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Physical and mechanical characteristics ofcement mortars and concretes with additionof clinoptilolite from Beli Plast deposit
(Bulgaria), silica fume and fly ash
V. LILKOV1 ,* , I . ROSTOVSKY2AND O. PETROV3
1 University of Mining and Geology ‘‘St. Ivan Rilski’’, Sofia, Bulgaria, 2 Central Laboratory of Physico�Chemical
Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria, and 3 Institute of Mineralogy and Crystallography,
Bulgarian Academy of Sciences; Sofia, Bulgaria
(Received 15 September 2010; revised 9 December 2010; Editor: John Adams)
ABSTRACT: Cement mortars and concretes incorporating clinoptilolite, silica fume and fly ash
were investigated for changes in their physical and mechanical properties. It was found that additions
of 10% clinoptilolite and 10% Pozzolite (1:1 mixture of silica fume and fly ash) were optimal for
improvement of the quality of the hardened products, giving 8% and 13% increases in flexural and
compressive strength respectively. The specific pore volume of the mortars incorporating zeolite
decreased between the 28th and 180th day to levels below the values for the control composition due
to the fact that clinoptilolite exhibits its pozzolanic activity later in the hydration. In these later
stages, pores with radii below 500 nm increased at the expense of larger pores. The change in the
pore-size distribution between the first and sixth months of hydration occurs mostly in the mortars
with added zeolite.
KEYWORDS: cement, pozzolan, pozzolanic activity, pozzolanic additive, zeolite, mortar, concrete,cement�zeolite mortar, clinoptilolite, silica fume, fly ash, Beli Plast deposit, Bulgaria.
Powdered minerals have been added to Portland
cement and mortar for a variety of purposes. Active
mineral additions interact with hydration products,
mainly with calcium hydroxide (portlandite), and
transform it into practically insoluble calcium
hydrosilicates and hydroaluminates (cement gel).
For high-performance concrete, addition of silica
fume produces more discontinuous and imperme-
able structures in concrete due to pore-size
modification and matrix densification, reduction in
the content of Ca(OH)2 and changes to the cement
paste�aggregate interface. During the hydration
process, the transition interfacial zone increases
gradually in density due to pozzolanic reaction
between silica fume and calcium hydroxide (Min-
Hong & Odd, 1991; Wild et al., 1995; Toutanji &
El-Korchi, 1995; Persson, 1998; Mazloom et al.,
2004; Qing et al., 2007; Song et al., 2010).
When both silica fume and fly ash are added to
cement, the resultant cement composition and
concrete have higher strength (especially in
respect to their flexural strength), higher water
permeability and higher corrosion resistance. In
addition, technological processing in production is
simplified compared to the situation when only
silica fume is used (Stoitchkov et al., 1996; Lilkov
& Stoitchkov, 1996). At the same time, the
viscosity of the cement paste is reduced compared
to the situation when the same amount of silica* E-mail: lilkov@abv.bgDOI: 10.1180/claymin.2011.046.2.213
ClayMinerals, (2011) 46, 213–223
# 2011 The Mineralogical Society
fume is used (Dimitrova & Lilkov, 1996). The
contact zone between the cement paste and the
coarse aggregate is also increased in density
(Robins & Austin, 1986; Marsh & Day, 1988).
The use of natural zeolites as pozzolanic mineral
additives for cement increases the corrosion
resistance of the cement because the pozzolanic
reactions between the zeolite particles and the
cement hydration products changes the pore
structure and strength of the cement (Li et al.,
1990; Janotka & Krajci, 2000; Akira et al., 2001;
Janotka et al., 2003; Kontori et al., 2009; Mertens
et al., 2009), increases the water demand and
viscosity of the cement solutions and influences
their plasticity (Sahmaran et al., 2008; Baldino et
al., 2008; S� ahmaran, 2008).
The addition of zeolites and fly ashes to concrete/
mortar mixes leads to a reduction in the heat
released, whereas the use of silica fume achieves
the opposite, which is related to its higher initial
pozzolanic activity (Sanchez de Rojas & Frıas,
1996; Lilkov et al., 1997; Langan et al., 2002;
Snellings et al., 2010).
