Extra water (%) 12
Sodium Silicate/Sodium hydroxide by mass 2.5
Alkaline to Fly ash ratio 0.5
Curing time (hrs) 24-96
Curing temperature (°C) 60-90
C.
Mixing
Procedure
Mixing was carried out in two
stages. Initially, Fly ash, Fine
aggregate (in dry condition) and coarse aggregate (in saturated surface dry condition) were mixed in a pan
mixture for about
2.5 minutes. At the end of this
mixing, the liquid component of
the geopolymer concrete
mixture comprising alkaline
solution, superplasticiser and the extra water, was added to the solid particles and the mixing continued for
another 3 minutes.To ensure the
mixture homogeneity, fresh
concrete mix was
hand mixed for
further 2 to
3 minutes. The
freshly prepared concrete
mix was then
assessed for the
essential workability tests
required for characterizing self-compacting concrete(SCC). Tests such as slump flow,
slump flow at T 50 , V-funnel, L-box, and J-ring were performed for this
purpose.
D.
Casting
and Curing of Test Specimens
After assessing
the necessary workability
properties as guided
by EFNARC [11],
the fresh concrete
was placed in
steel moulds of dimensions 100 x 100 x 100 mm and allowed to
fill all the
spaces of the
moulds by its
own weight. Three
cubes were prepared
for each test
variable. After casting
the moulds, without
any delay, they
were kept in
the oven at
a specified temperature
for a specified
period of time
in accordance with the test
variables selected. At the end of the
curing period, the
moulds were taken
out from the
oven and left undisturbed for about 15 minutes. The
test specimens were removed from
the moulds and
left to air
dry in the
room temperature conditions
until tested for
direct compression at the
specified age.
III. RESULTS AND DISCUSSION
A.
Fresh Properties of SCGC
The
properties of fresh SCC differ significantly from that of conventional
fresh concrete. There
are three distinct
fresh properties of
SCC which are
fundamental to its
performance both in fresh and
hardened state. According to EFNARC [11],
a concrete mix
can only be
classified as SCC
if the requirements
for all the
three workability properties
are fulfilled. The three
essential fresh properties required by SCC
are filling ability, passing ability and resistance to segregation. To
accomplish the workability properties, tests such as slump flow,
slump flow at
T50, V-funnel, L-box,
and J-ring were
carried out. All
the tests were
performed by following
the European Guidelines
for SCC. The
test results of
fresh properties of SCGC are
presented in Table III. The results of
the quantitative measurements and visual observations showed that
freshly prepared concrete
mix had good
flow, filling & passing ability, and produced desired
results and were within the EFNARC range
of SCC.
TABLE III
WORKABILITY TEST RESTULTS
Workability
Test Results Acceptance Criteria for SCC
As per EFNARC [11]
Minimum Maximum
Slump
flow (mm) 710 650 mm 800
mm
T
50 cm Slump flow (sec.) 4.0 2 sec. 5 sec.
V-Funnel
Flow time (sec.) 7 6 sec. 12 sec.
L-Box
(H 2 /H 1 ) 0.96 0.8 1.0
J-Ring
(mm) 5 0 mm
10 mm
B.
Compressive
Strength of SCGC
Compressive
strength is one of the most common measures
used to evaluate
the quality of
hardened concrete and
is considered as
the characteristic material
value for the
classification of concrete.
Many researchers have
used compressive strengths measurements as a tool
to assess the
success of geopolymerisation process
[12]. Compressive strength test was performed in accordance
with BS EN 12390-3:2002 using 2000 KN
Digital Compressive &
Flexural Testing Machine. At the
end of specified oven curing period, a
set of three cubes for each test variable was tested at the ages of 1, 3, 7 and 28 days. The compressive
strength test results are presented in
Table IV. The reported compressive strength
is the average strength of three specimens.
