The influence of elevated temperature on strength and microstructure of high strength concrete containing ground pumice and metakaolin

Construction and Building Materials 124 (2016) 244–257

Contents lists available at ScienceDirect

Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

The influence of elevated temperature on strength and microstructure
of high strength concrete containing ground pumice and metakaolin
M. Saridemir a, M.H. Severcan a, M. Ciflikli b, S. Celikten a, F. Ozcan a, C.D. Atis c,⇑
a

Department of Civil Engineering, Nigde University, 51240 Nigde, Turkey
Department of Geology Engineering, Nigde University, 51240 Nigde, Turkey
c
Department of Civil Engineering, Erciyes University, 38039 Kayseri, Turkey
b

h i g h l i g h t s
 Influence of elevated temperature on mechanical properties of HSC is examined.
 The changes in microstructure of concrete were examined by XRD, SEM and PLM.
 Increase in temperature result with decrease in mechanical properties of concrete.
 High temperature caused cracks and alterations in microstructures of materials.
 Under elevated temperature concrete containing GP and MK blend behaved better.

a r t i c l e

i n f o

Article history:
Received 22 July 2015

Received in revised form 18 July 2016
Accepted 22 July 2016

Keywords:
High strength concrete
Elevated temperature
Microstructure
Interface

a b s t r a c t
A laboratory study is performed to evaluate the influence of elevated temperature on the strength and
microstructural properties of high strength concretes (HSCs) containing ground pumice (GP), and blend
of ground pumice and metakaolin (MK) mixture. Twelve different mixtures of HSCs containing GP and
MK were produced, water-to-binder ratio was kept constant as 0.20. Hardened concrete specimens were
exposed to 250 °C, 500 °C and 750 °C elevated temperatures increased with a heating rate of 5 °C/min.
Ultrasound pulse velocity (Upv), compressive strength (fc), flexural strength (ffs) and splitting tensile
strength (fsts) values of concrete samples were measured on unheated control concrete and after
air-cooling period of heated concrete. The crack formation and alterations in the matrix, interface and
aggregate of HSCs were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and
polarized light microscope (PLM) analyses. XRD, SEM and PLM analyses have shown that, increasing
target temperature result with decrease in mechanical properties i.e. Upv, fc, ffs and fsts values. Elevated
temperature also results with crack formation, and increasing target temperature caused more cracks.
Alterations in the matrix, interface and aggregate were, also observed by these analyses. The experimental results indicate that concrete made with MK + GP blend together as a replacement of cement in mass
basis behaved better than control concrete made with cement only, and concrete containing only GP as a
cement replacement.
Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction
In recent years, high strength concretes (HSCs) containing
natural pozzolanas, which are either in raw or calcined condition,

such as metakaolin, zeolite, volcanic tuff and diatomite, are used
widely in the world. The columns, shear walls, foundations,
bridges, skyscrapers, nuclear and power structures are among
the major areas for high strength concrete applications. The
⇑ Corresponding author.
E-mail address: (C.D. Atis).
/>0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

application fields of high strength concretes are expanding in
time, since they show extraordinary structural performance,
protect the environment, save energy by using pozzolanas [1,2].
Moreover, the natural pozzolanas provide more advantage i.e.
the reduction of cost, reduction of heat leak, decrease of
permeability and increase of chemical resistance, since they
intensify the microstructure of concrete when used as cement
replacement material [3]. They also provide extra strength in
the concrete by reacting with the cement hydration product
Ca(OH)2 to form extra calcium-silicate-hydrate (C-S-H) gels
particularly in transition zone [4,5].


245

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

Over the past several decades, due to its high pozzolanic
properties, the influence of using MK as supplementary cementing materials in concrete on the mechanical and durability
related properties of concrete was studied by numerous
researchers [6–10]. MK is an ultrafine pozzolanas, produced by
calcination of purified kaolin clay, heating in the temperature

range of 600–900 °C [5,10]. The main ingredients of MK are
amorphous SiO2 and Al2O3. MK has high pozzolanic activity
due to its glassy components. Apart from the filling effect in
concrete, MK reacts with Ca(OH)2 to produce C-S-H gels in the
main bonding phase of concrete [11–13]. It is known that Ca
(OH)2 is the main element which causes the weakness of interface between the aggregate particles-cementitious materials.
Thus, it influences the strength, porosity, permeability and durability related properties of concrete [13,14]. The replacement of
natural puzzolanas with cement, such as MK, consumes Ca
(OH)2 and improves the above-mentioned properties of interfacial zone of concrete [13].
Concrete, which is one of the most widely used as building
materials in the world, has higher resistance to elevated temperature, when compared to other building materials i.e. steel and
wood. Nevertheless, this resistance is valid up to a certain temperature level and exposure duration [15,16]. When a certain time is
exceeded under elevated temperature, it brings about important
physical and chemical changes and resulting in deterioration of
concrete such as forming cracks, causing large pores, spalls and
reduction of the adherence between the aggregate particlescementitious materials in the concrete [16,17]. Therefore, the
mechanical properties of concrete are decreased due to these
changes. The reduction in these properties due to elevated temperatures was also associated with the heating rate of specimens.
When the specimens are heated up to approximately 250 °C temperature, free water present in the specimens evaporates slowly,
and no structural damage occurs in the specimens. Nevertheless,
rapid heating rate results in higher vapor pressure and causes
cracks in concrete [18]. When the temperature of specimens
reaches approximately at 300 °C, the water in the interface of CS-H gels is evaporated. Micro-cracks occur approximately at
300 °C temperature in the cement matrix and the bond between
the aggregate particles-cementitious materials [16,19]. Therefore,
the mechanical properties of the specimens, exposed to higher
than 300 °C temperature, gradually decrease due to the crack
growth and deterioration of C-S-H gels, when compared to nonheated specimens.
Previous papers studied on the influence of MK were, in general,
on the properties of concrete as a cement replacement material.