The aim of this study was to develop a mineral
additive for cement based on silica fume (a
by-product from metal production at the
Kremikovtsi plant, Bulgaria), fly-ash from a
thermal power plant (Bobov dol, Bulgaria) and
clinoptilolite from Beli Plast deposit (East
Rhodopes, Bulgaria).
MATER IALS
Pure clinker cement CEM I 42.5 R (EN 197-1) was
used. The chemical and mineral compositions are
presented in Table 1.
The following were used as mineral additives:
clinoptilolite from the Beli Plast deposit (Bp); silica
fume (SF); Pozzolite (Pz) � a 1:1 mixture (by
mass) of silica fume and fly ash; a 1:1 (by mass)
mixture of clinoptilolite and Pozzolite. The zeolite
was ground to a grain size <160 mm. The specific
surface areas (BET method) were 18.5 m2 g�1 for
clinoptilolite and 19 m2 g�1 for silica fume,
respectively. The chemical compositions of the
additives are shown in Table 2.
The SEM images of the particles of the ground
zeolite (a), mixture of zeolite and silica fume (b),
silica fume (c) and Pozzolite (d) are presented in
Fig. 1. The analysis shows that the fine grains of
silica fume are attached to the surface of the fly ash
particles and, thus, a mixture which is relatively
stable against mechanical action, was formed. To a
lesser degree the same can be said for the particles
of silica fume and the zeolite. The sand used was
natural quartz (according to EN 196-1).
RESEARCH METHODS
The influence of the additives on the water demand
of the mortars was assessed according to
EN 1015-3. The cement-to-sand ratio was fixed at
1:3 and the quantity of water was such that the
mortars had flow diameters of 190�200 mm. The
flows of the mortars were within the limits of the
maximal precision of the test method.
TABLE 1. Chemical and mineral composition of the
Portland cement clinker.
Chemical composition(wt.%)
Mineral composition(%)
CaO 61.95 3CaO.SiO2 40.53SiO2 20.69 2CaO.SiO2 28.69Al2O3 5.74 3CaO.Al2O3 9.36Fe2O3 3.42 4CaO.Al2O3.Fe2O3 10.32SO3 2.6 Insoluble residue 1.93Na2O 0.33 Loss on ignition 2.42K2O 0.62 CaO-free lime 1.15Cl 0.0176 LSF 90.34Naeq 0.74
TABLE 2. Chemical compositions of the additions.
Chemicalcomposition(wt.%)
Clinoptilolite Fly ash Silicafume
SiO2 62.74 58.50 89.70Fe2O3 0.74 7.95 2.27TiO2 0.12 0.73 <0.05Al2O3 9.68 25.31 1.13MnO 0.03 0.05 0.02CaO 6.73 2.13 1.38MgO 2.90 1.78 1.75Na2O 0.29 0.86 0.49K2O 2.79 2.02 0.86P2O5 <0.03 0.05 0.12SO3 <0.03 0.25 0.40Loss on ignition 13.47 0.88 2.40
214 V. Lilkov et al.
The pozzolanic activity of the cement samples
was measured according to EN 196-5 by comparing
the quantity of calcium hydroxide in the water
solution that contacted the hydrated cement with a
composition containing 90% cement and 10%
mineral additive. The test is positive when the
concentration of Ca(OH)2 in the solution is lower
than the saturation concentration at 40ºC.
A standardized method for determining the
depth of penetration of water under pressure
according to EN 13390-8 was used. Three
samples were tested using water at a pressure of
0.5 MPa on their lower surfaces for 72 h.
Afterwards the samples were split and the
maximal depth of penetration of the water was
measured.
The heat of hydration was calculated using a
semi-adiabatic method on cement mortars with
mass 1575M1 g, binder content 360M0.5 g,
1080M1 g standard sand and 180M0.5 g distilled
water, according to EN 196-9. Pure Portland cement
and Portland cement in combination with mineral
additives (zeolite, silica fume and Pozzolite) were
used as the hydraulic binder.