TABLE IV
COMPRESSIVE STRENGTH TEST
RESTULTS
Compressive Strength Test Results
Mix 1-Day 3-Day 7-Day 28-Day
(MPa)
S1
45.01 45.85 46.94 48.53
S2 51.03 51.98
52.26 53.80
S3
51.41 52.20
52.69 53.92
S4
51.68 52.33 52.72 53.99
S5
44.81 45.64 45.98 47.54
S6
51.03 51.98 52.26 53.80
S7
48.56 49.22 49.80 50.77
S8
47.99 48.83 49.67 50.42
1.
Effect
of Curing Time on Compressive Strength
Fig. 1
shows the influence
of curing time
on the compressive strength of self-compacting
geopolymer concrete.The test specimens were cured in the oven at a temperature
of 70°C.
The curing time
varied from 24
hours to 96
hours (4 days). It
is believed that
longer curing time
improved the geopolymerisation process resulting in
higher compressive strength.
The test results
shown in Fig.
1 indicate that
the compressive strength
increases with increase
in curing time. The
test specimens cured
at 70°C for
a period of
96 hours produced the highest compressive strength at
all ages. The rate of increase in
strength was rapid up to 48 hours of curing time; however, the
gain in strength
beyond 48 hours
is not significant. The
results shown in
Fig. 1 clearly
demonstrate that longer
curing time does
not produce weaker
material as claimed .
The trend of
these test results
is similar to
those observed by in their study on Fly ash-based geopolymer
concrete.
2.
Effect
of Curing Temperature on Compressive Strength
Curing temperature
plays a significant
role in the
setting and hardening of the
geopolymer concrete [2]. Hardjito et al. [14], in their study on low-calcium
Fly ash-based geopolymer mortar have
reported that curing
temperature plays an
important role in the geopolymerization process
of Fly ash-based geopolymer. They have concluded
that higher the curing temperature, higher
will be the
rate of geopolymerization process,
which eventually accelerates
the hardening of
geopolymer
mortar.
The effect
of curing temperature
on the compressive
strength is illustrated
by the test
data shown in
Fig. 2. As
curing of fly ash-based geopolymer concrete is usually carried out
at an elevated
temperature in the
range of 60-90°C;
therefore, in this
experimental study the
curing temperature was varied from 60 to 90°C. The test
specimens were cured in the oven at a
temperature of 60, 70, 80 and 90°C for a period
of 48 hours. All the other test parameters were kept constant.
Compressive strength
results shown in
Fig. 2 indicate
that higher curing temperature
does not ensure higher compressive
strengths as claimed
by in their
study on Fly ash-based geopolymer concrete. Test results
show that an increase in
the curing temperature
from 60°C to
70°C increases the compressive strength of the concrete. However, increasing the curing temperature beyond 70°C decreases
the compressive strength of
self-compacting geopolymer concrete.
It is
believed that increase
in the curing
temperature from 60°C to 70°C increased the rate and extent of
reaction through an increase in the heat
of reaction; consequently increased the
compressive strength of the concrete.
Fig. 2 Effect of Curing
Temperature on Compressive Strength
V.
CONCLUSION
In
this experimental work, the effect of curing conditions on the
compressive strength of
fly ash-based self-compacting geopolymer
concrete was investigated.
Test results indicate
that curing time and curing temperature significantly affect the compressive strength of hardened concrete.
Based on the test results reported here,
the following conclusions can be drawn.
1. Longer
curing time improves
the geopolymerisation process
resulting in higher compressive
strength. Increase in compressive
strength was observed
with increase in
curing time. The
compressive strength was
highest when the
specimens were cured
for a period
of 96 hours; however, the
increase in strength
after 48 hours
was not significant.
2. Compressive
strength of concrete
increased with the
increase in curing
temperature from 60°C
to 70°C;however an
increase in the
curing temperature beyond
70°C decreased the
compressive strength of
self- compacting geopolymer concrete.
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About Author:
I am Thomas Britto here to share my experiences in the civil engineering field to all my readers.Today many students are struggling to buy books at high prices. So I decided to start a blog and share my experience and knowledge with all my readers.
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