Using MK as a cement replacement in concrete improved mechanical and durability related properties at optimal replacement ratio,
which depends on the fineness and properties of MK used. The
effect of ternary blend of MK and silica fume or fly ash on the properties of HSCs was also studied by many researchers. Nevertheless,
there are no study investigating the effect of ternary blend of MK
and GP on the mechanical and microstructural properties of HSCs
exposed to elevated temperatures. The aim of this paper was to
investigate the effect of 5%GP, 10%GP, 15%GP, 20%GP, 2.5%MK
+ 2.5%GP, 5%MK + 5%GP, 5%MK + 10%GP, 5%MK + 15%GP, 10%MK
+ 5%GP, 10%MK + 10%GP and 15%MK + 5%GP (5GP, 10GP, 15GP,
20GP, 2.5MK + 2.5GP, 5MK + 5GP, 5MK + 10GP, 5MK + 15GP,
10MK + 5GP, 10MK + 10GP and 15MK + 5GP) blend, as cement
replacement in concrete, on the ultrasound pulse velocity (Upv),
compressive strength (fc), flexural strength (ffs) and splitting tensile strength (fsts) values of concrete studied. In addition, investigating the influence of elevated temperature on residual Upv, fc,
ffs, and fsts values of HSCs containing GP and MK + GP blend was
another aim of this work. Crack formation, alterations in
microstructural properties of cementitious matrix, interfacial zone

between the aggregate particles and cementitious materials were
also to be investigated by XRD, SEM and PLM analyses.
2. Experimental study
2.1. Materials
The cementitious materials used in the mixtures were ordinary
Portland cement (CEM I 42.5 R) complying with relevant TS EN
197-1 [20], GP and MK complying with relevant ASTM C-618
[21]. Portland cement, GP and MK were procured from Nigde
cement plant of CIMSA, Nevsehir Mikromin Company and BASFThe Chemical Company in Turkey, respectively. The chemical compositions, physical and mechanical properties of the cementitious
materials are presented in Table 1.
The fine aggregates used in the mixtures were natural sand-I
(NS-I) and natural sand-II (NS-II). The coarse aggregates used in
the mixtures were crushed limestone-I (CL-I) and crushed

limestone-II (CL-II). The aggregates used were compatible with
the requirements of TS 706 EN 12620+A1 [22]. The particle size,
mixing ratio and specific gravity of aggregates used in mixtures
are given in Table 2. In addition, the gradations of aggregates used
are provided in Table 3, with the standard limits.
2.2. Mix proportions
Twelve HSC mixtures with 0.2 water binder ratio and 500 kg
cementitious materials for a cubic meter were prepared. These
mixtures include one control concrete (C), four concretes containing up to 20% GP and seven concretes containing up to 20% MK
+ GP blend. The details of mixture proportions of concretes containing GP and MK + GP are given in Table 4. The modified polycarboxylic ether polymers based high range water reducing admixture

Table 1
Properties of cement, GP and MK admixtures.
Component (%)

Cement

GP

MK

SiO2
Fe2O3
Al2O3
CaO
MgO
SO3
Na2O
K2O
Loss on ignition

Physical properties
Initial-final setting time (min)
Specific gravity
Specific surface area (m2/kg)
Mechanical properties
Compressive Strength (MPa)
3 days
7 days
28 days

25.10
2.30
5.10
61.35
1.50
1.65
0.70
0.95
1.25

55.23
0.83
42.05
0.31
0.45
0.32
0.61
0.20

68.14

2.50
13.94
3.23
1.06
1.24
4.46
2.63
2.78

2.54
16.700

2.50
13.150

125–215
3.16
365

32.65
44.80
53.45

GP = Ground pumice, MK = Metakaolin.

Table 2
The particle size, mixing ratio and specific gravity of aggregates.

Particle size (mm)
Mixing ratio (%)

Specific gravity

Fine aggregate

Coarse aggregate

NS-I

NS-II

CL-I

CL-II

0–1
10
2.55

0–5
30
2.47

5–12
25
2.69

12–22
35
2.71

NS = Natural sand, CL = Crushed limestone.


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M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

Table 3
Total used aggregate grading with standard limits.
Sieve size (mm)

31.5
22.4
16
11.2
8
4
2
1
0.5
0.25
0.15
0.63

Passing from sieve (%)
A limit

B limit

C limit

Total used aggregate

100.00
98
85
68
48
33
22
15
10
6
3
1

100.00
99
92
79
63
49
37
28
20
13
7
3

100.00

100.00
99
90
77
64
52
41
30
20
11
5

100
100
89.55
71.25
59.60
42.05
32.55
25.38
17.35
8.04
3.76
0.96

called as Glenium 51 was used in the concrete mixtures to maintain desired slump of 80 ± 20 mm.
2.3. Mixing, casting, curing, heating and cooling details
The mixing, casting and compacting of concretes containing GP
and MK + GP blend were performed complying with relevant standard ASTM C192/C192M-14 [23]. A power driven rotating pan
mixer was used for mixing, and a vibrating table was used in casting and compacting the samples. After casting the fresh concrete

mixture samples into the molds, they were covered with wet burlaps for 24 h in the laboratory condition. Afterwards, hardened
specimens were removed from the molds after a day, and were
placed in water tank with 24 ± 1 °C temperature, until testing.
The heating of concrete specimens carried out by exposing
them to 250 °C, 500 °C and 750 °C each target temperatures. Before
elevated temperature testing, specimens were removed from
water tank, and conditioned in laboratory condition for a week,
then dried for 24 h in an oven at 105 °C. The specimens were put
in a furnace at room temperature, and temperature was elevated
at a rate of 5 °C/min up to target temperatures. The specimens
were exposed to target temperature for 2 h in steady-state condition. Then the power button on the furnace was shut off. At the
end of heating process, the door of furnace was opened, and the
specimens were exposed to slow cooling in the air for 24 h.
2.4. Testing procedure and methods
The Upv, fc, ffs and fsts values were determined on the control
concrete and concretes containing 5GP, 10GP, 15GP, 20GP,
2.5MK + 2.5GP, 5MK + 5GP, 5MK + 10GP, 5MK + 15GP, 10MK
+ 5GP, 10MK + 10GP and 15MK + 5GP. The Upv and fc tests were