Two series of cement mortars were prepared for
strength measurement. The ratio of binder cement:-
standard quartz sand was 1:3 by mass. The first
series had constant consistency and a variable
water-to-binder ratio. The second series had a
constant water-to-binder ratio and constant consis-
tency. The constant consistency for the second
series of mortars was achieved by using a
polycarboxylate water reducing admixture.
The flexural and compressive strength of the
mortars was determined using the EN 196-1
standard. The split test of the concretes was
performed using the EN 12390-6 standard.
The pore structure of the hardened mortars was
investigated over the range 50�7500 nm by
mercury porosimetry using a ‘‘Carlo Erba’’ porosi-meter. The integral and differential curves of the
pore-size distribution were obtained.
FIG. 1. SEM images of the mineral additions (Philips SEM 515): (a) particles of the ground zeolite; (b) mixture of
zeolite and silica fume; (c) silica fume; (d) particles of Pozzolite.
Cement mortars with zeolite and Pozzolite 215
RESULTS AND DISCUSS ION
Water demand results are presented in Table 3. The
water demand of the zeolite-containing mortars
increased proportionally to the dosage in the mortar
and were comparable to the water demand when the
Pozzolite additive was used. This results from the
high sorption ability and specific surface area of
clinoptilolite. The increased water demand has a
negative influence on the numerous properties of
the cement-based composites � mechanical para-
meters, chemical and corrosion resistance, perme-
ability, etc. This requires the parallel use of these
materials with admixtures which reduce water
demand.
Pozzolanic activity test results are presented in
Table 4. The pure clinker cement CEM I 42,5R
used in combination with the mineral additives
(zeolite, Pozzolite and silica fume) conforms to the
standard requirement for pozzolanic cement.
The depths of penetration of water under pressure
are presented in Table 5. The compositions exam-
ined were: reference concrete according to
EN 480-1 (0), concrete with addition of zeolite �10% (1), 15% (2), and 20% (3), with addition of
Pozzolite (4) and addition of 5% zeolite and 5%
Pozzolite (5). The mineral additions were intro-
duced as substitutes for part of the cement. The
concrete mixes had a slump of 6 to 8 cm according
to the EN 12350-2 standard. The quantity of the
mixing water necessary to achieve these desired
slump values grew with the dosage of zeolite. As a
result, reductions in strength (particularly split
tension strength) and increases in water perme-
ability (measured by depth of penetration of water
under pressure) occurred.
Calorimetric studies over the first seven days of
hydration are given in Table 6. The quantity of
released heat was expressed in two forms � per
TABLE3.Water
dem
andoftheinvestigatedcementmortars.
‘‘0’’
‘‘1’’
‘‘2’’
‘‘3’’
‘‘4’’
‘‘5’’
Composition
100%
cement
andsand
(control)
90%
cement,
10%
zeolite
andsand
85%
cement,
15%
zeolite
andsand
80%
cement,
20%
zeolite
andsand
90%
cement,
10%
Pozzolite
andsand
90%
cement,
5%
zeolite,
5%
Pozzolite
andsand
Flow
diameter,mm
200
195
202
195
197
198
W/C
�ratio
0.56
0.60
0.63
0.66
0.61
0.60
Relativegrowth
ofwater,%
�7.9
14.1
19.0
10.4
8.0
TABLE 4. The results from the pozzolanicity tests.
Sample Hydroxyl ionsconcentration(mmol l�1)
Calcium hydroxideconcentration(mmol l�1)
PC+Bp 46.80 4.88PC+SF 44.55 3.28PC+Pz 48.62 4.07PC+Bp+Pz 46.80 3.82
216 V. Lilkov et al.
gram of binder (cement + mineral addition) and per
gram of cement. The lower heat of hydration of the
cement with the addition of zeolite (compared with
pure cement) is due to the smaller quantity of
cement in the mortar and relatively slow pozzolanic
reaction between clinoptilolite and the products of
hydration. The heat of hydration of the composi-
tions with Pozzolite and Pozzolite and clinoptilolite
was comparable to those of the mortar with added
clinoptilolite. The influence of silica fume on the
hydration process is obvious � an acceleration of
the interaction and an increase in the heat of
hydration.
