S. KOLTKA et al.: THE USE OF NATURAL SEPIOLITE FIBER IN CONCRETE
65–74
THE USE OF NATURAL SEPIOLITE FIBER IN CONCRETE
UPORABA NARAVNEGA SEPIOLITNEGA VLAKNA V BETONU
Selçuk Koltka1, Tayfun Uygunoðlu2, Eyüp Sabah1, Muhammed Fatih Can1
1Afyon Kocatepe University, Department of Mining Engineering, 03200 Afyonkarahisar, Turkey
2Afyon Kocatepe University, Deparment of Civil Engineering, 03000 Afyonkarahisar, Turkey
uygunoglu@aku.edu.tr
Prejem rokopisa – received: 2015-07-08; sprejem za objavo – accepted for publication: 2015-12-23
doi:10.17222/mit.2015.210
To determine the effect of sepiolitic fibers on the characteristics of fresh and hardened concrete, i.e., slumping, density, air con-
tent, ultrasonic pulse velocity, freeze-thaw, compressive strength, bending and splitting, tensile strength tests were conducted by
the addition of de-fibered sepiolite to the cement in ratios of 0.5 %, 1 %, 2 % and 3 % of mass fractions. The fibers support
higher slumping for fresh concrete and increased workability. Samples with 1 % and 3 % addition have similar compressive
strength values to the reference series after 28 d. The bending strength of the 2 % added sample is closer to the bending strength
of the 28-days reference concretes; however, it is reduced for 0.5 %, 1 % and 3 %. For all the reinforcement ratios the splitting
strength increased. The highest impact strength performed for a 3 % reinforcement sample increased by 29 % and scanning elec-
tron microscopy (SEM) investigations reveal that the white sepiolite fibers are dispersed homogenously in the concrete, and
therefore it, reinforce especially at the interface of the aggregate cement paste by increasing durability.
Keywords: sepiolite fiber, mechanical activation, fiber reinforced concrete.
Za dolo~itev vpliva sepiolitnih vlaken na zna~ilnosti sve`ega in strjenega betona so bili izvedeni; razlivanje pri padcu, gostota,
vsebnost zraka, hitrost ultrazvo~nega impulza, zmrznjenje in odtajanje, tla~na trdnost, upogibanje in cepilno natezna trdnost z
dodatkom razvlaknjenega sepiolita betonu v masnih dele`ih 0,5 %, 1 %, 2 % in 3 %. Vlakna pospe{ujejo razlivanje sve`ega
betona in izbolj{ajo njegovo oblikovalnost. Vzorca z dodatkom 1 % in 3 % sta pokazala podobno tla~no trdnost kot referen~na
serija po 28 d. Upogibna trdnost pri vzorcu z 2 % dodatka je zelo blizu upogibni trdnosti referen~nega betona po 28 d; vendar je
manj{a pri dodatkih 0,5 %, 1 % in 3 %. Pri vseh dodatkih pa je cepilna trdnost narasla. Najvi{ja udarna `ilavost izvedena pri
vzorcu s 3 % dodatkom je narasla za 29 % in preiskava z vrsti~nim elektronskim mikroskopom (SEM) je odkrila, da so bela
sepiolitna vlakna homogeno razpr{ena v betonu, zato oja~ajo stik agregata s cementno pasto in pove~ajo trajnost betona.
Klju~ne besede: sepiolitna vlakna, mehanska aktivacija, z vlaknom oja~an beton
1 INTRODUCTION
The term fiber reinforced concrete (FRC) is defined
by the American Concrete Institute (ACI) Committee
5441 as a concrete made of hydraulic cements containing
fine or fine and coarse aggregates and discontinuous dis-
crete fibers. Inherently, concrete is brittle under tensile
loading. The mechanical properties of concrete can be
improved by reinforcement with randomly oriented short
discrete fibers, which prevent and control the initiation,
propagation, or coalescence of cracks.2,3 The character
and performance of FRC changes depending on the ma-
trix properties as well as the fiber material, fiber concen-
tration, fiber geometry, fiber orientation, and fiber distri-
bution.4,5
In the construction industry, which has undergone a
very rapid development period since the 1980s, fiber-re-
inforced precast concrete products have been fulfilling
an important role for both designers and contractors by
offering technical and aesthetic convenience. Because of
the health problems it caused, asbestos, once widely used
as a provider of fibrous structure, left its place to fibrous
synthetic materials.6 Straw was used to reinforce
sun-baked bricks, and horsehair was used to reinforce
masonry mortar and plaster. Human beings have always
been remarkably adaptable, working with the materials
around them to make whatever is required. A wide
variety of fibers have thus been used with cement-based
matrices. They include metallic fibers, polymeric fibers,
mineral fibers and vegetable fibers. In recent years, a
great deal of interest has been created worldwide in the
potential applications of natural fiber-reinforced,
cement-based composites7,8. Wood, grasses, clay and
stone have all been used as they occur naturally, but
man’s ability to process natural resources has improved
in parallel with man’s own development. In modern
times, a wide range of engineering materials (including
ceramics, plastics, cement, and gypsum products)
incorporate fibers to enhance the composite properties.1,9
F. O. Slate10 investigated the compressive and flexural
strength of coconut-fiber-reinforced mortar. Two