performed, in accordance with ASTM C 597-09 [24] and TS EN
12390-3 [25], on cubic specimens with a 10 cm side, at the ages
of 7, 28 and 56 days. In addition, the Upv, fc, ffs and fsts tests were
also performed on the same size cubic specimens after exposing
them to 250 °C, 500 °C and 750 °C temperatures at 56 days. The
fc values on the concrete specimens were measured by compression load applied with a rate of 0.10 MPa/s by using a 3000 kN
capacity compression machine. The ffs and fsts values were
obtained by using flexural tensile testing and split tensile strength
apparatus. The ffs and fsts tests were carried out in accordance with
TS EN 12390-5 [26] and TS EN 12390-6 [27], respectively. Flexural
(ffs) and split tensile strengths (fsts) were measured at 56 days, by

using prism specimens with dimension of 10 Â 10 Â 40 cm, and
cubic specimens with a 15 cm side, respectively.
In this study, microscopic analyses of HSCs containing GP and
MK + GP exposed to elevated temperature were performed by
using a Philips Panalytical EMPYREAN type XRD, Zeiss EVO
40XVP type SEM, and Nikon ECLIPSE E400 Pol type PLM.
XRD and SEM analyses were used to investigate the changes in
the chemical component, mineralogical structure, microstructure
and interface between the aggregate particles and cementitious
materials of the control concrete and concretes containing 5GP
and 5MK + 5GP exposed to 25 °C, 500 °C and 750 °C temperatures.
These analyses were performed on the small pieces taken from the
specimens used for the PLM analysis. For the SEM analyses, the
small pieces were mounted on the brass stubs using carbon tapes
and, were covered with gold.
PLM analyses were used to investigate the cracks and alterations in the cementitious matrix, interface between the aggregate
particles and cementitious materials, and aggregate microstructures on the thin segments taken from cubic sample with a
10 cm side. The analyses were carried out on the control concrete
and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP
after exposing them to 25 °C, 500 °C and 750 °C temperatures.
After exposing the concrete specimens to the target temperatures
and cooling, they were cut in four equal parts using a rotary saw
and 5 Â 5 Â 10 cm prism specimens were prepared as shown
Fig. 1a, b and d. One of four equal parts was selected. The selected
part was divided into two equal parts using a rotary saw and one of
the parts was overlaid in the acetone to clean the free particles and
pores. This cleaned part was embedded in resin to absorb resin in a
vacuum desiccator until there is no micro-air bubble in the part.
The part embedded in resin for the PLM analyses were left to
harden in the laboratory condition as seen Fig. 1d. The hardened

parts were adhered to the 5 Â 5 Â 0.5 cm size glass to obtain thinner section. The adhered parts were cut very thin by a cutting
machine to make thin parts 5 Â 5 Â 0.03 cm in size by using a sensitive diamond saw as seen Fig. 1c and e. Then, thin sections were
eroded for use in the PLM analysis size. The surfaces of thin

Table 4
Mixture proportions of concretes containing GP and MK + GP (kg/m3).
Mixtures No

Meaning

Cement
kg/m3

GP
%

C
5GP
10GP
15GP
20GP
2.5MK + 2.5GP
5MK + 5GP
5MK + 10GP
5MK + 15GP
10MK + 5GP
10MK + 10GP
15MK + 5GP

Control concrete

%5GP
%10GP
%15GP
%20GP
%2.5MK + %2.5GP
%5 MK + %5 GP
%5MK + %10GP
%5MK + %15GP
%10MK + %5GP
%10MK + %10GP
%15MK + %5GP

500
475
450
425
400
475
450
425
400
425
400
400

5
10
15
20
2.5

5
10
15
5
10
5

MK
%

2.5
5
5
5
10
10
15

Water
kg/m3

NS-I
(0–1 mm)

NS-II
(0–5 mm)

CL-I
(5–12 mm)

CL-II
(12–22 mm)

SP
kg/m3

100

255.10
254.13
253.16
252.18
251.21
254.28
253.46
252.49
251.52
252.80
251.83
252.13

537.66
535.61
533.56
531.52
529.47
535.94
534.21
532.16
530.12

532.81
530.76
531.41

489.60
487.74
485.87
484.01
482.14
488.03
486.46
484.60
482.73
485.19
483.32
483.91

690.50
687.87
685.24
682.61
679.98
688.29
686.07
683.44
680.81
684.28
681.64
682.48

15.00
15.00
18.33
20.00
21.67
15.00
15.83
20.00
21.67
18.33
20.83
19.17

GP = Ground pumice, MK = Metakaolin, NS = Natural sand, CL = Crushed limestone, SP = Superplasticizer.