If, however, the quantity of heat released is
calculated per gram of cement in the mortar, it can
be seen that after the fourth day of hydration it is
greater in the composition with added zeolite
compared to the control mix and it is comparable
to mortars containing Pozzolite. This means that the
zeolite particles influence the intensity of the
processes of formation of hydration products.
The mechanical test results after 28 and 180 days
of hydration are presented in Table 7. The
substitution of part of the cement (10�20 %) with
zeolite in the mortars with equal consistency and
variable water-to-cement ratio leads to a reduction
both in the flexural strength (15�25%) and in the
compressive strength (20�36%). These differences
are characteristic for 28 days age as well as for 6
months age. It cannot be deduced from these data
how much of the strength reduction is due to a
decrease in the cement content and how much to
the increased water-to-binder ratio. The results from
the testing of samples with equal consistency and
equal water-to-binder ratio provide the answer to
this question. At 28 days, the reduction in flexural
strength is 15�18% and in the compressive strength
is 13�27%.
After 180 days, the reduction is between 7 and
22% for the compressive strength and between 8
and 12% for the flexural strength, which means
there is a contribution from the pozzolanic reaction,
TABLE 5. Results from water penetration test.
Series Average splitting forcefor three samples
(N)
Splitting tensionstrength
(N mm�2)
Depth of penetrationof the water
(mm)
‘‘0’’ 99700 2.77 9‘‘1’’ 98150 2.73 10.3‘‘2’’ 89000 2.72 13‘‘3’’ 98000 2.47 15‘‘4’’ 112800 3.13 8‘‘5’’ 109500 3.04 9
TABLE 6. Heat of hydration per 1 g binder (in numerator) and per 1 g cement (in denominator).
—————— Released heat of hydration J g�1 for a period of time ——————Composition 1 h 2 h 3 h 4 h 5 h 6 h 1 d 2 d 3 d 4 d 5 d 6 d 7 d
PC (control)2121
2525
2828
3636
4444
5252
� � 365365
385385
398398
407407
414414
PC + Bp1618
2022
2528
3033
3943
4853
234260
� 313348
340378
362402
377419
389432
PC+Bp+Pz1618
2022
2427
3134
3943
4853
� � 330367
354393
371412
384427
393437
PC+Pz1517
2022
2326
2932
3438
4146
� � 340378
364404
379421
392436
402447
PC + SF2022
2427
2831
3640
4247
5056
� � 366407
391434
408453
421468
430478
Cement mortars with zeolite and Pozzolite 217
which
TABLE7.Resultsfrom
testingofcementmortars
withmineral
additionsat
constantconsistencyandvariable
water-to-binder
ratio(innumerator)
andat
constant
consistency
andconstantwater
tobinder
ratio(indenominator)
100%
cement
andsand
(control)
90%
cement,
10%
zeolite
andsand
85%
cement,
15%
zeolite
andsand
80%
cement
,20%
zeolite
andsand
90%
cement,
10%
Pozzolite
andsand
90%
cement,
5%
zeolite,
5%
Pozzolite
andsand
Cem
entmortarswithequal
consistence
comp.‘‘0’’
comp.‘‘1’’
comp.‘‘2’’
comp.‘‘3’’
comp.‘‘4’’
comp.‘‘5’’