cement-sand ratios by weight, 1:2.75 with a water
cement ratio of 0.54 and 1:4 with a water cement ratio of
0.82 were considered. The fibre content was 0.08 %,
0.16 % and 0.32 % by total weight of cement, sand and
water. The mortar specimens were cured for 8 d only. It
was found that, compared to that of plain mortar of both
mix designs, all the strengths were increased in the case
of fiber-reinforced mortar with all the considered fiber
contents. However, a decrease in the strength of the
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Materiali in tehnologije / Materials and technology 51 (2017) 1, 65–74 65
UDK 625.821.5:625.843:666.97.031.9 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(1)65(2017)
mortar with an increase of the fiber content was also ob-
served. Z. Li et al.11 studied the fiber-volume fraction
and the fiber-surface treatment with a wetting agent for
coir-mesh-reinforced mortar using non-woven coir mesh
matting. They performed a four-point bending test and
concluded that cementitious composites, reinforced by
three layers of coir mesh with a low fiber content of
1.8 %, resulted in a 40 % improvement in the maximum
flexural strength. The composites were 25 times stronger
in flexural toughness and about 20 times higher in
flexural ductility when compared to plain composites. R.
Siddique12 used the natural san fibers (its botanical name
is Crotalaria juncea) having a length of 25 mm and three
percentages by volume of concrete (0.25 %, 0.50 % and
0.75 %) in concrete. The author investigated the fresh
and hardened concrete properties such as slump, Vebe
time, compressive strength, splitting tensile strength,
flexural strength and impact strength. As result, it was
found that the slump flow and compressive strength
decreased with an increase in the percentage of san fibers
when the flexural and impact strengths increased. How-
ever, some fibers can negatively affect the mechanical
properties of composites. For example, the addition of
sisal fibers to cement mortar matrices reduced the com-
pressive strength by 18.4 % to 32 %, the elastic modulus
by 1.3 % to 15 %, the longitudinal strain capacity at ulti-
mate stress by 15.2 % to 32.9 %, and the lateral strain
capacity by 4.2 % to 24.9 %. S. K. Al-Oraimi and A. C.
Seibi13 compared to natural and glass-fiber-reinforced
concrete properties. They also report that strength is
generally marginally decreased by the addition of glass
and natural fibres. New types of fibers, new methods of
fabrication and different types of applications are
continuously being developed. One of them is sepiolite.8
Sepiolite is a natural clay mineral with a formula of
magnesium hydrosilicate Si12Mg8O30(OH)6(OH2)4·8H2O
characterized by its fibrous morphology and intra-crys-
talline channels, extending in the fiber direction
(c-axis).14 It has a fibrous structure formed by the alter-
nation of blocks and channels that grow in the fiber di-
rection. Each structural block is composed of two in-
verted tetrahedral silica sheets and a central octahedral
sheet containing Mg. In the inner blocks, all the corners
of the silica tetrahedral are connected to adjacent blocks,
but in the outer blocks, some of the corners are Si atoms
bound to hydroxyls (Si–OH).15 This unique structure im-
parts sepiolite with a fibrous matrix with channels
(3.6×105 nm) oriented in the longitudinal direction of the
fibers. The fibrous structure of sepiolite induces sorptive,
colloidal/rheological and catalytic properties, which find
a variety of diverse applications. The fiber length, width
and thickness of layered sepiolite as bundles of fiber can
range between 10 nm and 5 μm, between 10 and 30 nm
and between 5 and 10 nm, respectively. However, the
length of the fibers in sepiolite varies according to the
source of sepiolite.16 For example, the length of Ampan-
drandawa and China sepiolites reaches up to a few milli-
meters, and sometimes even a centimeter. While the fiber
dimensions of Vallecas (Spain) sepiolite are 800 nm ×
25 nm × 4 nm,17 the fiber length of the original brown
sepiolite from the Türktaciri region of Turkey is deter-
mined as 5–10 μm.18
The sepiolite was used in the production of cement as
natural clay (raw material) by Kavas et al.6 Sepiolite was
replaced with clinker in proportions of (3, 5, 10, 15, 20
and 30) % by weight. Mortar specimens were prepared
with sepiolite-blended cement at a water to cement ratio
of 0.5. The addition of 10 % sepiolite is found to
increase both the compressive and bending strengths of
the mortar. However, there is very limited study in the
literature on the performance of sepiolite as fiber in
concrete. Therefore, in this study, it was decided to use
sepiolite fibers in order to prevent micro-cracks
occurring in the concrete matrix under load so that it
improve the physico-mechanical properties of concrete,
such as compressive, bending, and splitting strengths.