M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

247

Fig. 1. Preparing of thin part for microstructure analyses.

sections were moistened to increase the quality of images using
the microscope camera. In this way, the microstructure images of
thin sections between the lines seen in Fig. 1f were obtained to
investigate the cracks and alterations in the cementitious matrix,
interface between the aggregate particles and cementitious materials, and aggregate microstructures.
3. Test results and discussion
3.1. Ultrasound pulse velocity and compressive strength
The normalized Upv and fc values of HSCs containing GP and MK

+ GP at the ages of 7, 28 and 56 days are given in Table 5. Besides,

the effects of GP on the Upv and fc values of HSC are shown in
Figs. 2a and 3a in 3D graphs, and also the effects of MK + GP on
the Upv and fc values of HSC are shown in Figs. 2b and 3b in 3D
graphs at the ages of 7, 28 and 56 days. As shown in
Figs. 2a and 3a, the Upv and fc values of concrete containing 5%
GP increases at all ages, while these values for concrete containing
10%, 15% and 20% GP decrease. These figures shows that, the Upv
and fc values of the control concretes varied between
5.38–5.44 km/s and 75.50–81.35 MPa, while these values for
concretes containing GP ranged between 5.28–5.45 km/s and
65.13–84.19 MPa, respectively, depending on the curing time
and replacement level of GP. The effect of MK + GP on the Upv
and fc values of concrete can obviously be observed from


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M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

Table 5
The normalized fc, fsts and ffs values of HSCs.
Mixtures

Upv (km/s)

C
5GP
10GP

15GP
20GP
2.5MK + 2.5GP
5MK + 5GP
5MK + 10GP
5MK + 15GP
10MK + 5GP
10MK + 10GP
15MK + 5GP

fc (MPa)

fsts (MPa)

ffs (MPa)

7 days

28 days

56 days

7 days

28 days

56 days

56 days

56 days

1.00
1.00
0.99
0.99
0.98
1.01
1.01
1.00
1.00
1.01
1.01
1.00

1.00
1.00
1.00
0.99
0.98
1.01
1.01
1.00
1.00
1.01
1.01
1.00

1.00
1.00

0.99
0.99
0.97
1.01
1.01
1.00
0.99
1.01
1.00
1.00

1.00
1.03
0.97
0.93
0.86
1.09
1.10
1.04
1.00
1.08
1.03
1.00

1.00
1.03
0.97
0.93
0.85
1.08

1.09
1.02
0.98
1.06
1.03
0.99

1.00
1.03
0.95
0.92
0.84
1.07
1.08
1.03
0.97
1.07
1.03
0.99

1.00
1.02
0.99
0.97
0.95
1.05
1.07
1.02
0.99
1.05

1.02
0.99

1.00
1.04
0.95
0.90
0.83
1.05
1.09
1.02
0.94
1.05
0.99
0.95

fc = Compressive strength, fsts = Splitting tensile strength, ffs = Flexural strength.

5.44-5.46
5.42-5.44
5.40-5.42
5.38-5.40
5.36-5.38
5.34-5.36
5.32-5.34
5.30-5.32
5.28-5.30
5.26-5.28

5.46

5.44
5.42
5.40
5.38
5.36
5.34
5.32
5.30
5.28
5.26

56
28
7
0

5

10

15

84-85
82-84
80-82
78-80
76-78
74-76
72-74
70-72

68-70
66-68
64-66

(a)

f c , MPa

U pv , km/s

(a)

Time, days

20

84
82
80
78
76
74
72
70
68
66
64

56
28

7
0

GP, %

5

10

15

Time, days

20

GP, %

(b)

5.48-5.50

5.44-5.46

5.50

5.42-5.44

5.48

5.40-5.42

5.46

5.38-5.40

5.44

5.36-5.38

5.42

56

5.40
5.38

28

5.36

Time, days

7
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5

MK+GP, %
Fig. 2. The Upv values of HSCs: a) containing GP and b) containing MK + GP.

86-88
84-86

82-84

88

80-82

86

78-80

84

f c , MPa

U pv , km/s

(b)

5.46-5.48

76-78

82

74-76

80
56

78

28

76
74

7
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5

Time, days

MK+GP, %
Fig. 3. The fc values of HSCs: a) containing GP and b) containing MK + GP.

Figs. 2b and 3b. The concretes containing MK + GP had higher Upv
and fc values than the control concretes at the same ages, except
for 20% replacement level of MK + GP. The Upv and fc values of concretes containing MK + GP ranged between 5.39–5.50 km/s and
75.62–87.84 MPa as seen in these figures, respectively. The highest
Upv and fc values were obtained between 5.44–5.50 km/s and
82.77–87.84 MPa for the concretes containing 5MK + 5GP. In the
Upv and fc values of HSCs containing GP and MK + GP at the ages
of 7, 28 and 56 days are evaluated, it was concluded that the concrete containing MK + GP shown better performance than that of
concrete containing only GP. Rashiddadash et al. [28] investigated
the fc values of the polypropylene fiber and steel fiber reinforced
concretes containing MK and GP. They prepared concrete containing 10%GP and 15% GP, 10%MK and 15% MK, 7.5% MK + 7.5% GP,
and a control concrete. They reported that the fc values varied
between 18–44 MPa and 16.7–37.6 MPa for control concretes and
concretes containing GP, respectively. They observed that the early
and long-term fc values of concretes containing GP were lower
than that of the control concretes, depending on the replacement

level of GP. However, they reported the fc values varied between
18.7 and 46.5 MPa for concrete containing MK were higher than
that of the control concretes. They concluded that, due to high fineness and reactivity of MK, the concretes containing MK had relatively higher fc development than that of the control concrete
and concrete containing GP.
A high temperature furnace used in this study to heat, cubic
concrete specimens with a 10 cm a side, up to 750 °C temperature
is shown in Fig. 4b. The Upv and fc values of concretes containing GP
and MK + GP exposed to 250 °C, 500 °C and 750 °C temperatures
are normalized according to the Upv and fc values obtained from
unheated (25 °C) specimens at the age of 56 days, and these values
are presented in Table 6. In addition, the changes in the Upv and fc
values of concretes containing GP and MK + GP exposed to 250 °C,
500 °C and 750 °C temperatures, compared with unheated counterpart control concrete samples which is shown in Figs. 5 and 6
as 3D graphs.


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M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

Fig. 4. a) Broken specimen in the normal temperature, b) electric furnace for high temperature, and c) broken specimen after high temperature.