Flexuralstrength
at28-day
age,
Nmm
�2
9.69.9
8.28.3
7.78.3
7.18.1
9.4
10.7
8.8
10.3
Compressivestrength
at28-day
age,
Nmm
�2
43.6
43.5
34.9
37.7
29.7
34.6
27.8
31.8
40.9
49.1
40.6
45.5
Flexuralstrength
at6-m
onthsage,
Nmm
�2
9.6
10.5
9.29.7
8.19.8
7.79.2
9.8
10.8
9.2
10.9
Compressivestrength
at6-m
onthsage,
Nmm
�2
59.2
52.1
44.8
48.5
42.6
44.6
37.7
40.6
59.8
57.8
55.8
53.8
Relativegrowth
ofthecompressivestrength,
%from
28th
to180th
day
ofhydration
+35.8
+19.8
+28.4
+28.6
+43.4
+28.9
+35.6
+27.6
+46.2
+17.7
+37.4
+18.2
218 V. Lilkov et al.
increases the strength when zeolite is used. This is
confirmed by the larger relative growth of the
strength of the zeolite-containing compositions
between 28 and 180 days of hydration.
The replacement of 10% of the mass of the cement
in the cement mortars with Pozzolite in mixtures
with equal consistency and water-to-binder ratio led
to increases in both flexural and compressive strength
(of 8% and 13%, respectively). The rise of the
compressive strength up to six months of age
(compared with the control mix) is evidence for the
interactions between the additive and the products of
hydration of the Portland cement.
The replacement of 10% of the mass of the
cement in the cement mortars with the 1:1 mixture
of Pozzolite and clinoptilolite from the Beli Plast
deposit led to increased strength (~3�5%) in each
case.
The pore structures of the cement mortars
(cement/sand ratio 1:3) evolved over time. The
integral curves of the pore-size distribution of the
cement mortars with equal consistency and different
water-to-binder ratio for hydration ages of 28 and
180 days are shown in Figs 2 and 3 respectively.
Increasing the quantity of the zeolite goes hand-in-
hand with increases in the water-to-binder ratio, the
FIG. 2. Integral curve of pore-size distribution for the mortars from first series for the 28th day of hydration.
FIG. 3. Integral curve of pore-size distribution for the mortars from the first series for 6-months age.
Cement mortars with zeolite and Pozzolite 219
relative specific pore volume and the average pore
radius. The replacement of part of the cement with
Pozzolite leads to a decrease in the total pore
volume compared with pure cement mortars. The
same was also observed when the 1:1 mixture of
Pozzolite and zeolite was used at the 10% level.
It is clear that the changes in the differential
pore-size distribution occur between the first and
sixth month of hydration, mostly shown in mortars
containing zeolite additives. Table 8 shows data for
the differential pore-size distribution and specific
pore volume of the mortars with and without
mineral addition. The mortars are of equal
consistency and different water-to-binder ratio
(series 1). The specific pore volumes of the
mortars with addition of zeolite were greater than
that of the control after hydration for 28 days. The
substitution of 10% from the cement with Pozzolite
or equal quantities of zeolite and Pozzolite led to a
reduction of the specific pore volume compared
with the control cement mortar.
Between the 28th and 180th days of hydration,
the specific pore volumes of the mortars with 10%,
15% and 20% clinoptilolite decreased by factors of
1.77, 1.34 and 1.30, respectively. The control
decreased by a factor of 1.11 over the same
period. The specific pore volumes of the mortars
with 10% Pozzolite and with 5% Pozzolite and 5%
zeolite decreased by factors of 1.19 and 1.28,
respectively. This is due to the fact that the zeolite
exhibits its pozzolanic activity at later stages of
hydration than silica fume and Pozzolite. The
zeolite particles interact with the products of the
cement hydration, increasing the quantity of the
hydrosilicates and hydroaluminates, decreasing the
quantity of portlandite, and compacting the contact
area between the zeolite particles and cement paste,
and also between the grains of sand and the cement
paste. Thus, at the later stages of hydration, the
fraction of the pores having radii below 500 nm
increased at the expense of the fraction with larger
pores.
SEM images of the hydrated cement paste
containing zeolite are presented in Fig. 4. The age
of the paste is six months. The right-hand part of
Fig. 4a represents broken zeolite particles.
Microscopic pores having dimensions <1 mm are
visible on its surface. The left-hand part of the
figure shows the hydration products on the surface
of the zeolite particles and the pores in the
hardened cement paste with diameters <10 mm.