2 EXPERIMENTAL PART
2.1 Materials
2.1.1 Aggregate
The maximum 22 mm nominal size of crushed
aggregate from Afyonkarahisar, Turkey was used. The
coarse aggregates were calcareous stone as crushed stone
I in 4–11 mm; and crushed stone II in 11–22 mm. The
fine aggregates were crushed stone dusty in 0–4 mm.
The characteristic properties of the aggregates are given
in Table 1 according to standards.
Table 1: Aggregate properties
Tabela 1: Lastnosti agregata
Particle
size
(mm)
Specific
gravity
(Mg/m3)
Water
adsorp-
tion (%)
Particle
shape
(%)
Los
Angeles
(%)
Ultra fine
material
22.4-11.2 2.69 0.13 13.4 30 0.56
11.2-4 2.69 0.28 14.7 0.31
4-0 2.69 0.93 5.58
2.1.2 Cement
Ordinary Portland cement (OPC) was used in the ex-
periments with a minimum strength of 45 MPa at 28 d
(CEM I 42.5 R). It complies with the requirement of Eu-
ropean Standards EN 197-1.19 The characteristic proper-
ties of cement are given in Table 2.
2.1.3 Sepiolite
The dolomitic sepiolite samples taken from the
Eskisehir–Sivrihisar Region (purity grade is 40 %) are
subject to reinforcement procedure, and the sepiolite
samples with fibers uncoupled by a mechanical activa-
tion (Figure 1) are used in the manufacture of the fi-
ber-reinforced concrete. After the dolomitic sepiolite
having a 10,000 mPa·s times 10 % (poor quality) is rein-
forced by gravitational methods, its viscosity increases
S. KOLTKA et al.: THE USE OF NATURAL SEPIOLITE FIBER IN CONCRETE
66 Materiali in tehnologije / Materials and technology 51 (2017) 1, 65–74
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
up to 19,000 mPa·s. The sepiolite to be used as a fiber in
a concrete is a reinforced white sepiolite having
uncoupled fibers and a viscosity of 19,000 mPa·s. A
blender was used to disperse the fiber bundles that form
the structure of the sepiolite and provide the gelation at
high speed (max 20,000 min–1). Sepiolite suspensions
prepared in the 2000 mL steel cup of a blender.
2.1.4 Superplasticizer
A polycarboxylic-based new generation superplasti-
cizer admixture was used to obtain a good workability
for the fresh SEPRC mixture. Its specific gravity was
1.2 kg/dm3.
2.2. Preparation of SEPRC specimens
Several series of samples were prepared to test the in-
fluential variables on the mechanical and physical prop-
erties of fresh and hardened concretes with sepiolite. The
cement dosage, water to cement ratio and chemical ad-
mixture content were 350 kg/m3, 0.55 % and 1.5 % (by
weight of cement), respectively, in the mixtures. The
volume of aggregate was determined for the reference
Portland cement concrete by assuming that approxima-
tely 1.5 % of air is trapped in the fresh concrete. The six-
teen different series were designed by using fiber con-
tents as 0 % (control), 0.5 %, 1 %, 2 % and 3 % by
weight of cement. The mixing of concrete batches was
carried out using a small drum mixer. The concrete dried
components were mixed for 2 min. Then, to encourage a
uniform distribution of fibers throughout of the concrete,
fibers were added to the mixing water slowly (with
diluted admixture). The mixing water was poured into a
drum; the aggregates and cement were fully mixed. The
fresh concrete was mixed for 3 min (Figure 2). The mix-
ture proportion per cubic meter of sepiolite-fiber-rein-
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Table 2: Characteristic properties of CEM I 42.5R cement
Tabela 2: Zna~ilne lastnosti cementa CEM I 42,5R
Component Content, %
CaO 63.56
SiO2 19.3
Al2O3 5.50
Fe2O3 3.46
MgO 0.86
SO3 2.96
K2O 0.80
Na2O 0.13
A.K. 1.15
Physical properties
Initial setting time, h 2.52
Final setting time, h 4.36
Volume expansion, mm 3.00
Blaine surface area, cm2/g 3212
Specific weight, g/cm3 3.07
Mechanical properties
Compressive strength, MPa 7 d 38.7
28 d 46.0
Figure 2: Concrete mixing procedure depends on the time
Slika 2: Me{anje betona je odvisno od ~asa
Table 3: Physico-mechanical tests on SEPRC
Tabela 3: Fizikalno-mehanski preizkusi na SEPRC
Experiments Specimen Size Standard
Fresh concrete
Slump test20 6 dm3 EN 206-1 (2000)
Unit weight21 7 dm3 EN 12350-6 (2009)
Air content22 7 dm3 EN 12350-7 (2009)
Hardened concrete
Compressive
strength23 (150×150×150) mm EN 12390-3 (2009)
Splitting strength24 (Ø150×300) mm EN 12390-6 (2009)
Flexural strength25 (100×100×350) mm EN 12390-5 (2009)
Ultrasonic pulse
velocity26 (150×150×150) mm EN 13791 (2007)