Table 6
The normalized Upv and fc values of HSCs exposed to elevated temperatures.
Mixtures

Upv (km/s)

C
5GP

10GP
15GP
20GP
2.5MK + 2.5GP
5MK + 5GP
5MK + 10GP
5MK + 15GP
10MK + 5GP
10MK + 10GP
15MK + 5GP

fc (MPa)

25 °C

250 °C

500 °C

750 °C

25 °C

250 °C

500 °C

750 °C

1.00

1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00

0.81
0.81
0.80
0.80
0.79
0.81
0.81
0.80
0.79
0.81
0.80
0.78

0.54
0.57
0.54
0.54
0.53

0.58
0.59
0.56
0.53
0.57
0.55
0.54

0.26
0.26
0.25
0.24
0.23
0.26
0.26
0.24
0.24
0.26
0.25
0.25

1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00

1.00
1.00
1.00

0.99
0.98
1.00
0.99
1.05
0.99
0.99
0.96
0.97
1.00
0.94
0.96

0.85
0.87
0.87
0.90
0.96
0.88
0.90
0.90
0.87
0.89
0.86
0.87

0.42
0.42
0.42
0.40
0.42
0.40
0.42
0.39
0.41
0.42
0.42
0.42

Upv = Ultrasound pulse velocity, fc = Compressive strength.

5.6
5.2
4.8
4.4
4.0
3.6
3.2
2.8
2.4
2.0
1.6
1.2

f c , MPa

U pv , km/sn

(a)

5.2-5.6
4.8-5.2
4.4-4.8
4.0-4.4
3.6-4.0
3.2-3.6
2.8-3.2
2.4-2.8
2.0-2.4
1.6-2.0
1.2-1.6

(a)

25
250

0

5

10

15

20

500
750 Temperature, ºC

85
80
75
70
65
60
55
50
45
40
35
30
25

500
750
5

10

15

Temperature, ºC

20

GP, %

25
250
500
750 Temperature, ºC
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5

MK+GP, %
Fig. 5. The effect of high temperature on the Upv values of HSCs: a) containing GP
and b) containing MK + GP.

85-90
80-85
75-80
70-75
65-70
60-65
55-60
50-55
45-50
40-45
35-40
30-35

(b)

f c , MPa

5.2-5.6

4.8-5.2
4.4-4.8
4.0-4.4
3.6-4.0
3.2-3.6
2.8-3.2
2.4-2.8
2.0-2.4
1.6-2.0
1.2-1.6

(b)

U pv , km/s

25
250

0

GP, %

5.6
5.2
4.8
4.4
4.0
3.6
3.2
2.8

2.4
2.0
1.6
1.2

80-85
75-80
70-75
65-70
60-65
55-60
50-55
45-50
40-45
35-40
30-35
25-30

90
85
80
75
70
65
60
55
50
45
40
35

30

25
250
500
750 Temperature, ºC
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5

MK+GP, %
Fig. 6. The effect of high temperature on the fc values of HSCs: a) containing GP and
b) containing MK + GP.


250

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

The Upv values of heated concretes containing GP and MK + GP
reduce gradually as the temperature increase, as shown in
Fig. 5a and b in 3D graphs. Furthermore, the GP and MK + GP contents of concretes have no significant effect on the decrease of Upv
values when compared to strength of control Portland cement concrete as shown in these figures.
As the temperature increases, the Upv values of concretes
decrease gradually together with the growth and increase of the
amount of pores and space within the concrete. As shown in
Fig. 6a and b, when concrete specimens are exposed to 250 °C,
while the average fc values of concretes containing GP show no
change, the average fc values of concretes containing MK + GP
reduce about 3%. It is reported that, near such temperatures, the
fc values of concretes decrease due to the occurrence of micro
cracks caused by evaporation of water and the growth of pore

structure inside the concrete [18].
When the temperature reached to 500 °C, while the average fc
values of concretes containing GP decrease about 11%, the average
fc values of concretes containing MK + GP reduce about 12%. Some
researchers stated that loss in fc is generally associated with the
dehydration of C-S-H gel and the volumetric expansion due to
changing shape of the chemical compound Ca(OH)2 to CaO that
is known to happen between 500 °C and 600 °C [18,29,30]. Moreover, in these temperatures, the adherence between the aggregate
particles and cementitious materials is impaired, due to the
cementitious materials shrinkage resulting from the loss of water
together with the expansion of the aggregates [30].
The fc loss in the concrete increased substantially when the
temperature was reached to 750 °C and over temperatures due to
the disintegration of C-S-H gel and increase of the macro cracks
[19,31,32]. In this study, as shown in Fig. 6a and b the greatest fc
loss was observed at 750 °C temperature. At this temperature,
while the average loss in the fc values of concrete containing GP
was about 58%, the average loss in the fc values of concrete containing MK + GP was 59% compared to fc before heating. It can be
seen from Fig. 6a and b that the replacements of GP and MK + GP
with cement have no significant influence on the fc loss occurring
due to elevated temperatures.

