06-Lilkov_et_al._1st_proof (1)

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: [email protected]: 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


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


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


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.


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.


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


%+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


%+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


%+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


%+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


%+14.3

�11.8

��75.0

�28.6

+123.5

�21.8


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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