Freezing-thawing
test27 (100×100×150) mm EN 1367-1 (2007)
Figure 1: SEM images of sepiolite fibers
Slika 1: SEM-posnetka sepiolitnih vlaken
forced concrete (SEPRC) is given in Table 2. The fresh
concrete was placed in the moulds with a shaker and they
were de-molded after 24 h. The specimens were cured in
water at 20 °C for 7 d and 28 d. The experiments that are
carried out on the SEPRC series are summarized in
Table 3 according to the related standard.21–28
For the SEPRC specimens, compressive strength was
defined according to the EN 12390-323 standard using a
2000 kN compressive machine with a rate of loading
controller on cubic specimens (Table 4) aged for 7 d and
28 d, respectively. Flexural strength was defined on the
prismatic specimens under the mid-point loading with
200 kN manual controlling bending test machine. For the
freeze–thaw testing, the SEPRCs were exposed to ASTM
C66628 Procedure A conditions: the specimens were kept
in a fully saturated condition with temperature cycling
between –17 °C and +20 °C, each cycle took 6 h. The
climatic chamber used consisted of cooling and heating
equipment producing continuous freeze–thaw cycles
with chamber temperatures ranging from –20 °C to +20
°C. The specimens were frozen for 1 h in air at –17 °C,
and then they were immersed into the water to thaw for 2
h at + 20 °C. In total, 30 freeze–thaw cycles were
performed on each of the SEPRCs.
Table 4: Quality of concrete with ultrasonic testing29
Tabela 4: Ocena kvalitete betona z ultrazvo~nim preizku{anjem29
Wave velocity (km/s) Quality of concrete
> 4.5 Perfect
3.5 – 4.5 Best
3.0 – 3.5 Suspect
2.0 – 3.0 Weak
< 2.0 Very weak
Moreover, freshly fractured surfaces of the SEPRC
chips were coated with gold in a vacuum evaporator.
They were examined using a Zeiss EVO LS 10 scanning
electron microscope (SEM) to determine the morpholog-
ical and mineralogical features.
3 RESULTS AND DISCUSSIONS
3.1 Fresh concrete tests
The slump test that makes it possible to determine the
inspection of viscosity of the fresh concrete easily and
properly is the slump test. The slump value of 0.5 %,
1 %, 2 % and 3 % of the SEPRC are 17, 16.3, 17 and 6
respectively, while the slump value of the reference con-
crete (0 % sepiolite) is 16. In 3 % SEPRC fiber-rein-
forced concrete, critical reason of a reduction in the
slump value is that the sepiolite fibers have large surface
areas and a capability to retain water particles29. Thus,
the concrete containing 3 % sepiolite has almost no
consistency because of high viscosity and becomes
unworkable. However, it is behind the limits specified in
the EN 206-120 standard for slump values. In lower
sepiolite reinforcement ratios, the slump values are
within acceptable limits for concrete manufacture. For
instance, 0.5 %, 1 % and 2 % sepiolite-reinforced
concretes are S3 grade, while the inspection series are S4
grade, in accordance with the EN 206-120 standard for
slump values. In the SEPRC, workability of the
concretes increases up to a fiber use at 2 % and their
slump values increase by approximately 6 % with re-
spect to the inspection series. But, as the fiber amount in-
creases more, the slump value decreases significantly.
The inspection series and SEPRCs up to a 2 % fiber ad-
dition are S4 grade, in accordance with the EN 206-120
standard for slump values, and the final series are S1
grade.
In Figure 3 it is clear that because slump value of
SEPRC 0.5 % and SEPRC 2 % is 17 cm, while slump
value of the reference concrete 16 cm, workability of the
concrete is much better than SEPRC 0. In other words,
sepiolite fibers increase the workability of plain con-
crete. R. Jarabo et al.30 also reported that sepiolite
modifies the rheological properties of cement when used
as an additive. This is an important result, because al-
most all the fiber types that were used to improve the
mechanical properties, generally reduced the workability.
However, 3 % fiber added to the concrete absorbs water
and cement and creates a powerful mesh structure.
Therefore, an increase in viscosity restricts the flow and
gives a lower slump value (6 cm)31,32. Thus, J. Edgington
et al.33 recommend a 2 % steel fiber content maximum
and show that as polypropylene fibers are added more
than 1 %, they cause a large decrease in workability. In
this study, it is determined that a 3 % fiber addition
reduces the workability.