The ffs and fsts tests on the concretes containing GP and MK + GP
at the age of 56 days were performed on prism specimens with
dimension of 10 Â 10 Â 40 cm, and cubic specimens with a 15 cm
side.
The ffs and fsts test results of concretes containing GP and MK
+ GP are shown in Figs. 7 and 8, each result being average of three
concrete specimens. Besides, the normalized ffs and fsts values of
these concretes are also provided in Tables 5 and 7. It can be seen

from the results that GP has not enhanced the ffs and fsts values
compared to the results of the control concrete, except for 5% GP
content, and it could be concluded from the results that the higher
amount of GP in the mixture leads to decrease in the ffs and fsts values. However, it can be seen from the results that MK + GP blend
has enhanced the ffs and fsts values up to 20% MK + GP content
compared to the results of the control concrete, and concretes containing GP. The highest ffs and fsts values were obtained as
10.31 MPa and 5.70 MPa from concrete containing 5MK + 5GP mixture, while the lowest ffs and fsts values were obtained as 8.91 MPa
and 5.29 MPa from a concrete containing 5%MK and 15%GP blend
together.
On the other hand, as the temperature increase, the ffs and fsts
values of concretes containing GP and MK + GP exposed to
250 °C, 500 °C and 750 °C temperatures decrease gradually as
shown in Figs. 7 and 8, in 3D graphs. Normalized ffs and fsts values
of these concretes are presented in Table 7, with respect to the ffs
and fsts values of unheated concrete at the age of 56 days. It is
observed that GP and MK + GP contents of concretes have no
important effect in a decrease of ffs and fsts values as shown in
these figures and table. When the temperature reached to 250 °C,
500 °C and 750 °C, while the average ffs value of concrete containing GP decrease about 6%, 20%, and 60%; the average ffs value of
concrete containing MK + GP decrease about 9%, 23%, and 66%
compared to none heated concrete ffs value, respectively. Similarly,
in these temperatures, while the average fsts values of concrete
containing GP decrease about 9%, 22%, and 67%; the average fsts

(a)

6.0
5.5
5.0
4.5

4.0
3.5
3.0
2.5
2.0
1.5
1.0

f fs , MPa

5.5-6.0
5.0-5.5
4.5-5.0
4.0-4.5
3.5-4.0
3.0-3.5
2.5-3.0
2.0-2.5
1.5-2.0
1.0-1.5

(a)
f sts , MPa

3.2. Flexural and splitting tensile strengths

25
250
500 Temperature, ºC
750

0

5

10

15

20

10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0

25
250
750
0

GP, %

10

15

20

25
250
500 Temperature, ºC
750
0 2.5+2.55+5 5+10 5+15 10+510+1015+5

MK+GP, %
Fig. 7. The effect of high temperature on the fsts values of HSCs: a) containing GP
and b) containing MK + GP.

10.0-11.0

(b)

f fs , MPa

f sts , MPa

6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0

1.5
1.0

5

500 Temperature, ºC

GP, %
5.5-6.0
5.0-5.5
4.5-5.0
4.0-4.5
3.5-4.0
3.0-3.5
2.5-3.0
2.0-2.5
1.5-2.0
1.0-1.5

(b)

9.0-10.0
8.0-9.0
7.0-8.0
6.0-7.0
5.0-6.0
4.0-5.0
3.0-4.0
2.0-3.0

9.0-10.0
8.0-9.0

11.0
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0

7.0-8.0
6.0-7.0
5.0-6.0

25
250
500 Temperature, ºC
750
0 2.5+2.5 5+5 5+10 5+15 10+5 10+10 15+5

MK+GP, %
Fig. 8. The effect of high temperature on the ffs values of HSCs: a) containing GP and
b) containing MK + GP.


251

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257
Table 7
The normalized fsts and ffs values of HSCs exposed to elevated temperatures.
Mixtures

C
5GP
10GP
15GP
20GP
2.5MK + 2.5GP
5MK + 5GP
5MK + 10GP
5MK + 15GP
10MK + 5GP
10MK + 10GP
15MK + 5GP

fsts (MPa)

ffs (MPa)

25 °C

250 °C

500 °C

750 °C

25 °C

250 °C

500 °C

750 °C

1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00

0.91
0.89
0.91
0.92
0.90
0.90
0.92
0.92

0.90
0.92
0.90
0.89

0.77
0.77
0.76
0.77
0.75
0.79
0.78
0.78
0.78
0.79
0.80
0.79

0.33
0.33
0.34
0.33
0.29
0.34
0.33
0.32
0.32
0.33
0.34
0.31

1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00

0.94
0.89
0.93
0.94
0.92
0.91
0.94
0.94
0.95
0.92
0.94
0.90

0.75
0.77
0.78

0.78
0.78
0.80
0.78
0.81
0.83
0.71
0.81
0.80

0.34
0.34
0.34
0.33
0.33
0.33
0.35
0.33
0.35
0.34
0.34
0.35

fsts = Splitting tensile strength, ffs = Flexural strength.

value of concrete containing MK + GP decrease about 11%, 22%, and
67%. As shown Figs. 7 and 8, the greatest ffs and fsts losses were
observed at 750 °C temperature. The explanation for this decrease
is the disintegration of C-S-H gels and macro cracks as stated in
compressive strength section.

the matrix and interface were doubled with increasing temperature. The increases of cracks and pores are caused to decrease in
the fc value. It is concluded that the reduction in the fc value is a
natural results of deterioration, large pores and crack formation
in the concrete specimens due to elevated temperature. Similar
conclusions and reports are revealed in the literature [31,36,37].

3.3. XRD analyses after elevated temperatures
3.5. PLM analyses after elevated temperatures
X-ray diffraction (XRD) analyses were performed on powdered
samples obtained from control concrete and concretes made with
5GP and 5MK + 5GP after exposing them to 25 °C and 750 °C temperatures as shown in Fig. 9a, b and c. Besides, the chemical component and mineralogical structures of these concretes were
determined by X-ray fluorescence and XRD semi-quantitative analysis as seen in Tables 8 and 9, respectively. The results of analyses
indicated that the major mineralogical structures were calcite and
quartz from the aggregates as the main impurity in the concretes,
as well as some traces of feldspar and dolomite. Moreover, the
analyses indicated that other mineralogical structures were Ca
(OH)2, C2S, C3S and C-S-H from the cementitious materials. The
reduction in the XRD peak for mineralogical structures of concretes
was observed when the temperature was 750 °C. The reduction in
the calcite and quartz may be due to transformation of amorphous
phase of SiO2 and lime of calcite [33,34]. The dehydration caused
by the decomposition of Ca(OH)2, C2S, C3S and C-S-H gel plays a
dominant role in the reduction [35]. The reduction in the XRD
peaks for the control concrete was higher than the concretes containing GP and MK + GP at 750 °C temperature as shown in Fig. 9.
These situations are supported by the chemical components and
mineralogical structures of concretes as seen in Tables 7 and 8.