The hydration products on the surface of the zeolite
grain were photographed in the pore between two
zeolite particles which serve as a base for oriented
growth of hydration products having different forms
and micron dimensions.
CONCLUS IONS
The water demand of the mortars with clinoptilolite
increases proportionally to zeolite dosage and is
comparable to the water demand of the mineral
additive Pozzolite. Mineral additives consisting of
clinoptilolite + Pozzolite and silica fume +
clinoptilolite conform to the standard requirements
for pozzolanic additives in cement.
The lower values of the heat of hydration of
cement + clinoptilolite are due to a number of
factors; the smaller quantity of cement in the
mortar, the slower pozzolanic reaction between
clinoptilolite and the products of hydration, and the
fact that the zeolite particles sorb part of the water
and slow the solubility of the cement particles.
The heats of hydration of the compositions with
Pozzolite and Pozzolite + clinoptilolite are compar-
able with that of the mortar with clinoptilolite.
Silica fume accelerates the interaction and increases
the heat of hydration.
FIG. 4. SEM images of cement paste containing zeolite after 6 months of hydration: (a) broken zeolite particle;
(b, c) hydration products on the surface of the zeolite grains.
220 V. Lilkov et al.
TABLE8.Datafordifferential
pore-sizedistributionandthespecific
pore
volume.
Pore
radius
(nm)
<100
100�250
250�500
500�750
750�1500
1500�7500
Specific
pore
volume
(cm
3g�1)
Pure
cemen
t28th
day
relative
volume%
10
21
35
10
321
0.0667
180th
day
relativevolume,
%5
11
30
519
30
0.0603
Relativechangeofthevolumeofthepores,
%�100
�47.6
�14.3
�100
+533
+42.9
�9.6
10%
zeolite
28th
day
relative
volume%
510
31
15
16
23
0.0884
180th
day
relativevolume,
%12
23
35
06
24
0.0500
Relativechangeofthevolumeofthepores,
%+140
+130
+12.9
��62.5
+4.3
�43.4
15%
zeolite
28th
day
relative
volume%
613
23
315
40
0.0797
180th
day
relativevolume,
%8
18
22
15
829
0.0597
Relativechangeofthevolumeofthepores,
%+33.3
+38.5
�4.3
+400
�46.7
�27.5
�25.0
20%
zeolite
28th
day
relative
volume%
316
22
14
14
31
0.0969
180th
day
relativevolume,
%12
20
19
64
39
0.0746
Relativechangeofthevolumeofthepores,
%+300
+25.0
�13.6
�57.1
�71.4
+25.8
�23.0
10%
Pozzolite
28th
day
relativevolume%
623
29
810
24
0.0592
180th
day
relativevolume,
%10
25
27
10
820
0.0496
Relativechangeofthevolumeofthepores,
%+66.7
+8.7
�6.9
+25.0
�20.0
�16.7
�16.2
5%
zeolite
and5%
Pozzolite
28th
day
relativevolume%
717
28
24
717
0.0740
180th
day
relativevolume,
%8
15
28
65
38
0.0579
Relativechangeofthevolumeofthepores,
%+14.3
�11.8
��75.0
�28.6
+123.5
�21.8
Cement mortars with zeolite and Pozzolite 221
The substitution of part of the cement with
clinoptilolite in mortars with equal consistency
and equal water-to-binder ratio leads to a reduction
in the flexural and compressive strength by the 28th
day of hydration. After 180 days the differences in
both strengths diminish, which means that there is a
contribution by the pozzolanic reaction, which
increases the strength when zeolite is used.
The replacement of 10% of the cement with
Pozzolite leads to an increase in both flexural and
compressive strength.
The replacement of 10% of the cement with a 1:1
mixture of Pozzolite and clinoptilolite leads to
increased strength characteristics.
The specific pore volume of the mortars with
addition of zeolite was greater after 28 days of
hydration but decreased between the 28th and 180th
day compared with the control composition. This
was due to the fact that clinoptilolite exhibits its
pozzolanic activity at later stages of hydration
compared to silica fume and Pozzolite.
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