A gap remains absolutely in fresh concrete during the
pouring of concretes in molds. Concretes are compressed
and placed by vibration or other methods in their molds
and thus such gaps are minimized. Gaps trapped in the
concrete both reduce the strength of the concrete and
cause the concrete to contain more gaps, and thus to be-
come more instable against any exterior environmental
effects while the concrete is hardened. Figure 3 shows
the air content in the white sepiolitic fiber-reinforced
concrete. Except in particular circumstances (such as
concretes subject to freeze-thaw), it is not assumed that a
fresh concrete contains a gap between 2–3 % and a hard-
S. KOLTKA et al.: THE USE OF NATURAL SEPIOLITE FIBER IN CONCRETE
68 Materiali in tehnologije / Materials and technology 51 (2017) 1, 65–74
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Figure 3: Slump and air content of sepiolite-reinforced concrete
Slika 3: Razlezenje in vsebnost zraka v betonu oja~anem s sepiolitom
ened concrete contains a gap more than 6–8 %. It can be
seen that the air content of SEPRC fibers is between
1.5 % and 2.1 %. Therefore, air content of the concretes
containing SEPRC is within the proposed range.
Unit weights of fresh SEPRCs are presented in
Figure 4. There is no change in the unit weight values of
fresh concrete depending on an increase in sepiolite
ratios added to concrete and such values of around
2360–2400 kg/m3.
3.2 Hardened concrete
3.2.1 Compressive strength
The cube samples manufactured in the sepiolite-rein-
forced concretes are subject to the compressive strength.
7- and 28-day strengths are determined for the time-de-
pendent strength of the samples. The age of the samples
in all concrete series gets along, depending on the
strength development and thus their compressive
strength increases depending on the development of any
hydration products. While their compressive strength re-
duces by 5 % when the sepiolite fibers are used at 0.5 %,
as shown in Figure 5, their strength remains the same as
the reference series, when they are at 1 % and 3 % at 28
d. The probable reason for this behavior includes poor
zones occurring due to poor homogeneity of the con-
crete, a high water ratio or an excessive amount of fibers.
Similar results were observed on natural coconut-fi-
ber-reinforced concrete by M. Ali et al.34. The mechani-
cal properties such as the static modulus of elasticity,
compressive strength, compressive toughness, splitting
tensile strength, modulus of rupture, total toughness in-
dex and density were investigated by the authors. They
reported that the properties can increase or decrease de-
pending on fiber length and content.
3.2.2 Flexural strength
In concrete series containing SEPRC, the results that
are closest to the reference series are obtained in the con-
cretes containing 1 % SEPRC. Upon the addition of 3 %
fiber, the bending strength values are increased by ap-
proximately 22.9 % (Figure 6). However, it is deter-
mined that the bending strength of the concretes was re-
duced due to difficulties in mixture and molding of the
3 % sepiolitic fiber-reinforced concrete when compared
to 2 % of fiber content. It is reported by M. Emiroðlu35
that, upon addition of sepiolite to the concrete, it reduces
the workability and there is a reduction in the bending
and compressive strengths due to its ability to retain an
excessive amount of water. It is determined that a large
number of micro acicular sepiolite fibers are dispersed
between the matrix and the aggregates, while the
sepiolitic fiber addition is 2 %. An increase in workabi-
lity of the SEPRC concretes confirms the homogeneity
in the microstructure. Its effects on rupture behavior are
clearer than an increase in the bending strengths of the
sepiolitic fibers.
3.2.3 Splitting strength
The weakest properties of the structural materials
such as cement-based concrete include a lower tensile
strength. Therefore, a different type and nature of fibers
are added to the concrete to increase or improve its ten-
sile strength. One of the tests, where the tensile strength
is determined, is the splitting strength that gives the ten-
sile strength indirectly in a real-like way. Aggregate dis-
persion of the SEPRC3 sample in the concrete that gives
a high splitting strength is homogenous. Furthermore,
the concrete is not separated from its matrix, but it is
ruptured and broken into two parts during the force ap-
plied in the splitting of the concrete. The reason for this
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 6: Flexural strength of SEP-reinforced concrete
Slika 6: Upogibna trdnost betona utrjenega s SEP
Figure 4: Fresh unit weight of sepiolite fiber-reinforced concrete
Slika 4: Enota sve`e mase betona oja~anega s sepiolitnimi vlakni
Figure 5: Compressive strength of SEP-reinforced concrete
Slika 5: Tla~na trdnost betona utrjenega s SEP
is that the matrix and the interface of the aggregate and
matrix are reinforced by the sepiolite fibers. In the con-
crete samples that give a low splitting strength, the ag-
gregate is ruptured and separated from the cement paste
due to the poor interface of the cement paste and
aggregate.36 It was observed that the sepiolitic fibers in-
crease the adherence of the aggregate and cement paste
and the splitting strength of the concrete (Figure 7). In
the samples containing SEPRC, their splitting tensile
strength increases by 6 % and 33 %, respectively, upon
the addition of 2 % and 3 % fiber, while their tensile
strength never changes significantly upon the addition of
0.5 % and 1 % fiber.