Microstructures of matrix, interface and aggregate on the thin
sections were examined by using polarized light microscope (PLM).

3.4. SEM analyses after elevated temperatures

3.5.1. The effect of elevated temperature on the matrix structure
The matrix structures of the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to
25 °C, 500 °C and 750 °C temperatures were examined by using
PLM image analyses as seen in Fig. 11. The changes of the crack formation and deterioration on the matrix of the specimens were
evaluated by these image analyses. As seen in Fig. 4c, the discoloration in the specimens was observed when the specimens
exposed to 500 °C and 750 °C temperatures. This discoloration
was the cause of altered and oxidized zones in the matrix. Nearly
no crack was observed on the matrix of the specimens when
exposed to 500 °C temperature. However, the cracks were
observed on the matrix of the specimens when exposed to 750 °C
temperature. These cracks caused considerable reduction in the
mechanical properties of the specimens. As shown in Fig. 11,
the GP and MK + GP contents have not an important effect on
the cracks and deterioration occurred on the matrix of these
specimens due to elevated temperature. Similar observations have
been reported by Akca and Zihniog˘lu [37], Akçaözog˘lu [38] and
Ingham [39].

Microstructural analyses of concrete specimens were carried
out by scanning electron microscope (SEM). The microstructure
and interface between the aggregate particles and cementitious
materials of the control concrete and concretes containing 5GP
and 5MK + 5GP exposed to 25 °C, 500 °C and 750 °C temperatures
were examined on the crushed sample surfaces. As seen in
Fig. 10, the changes were emerged in the C-S-H gels, matrix and
interface due to the increase of temperature. At 25 °C temperature,
the internal structure of concretes is compact, and the C-S-H gels

are as the shape of block. When exposed to 500 °C temperature,
internal structure of concretes still compact, but the pores in the
Ca(OH)2 and C-S-H gels start to increase. When the elevated temperature was 750 °C, the deterioration in the Ca(OH)2 and C-S-H
gels emerges in the internal structure of concretes. Particularly,
when compared to 500 °C temperature, the cracks and pores in

3.5.2. The effect of elevated temperature on the interface structure
The interfacial zones are the weakest bond of the concrete and
the cracks development commonly reveal in the interfacial zones
between the aggregate particles and cementitious matrix [38].
The weakening of bonding strength in these zones due to the
elevated temperature causes major decrease in the mechanical
properties of concrete [38,40]. Hence, the bonding strength in the
zones has a significant effect on these properties at elevated
temperature [38]. Because of this, in this part, the interfacial
structure between aggregate particles and cementitious matrix
is examined.
The interfacial structure between aggregate particles and
cementitious matrix of the control concrete and concretes containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when exposed to
25 °C, 500 °C and 750 °C temperatures were examined by using


252

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

Fig. 9. XRD analyses of HSCs exposed to 25 °C and 750 °C temperatures: a) control concrete, b) concrete containing 5GP and b) concrete containing 5MK + 5GP.

PLM image analyses as seen in Fig. 12. The changes of cracks,
spaces and deterioration on the interfacial structure of these

concretes were evaluated in these image analyses. As seen from
the image analyses, no cracks, spaces and deterioration were


253

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257
Table 8
The chemical components of concretes by X-ray fluorescence.

Table 9
The mineralogical structures of concretes by semi-quantitative analysis.

Component
(%)

C

750 °C C

5GP

750 °C
5GP

5MK + 5GP

750 °C
5MK + 5GP

Mineralogy

C

750 °C
C

5GP

750 °C
5GP

5MK
+ 5GP

750 °C 5MK
+ 5GP

SiO2
CaO
Al2O3
Fe2O3
TiO2
MgO
Na2O
K2O
SO3
LOIa
Total

30.43
40.54
4.94
2.13
0.35
1.78
0.63
0.68
0.26
18.13
99.87

27.96
42.62
4.78
2.05
0.31
1.65
0.49
0.59
0.37
19.03
99.85

29.65
40.45
4.85
2.03
0.45

1.99
0.62
0.69
0.47
18.67
99.87

28.84
42.49
3.83
1.57
0.21
2.03
0.43
0.57
0.39
19.54
99.90

29.42
38.84
5.57
2.14
0.36
1.53
0.75
0.75
0.45
20.03
99.84

28.22
42.05
4.85
1.52
0.21
1.22
0.42
0.55
0.36
20.48
99.88

Calcite
Quarts
Albeit
Feldspar
C3S
C2S
C4AF
Dolomite
Hematite
Amorphous
Phase
Other

52
20
6
5

5
3
1
1
1
2

50
18
5
5
5
3
2
2
2
3

53
21
5
3
4
3
2
2
1
1

51

19
5
5
5
2
2
2
1
3

52
22
6
4
4
2
1
3
2
–

49
21
5
5
4
2
2
3
2

3

4

5

5

5

4

4

a

LOI = Loss on
MK = Metakaolin.

ignition,

C = Control

concrete,

GP = Ground

pumice,

C = Control concrete, GP = Ground pumice, MK = Metakaolin.

observed between the aggregate particles and cementitious matrix
in the concretes not exposed to elevated temperature. Besides, as
seen from the image analyses of concretes subjected to normal
temperature (25 °C) in Fig. 12, the bonding strength between the
aggregate particles and cementitious matrix was very strong. However, the increase at the cracks, pores and deterioration and the
decrease at the bonding strength between the aggregate particles
and cementitious matrix were observed depending on the increase
in the temperature. The micro-cracks, micro-spaces, a little