3.2.1 Microstructure
In Figure 8a to 8e, it was seen in the concrete sample
that the hydrate cement matrix and the aggregate inter-
face have a fairly dense structure. When a crack in this
aggregate is observed by 20000× magnification, an
anti-crack effect of the sepiolitic fibers (A) is observed
clearly. Upon an increase of the sepiolitic fibers, it con-
tributes positively for the cement paste to become more
stable and the gaps in the concrete to reduce. The dense
calcium hydroxide hydrate (C-S-H) gels in the concrete
and micro sepiolite fibers between these gels makes it
possible to reduce the gap and to make the concrete
denser. Figure 8, A, B and C show calcium hydroxide
(CH), C–S–H gel and the ettringite needles, respectively.
Since it is a binder final product in a concrete, the
C–S–H gel is a critical component, and consequently it
contributes mostly to the strength. It is clear that strength
development is fairly fast in the inspection series. It is
very effective for any chemical additive used on these
dispersed cement particles and react better with water.
S. Kakooei et al.37 show that the macro fibers (L = 4 mm)
reduce the crack formation and development and
increase the compressive strength. But, a large majority
of studies conducted on polypropylene-reinforced
concretes show that there is no or very little compressive
strength.31,38 When the SEM image of the SEPRC0.5
concrete is analyzed in Figure 7, the nano-sepiolite
fibers contribute the adherence of the interfaces of the
aggregates. Also, the nano-fibers in the surface of the
even aggregates improve the weakness in interface of the
aggregate cement paste. In concretes, the interfaces of
the aggregate cement paste have a critical effect on the
mechanical properties of the concrete as much as the
component features.39
It is determined that there is a film layer formed by
calcium hydroxide on the aggregate surface in contact
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70 Materiali in tehnologije / Materials and technology 51 (2017) 1, 65–74
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Figure 7: Splitting strength of SEP-reinforced concrete versus fiber
content
Slika 7: Cepilna trdnost betona oja~anega s SEP glede na vsebnost
vlaken
Figure 8: Distribution of sepiolite fibers in hydrated concrete: a) 0 %, b) 0.5 %, c) 1 %, d) 2 %, e) 3 %)
Slika 8: Razporeditev vlaken sepiolita v hidriranem betonu: a) 0 %, b) 0.5 %, c) 1 %, d) 2 %, e) 3 %)
with the transition zone and such a film is coated by
another layer in a thin C-S-H (tobermorite) form. It is
observed that such a double coat of film layer is also
coated by a calcium hydroxide later in its interface.40
3.2.5 Ultrasonic pulse velocity
Although it is difficult to find the compressive
strength of any concrete by the ultrasonic test method in
a sufficiently accurate way, because the ultrasonic wave
velocity passing through any concrete is associated
closely with amount (and density) of the gap in that con-
crete, it may be possible to establish a relation based on
the resultant ultrasonic velocity and concrete grade. If
the ultrasonic velocity is determined as a result of the ex-
perimental studies conducted on the concrete with a den-
sity of 2400 kg/m³, the results on the concrete grade are
shown in Table 4.41 When the inspection series are
compared in terms of ultrasonic pulse velocities in
Figure 9, the ultrasonic velocity is obtained as 3.9 km/s,
4.3 km/s, 4.5 km/s and 4.4 km/s for the 0.5 %, 1 %, 2 %
and 3 % fiber contents in 28-day concretes. Since their
ultrasonic pulse velocity is very close to 4.5 km/s, the
concretes are in good and high-quality concrete grade.
The concrete series with 0.5 % fiber content are in a
good concrete grade. It is recommended that fibers are
used at a ratio of 2 % and more for high-quality concrete
manufacture in terms of ultrasonic pulse velocity.
3.2.6 Modulus of elasticity
The modulus of elasticity of concrete is a key factor
for estimating the stiffness and deformation of the build-
ings and members. The precise determination of the
modulus of elasticity of concrete is very important for
structures that require strict control of the deformability.
In order to make full use of the compressive strength po-
tential, the structures using high-strength concrete tend
to be slimmer and require a higher elastic modulus so as
to maintain its stiffness. Therefore, knowledge of the
modulus of elasticity of high-strength concrete is very
important in avoiding excessive deformation, providing
satisfactory serviceability, and avoiding the most cost-ef-
fective designs.42,43 In the codes and standards related to
the design of concrete structures, the modulus of elas-
ticity of concrete is usually proposed by empirical
equations depending on a function of the compressive
strength of the concrete. Therefore, the E-moduli values
were obtained with ACI codes and Turkish standards
(TS) depending on the compressive strength, and it was
observed that they are relatively similar when comparing
them with the recommendations found in ACI 31844, ACI
36345 and TS 500. The following equations are re-
commended by ACI 31836 and TS 50046 for the relation-
ships between compressive strength (fc) and the E-mo-
duli is as follows for concretes:
(ACI 318)44 Ec = 4730×(fc)
1/2 (1)
(ACI 363)45 E
g
fc c=
⎛
⎝
⎜ ⎞
⎠
⎟ +
2346
10500 70000
1 5
.