25 oC

(a) C

Matrix

C-S-H

deterioration and bonding strength are revealed in the interfacial
zones of the concretes when exposed to 500 °C temperature, while
the macro-cracks, macro-spaces, significant deterioration and
impaired bonding strength are shown in the interface zones of
the concretes when exposed to 750 °C temperature. The effect of
GP and MK + GP contents on the interface structure between
aggregate particles and the cementitious matrix decreased as the
temperature increased. Akca and Zihnioglu [37] investigated the
colour changes, cracks and spalls of HSC exposed to elevated

25 oC

(b) 5GP

o
C-S- 25 C
H

(c) 5MK+5GP

Aggregate

Aggregate
Interface
C-S-H

Aggregate

Interface

Interface

Matrix

Matrix

500 oC

(d) C

500 oC

(e) 5GP

500 oC

(f) 5MK+5GP

Matrix
Aggregate

GP
Aggregate

Matrix

Inter
face

Matrix
Aggregate

Interface

C-S-H

(g) C

750 oC

Space
Space

Inter
Aggregate
face
Cracks

C-S-H
Matrix
Cracks

Inter
face

C-S-H

750 oC

(h) 5GP

Inter
face

Cracks
Matrix Crack
s

Aggregate
Space

750 oC

(i) 5MK+5GP

C-S-H
Matrix
Aggregate
Crack Interface
s

Fig. 10. SEM micrographs of HSCs exposed to 25 °C, 500 °C and 750 °C temperatures.

Cracks


254

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

Fig. 11. The effect of high temperature on the cementitious matrix.

Fig. 12. The effect of high temperature on the interface.

temperatures. They observed that the cracks were on the interface
of the specimen heated to 600 °C and the porous were on the interface of specimens heated up to 900 °C. Akçaözog˘lu [38] reported

that the interfacial structure between the aggregate particles and
the cementitious matrix was deteriorated as the temperature
increased.

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

3.5.3. The effect of elevated temperature on the aggregate structure
The quartz and crushed limestone were used as an aggregate in the concrete mixtures. The oxidation, alteration and

255

cracks in aggregates of the control concrete and concretes
containing 5GP, 20GP, 5MK + 5GP and 10MK + 10GP when
exposed to 25 °C, 500 °C and 750 °C temperatures were

Fig. 13. The effect of high temperature on the aggregates.

Fig. 14. The effect of high temperature on the mineralogy.


256

M. Saridemir et al. / Construction and Building Materials 124 (2016) 244–257

investigated by using PLM image analyses as seen in Fig. 13.
The changes of the oxidation, alteration and cracks on the
aggregate particles of these concretes were evaluated by these
image analyses.
Some micro-cracks derived from the formation of the quartzes
were appeared by PLM image analyses in the concrete specimens
unexposed to elevated temperature (Fig. 13). These cracks in the
formation of the quartzes gradually increased depending on the
increase in temperature. Besides, the oxidation and alteration in
the quartzes were observed when the concrete specimens exposed

to 500 °C and 750 °C temperatures. The micro-cracks in the
crushed limestone aggregates were not appeared in the concrete
specimens unexposed to elevated temperature. However, the
micro-cracks in the crushed limestone aggregates were appeared
in the concrete specimens exposed to 500 °C temperature and
expanded in the 750 °C temperature. Besides, the oxidation and
alteration in the crushed limestone aggregates were generally
observed when the concrete specimens exposed to 750 °C temperatures. Moreover, biotite, feldspar, ferromagnesium and plagioclase minerals situated in aggregates were appeared as more
pronounced in the concrete specimens exposed to elevated temperature as seen in Fig. 14. The oxidation occurred in some aggregates are known to be caused from the Fe2O3 minerals. Akçaözog˘lu
[38] and Ingham [39] revealed similar observations. Akçaözog˘lu
[38] stated that some cracks were at the surface of the quartzes
used in the concrete specimen unexposed to the elevated temperature. The author also stated that the increase of cracks, fractures
and disintegrations in the quartzes depend on elevated temperatures. Moreover, the author stated that the disintegration emerged
in almost all of the quartzes in the concrete exposed to 600 °C temperature. Ingham [39] stated that actual concrete colour depends
on the types of aggregate used in the concrete. The author
expressed that the colour changes were most evident in the aggregates containing siliceous and were very little in the limestone and
granite aggregates. Besides, the author stated that the red colour
change is a function of (oxidation) iron content in the aggregate
and emerged at around 300 °C temperature.
4. Conclusions
Based on the experimental study and microstructural analyses
presented in the paper, the following results can be drawn from
the study.
 The enhancements of Upv, fc, ffs and fsts values of concretes containing MK + GP were proved comparatively better than control
concrete (except for the concrete containing up to 20% MK
+ GP). However, no improvements were observed on concretes
containing GP compared to control concrete (except concrete
containing 5% GP).
 When regarding the concretes containing GP, and blend of MK
+ GP, 5% and 5% + 5% replacement were found to be the most

effective substitution level for enhancing the concrete strength
for all curing time.
 The decrease in the Upv and fc values of concretes was importantly enormous when the concrete specimens exposed to temperatures higher than 500 °C. The reason for the decrease in
these values are the pores and deterioration in the Ca(OH)2
and C-S-H gels, changes in the morphology and formation of
micro-cracks.
 SEM analyses conducted on the concrete specimens confirmed
the increase of porosity and deterioration in the Ca(OH)2 and
C-S-H gels as the temperature increased.
 PLM image analyses showed that the oxidation, alteration and
cracks in the matrix, interface and aggregate structures of all
concrete specimens gradually increased depending on exposed

to elevated temperatures. The crushed limestone aggregates
exhibited higher resistance to oxidation, alteration and crack
formation in the concrete specimens than that of quartz
aggregate.

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