( )
.
(2)
(TS 500)46 Ec = 14000 + 3250×(fc)
1/2 (3)
Where fc, Ec, and  represent the compressive
strength (mPa) elastic modulus of concrete (mPa), and
unit weight (kg/dm3) of sepiolite reinforced fiber
concrete, respectively.
A comparison of static modulus of elasticity obtained
from the empirical expressions given by the various de-
sign codes for both plain concrete and sepiolite fiber re-
inforced concrete is presented in Figure 10. It shows the
modulus of elasticity predicted by TS 50046 is higher
than compare to other code prediction. Values of elastic-
ity modulus vary from 28 to 27 GPa for ACI codes when
it was 33 GPa according to the TS 50046 standard. A
slight decrease in the results is observed for ACI codes
with an increasing addition of sepiolite volume fraction.
However, there was no change for the power function of
the TS 50046 standard. The modulus of elasticity of the
concrete depends upon the modulus of elasticity of the
hydrated cement matrix, type and content of aggregates,
the water to binder ratio, and the volume of the cement41.
The E-moduli of fiber-reinforced concretes may decrease
or increase depending on the orientation or distribution
of the fibers in the concrete. This is because the concrete
can be assumed to be a highly heterogeneous material
due to its composite structure. As the orientations and
distributions of the fibers in the concrete mixture are ran-
dom, the fiber-reinforced concrete is considered more
heterogeneous than the plain concrete. Therefore, the
E-moduli of sepiolite-fiber-reinforced concretes under
compression have very similar changes depending on the
distribution of sepiolite in concrete. The principal role of
sepiolite fibers is resisting the formation and growth of
cracks by providing pinching forces at the crack tips.
3.2.7 Freezing-thawing test
An expedited freeze-thaw test consisting of 30 peri-
ods is conducted for the purposes of revealing the dura-
bility properties of the sepiolite-fiber-reinforced con-
cretes under freeze-thaw circumstances, under which
they may be subject to, and then weight loss, density
loss, a variation in water-absorption ratios and compres-
S. KOLTKA et al.: THE USE OF NATURAL SEPIOLITE FIBER IN CONCRETE
Materiali in tehnologije / Materials and technology 51 (2017) 1, 65–74 71
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 9: UPV values of sepiolite fiber-reinforced concrete versus fi-
ber content
Slika 9: UPV vrednosti pri betonu oja~enem z vlakni sepiolita v
odvisnosti od vsebnosti vlaken
sive strength are determined. In Figure 11, the density
loss of 0.5%, 1 %, 2 % and 3 % fiber contents is respec-
tively 1.28 %, 1.17 %, 1.15 % and 1.39 %, while the
density loss of the reference concrete is 1.09 %. There is
no significant change in the density loss of the white
sepiolitic-fiber-reinforced concrete with regards to the
reference concrete. Any observations similar to weight
loss are conducted in the density of the concretes. How-
ever, density loss represents an increase of 7 %, 6 % and
28 %, respectively, upon the addition of 1 %, 2 % and
3 % fiber. A decrease in weight loss and density loss
shows that there is a concrete more resistant against any
freeze-thaw effects.
In Figure 12, it is determined that the compressive
strength of the 3 % concrete sample obtained from the
sepiolite-fiber-reinforced concretes is very close to the
reference concrete before and after the freeze-thaw cy-
cle. A maximum difference between the compressive
strength values are obtained in the sepiolitic concretes
upon the addition of 0.5 % and 1 % fibers with respect to
the reference series. Fibers as high as 2 % and 3 % are
used in both types of fiber, upon an increase in the
strength of the interface of the aggregate-cement paste of
the manufactured concretes and also of the cement paste
under stresses, the strength losses of the concretes under
a freeze-thaw effect are similar to the inspection series.
In the inspection series, the strength loss is 2 %. In the
concrete series containing SEPRC, in which 2 % and
3 % fibers are used, the strength losses are determined as
8 % and 5 %. Internal stresses occur as a result of the
water in the concrete subject to freeze-thaw. Therefore,
any cracks tend to occur previously in the cement paste.
The sepiolitic fibers make it possible to overcome such
internal stresses caused by the frozen water and prevent
the concrete from being cracked. Since short fiber types
greatly increase the number of fibers used in the con-
crete, they are used to decrease cracking and increase du-
rability depending on the properties of the materials
used; whereas, long fibers aim more often to increase the
mechanical properties of the concrete.47
Upon use of the white fibers at 0.5 %, 1 % and 3 %, a
reduction of 3 % occurs in the 2 % fiber content, in-
creases of 3 %, 97 % and 44 % are observed with regard
to the reference concrete (Figure 12). As a result of such
a freeze-thaw procedure, it is shown that dispersion in
the white sepiolitic fiber-reinforced concrete is more. In
Figure 13, water absorption values are 0.69 %, 2.33 %,
1.23 % and 0.97 %, respectively, in 0.5 %, 1 %, 2 % and
3 % fiber contents, while the water absorption value of
the reference concrete is 0.63 % after the freeze-thaw cy-
cle. Porous structural materials such as concrete have a
capability to absorb water as much as open and semi-
S. KOLTKA et al.: THE USE OF NATURAL SEPIOLITE FIBER IN CONCRETE
72 Materiali in tehnologije / Materials and technology 51 (2017) 1, 65–74
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967-2017) – 50 LET/50 YEARS
Figure 12: Change of strength of sepiolite reinforced concrete de-
pending on freeze-thaw test
Slika 12: Spreminjanje trdnosti betona, oja~anega z vlakni sepiolita, v
odvisnosti od preizkusa zmrznjeno-odtaljeno
Figure 10: Modulus of elasticity of sepiolite fiber-reinforced concrete
Slika 10: Modul elasti~nosti betona, oja~anega z vlakni sepiolita
Figure 13: Change of water absorption of sepiolite reinforced con-
crete depending on freeze-thaw test
Slika 13: Spreminjanje absorpcije vode v betonu, oja~anem z vlakni
sepiolita, v odvisnosti od preizkusa zmrznjeno-odtaljeno
Figure 11: Loss of density of sepiolite reinforced concrete depending
on freeze-thaw test
Slika 11: Zmanj{anje gostote betona, oja~anega z vlakni sepiolita, v
odvisnosti od preizkusa zmrznjeno-odtaljeno
open gaps included in them. If the gap diameter in-
creases after the internal stress caused by concrete
around the frozen water in its gaps before and after
freeze-thaw cycle, their water absorption capability in-
creases accordingly. Thus, since mixture and molding of
the concretes become difficult upon use of fiber content
over 1 %, they contain gaps more according to other fi-
ber ratios and consequently the free water in gaps freezes
and its volume increases in the freeze-thaw cycles, gaps
in the concrete increase and cause absorption of more
water.48 In the concrete series containing SEPRC, it is
observed that there is a smaller increase than the others
with respect to water absorption in the inspection and
0.5 % fiber contents.
4 CONCLUSIONS
In the manufacture of sepiolite fiber-reinforced con-
crete, fiber is added to the concrete at 0.5 %, 1 %, 2 %
and 3 % by weight of the cement added by fiber. In the
concrete tests, it is ensured that, upon settlement of the
fresh concrete, the sepiolite fibers increase the work-
ability of the concrete and more settlement is obtained.
Furthermore, although the white sepiolitic fibers trap
more air in the concrete, such an amount is within an ac-
ceptable limit (approximately 0.5 %). The sepiolitic fi-
bers have no effect unit weight of the concrete in fresh
state. In the hardened concrete tests, it is determined that
the white sepiolitic fibers reduce the compressive
strength in comparison to the reference concrete. But,
upon addition of the white sepiolitic fibers at 1% and 3%
to the concrete, the compressive strength gives a value
similar to 28-day reference concretes. The bending
strength of the white sepiolitic fibers added at 2 % is
fairly close of the bending strength of 28-day reference
concretes. However, the bending strength reduces at con-
tent ratios of 0.5 %, 1 % and 3 %.
In the white sepiolitic fibers, all the content ratios in-
crease the splitting strength. In particular, the splitting
strength in the 3 % white sepiolitic fiber content in-
creases by 29 %, and as a result of that the interface of
the aggregate and cement paste becomes more stable.
The 28-day concretes manufactured with the white
sepiolitic fiber according to the ultrasonic pulse veloci-
ties are located in a high-quality concrete grade. From
SEM studies, it was observed that the white sepiolite fi-
bers are dispersed homogeneously and reinforce espe-
cially the interface of the aggregate and cement paste.
In general, it was seen that the sepiolitic fibers have
an effect on the fresh concrete. However, it was deter-
mined that a 2 % white sepiolitic fiber content gives
more positive effects on the tensile strength among the
concrete characteristics. Although any chemical addi-
tives are used in the concrete upon the addition of 3 % fi-
ber, it is suggested that concrete is used up to 2 % com-
plete with such chemical additives due to difficulties in
the mixture.
Acknowledgement
The investigation was funded by the R&D Support
Program (SAN-TEZ) of Ministry of Science, Industry
and Technology with the project number
00523.STZ.2010-1, Republic of Turkey. The authors are
grateful for the financial support provided by SAN-TEZ.
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