HYDRATION PROCESS AND COMPRESSIVE STRENGTH OF CEMENT PASTES CONTAINING NATURAL ZEOLITE

Beton je jedan od najznačajnijih građevinskih materijala, koji bolje nego bilo koji drugi materijal može da prilagodi svoja svojstva unapred definisanim zahtevima, ukoliko su oni definisani kao realan skup performansi. Ali upravo zbog njegove atraktivnosti često se zanemarujea činjenice da beton ima enorman uticaj na životnu sredinu. Prvo, ogromna količina prirodnih resursa je potrebna za proizvodnju milijardi tona betona godišnje. Drugo, cement predstavlja komponentu betona koja se u svetu najviše proizvodi i koristi (1.6 milijardi tona godišnje) [1]. Zatim, cementna industrija ima vodeću ulogu u globalnom zagrevanju, i odgovorna je za 7% ukupne emisije CO2 [1]. Kako bi se smanjila potrošnja električne

Kada se upotrebljava prirodni zeolit kao DCM, treba očekivati pucolansku reakciju koja se razlikuje od ostalih iz sledećih razloga: • struktura zeolita apsorbuje značajnu količinu vode; • difuzija negativno naelektrisanih hidroksilnih jona kroz pore zeolita otežana je zbog negativno naelektri-emission [1].In order to reduce energy consumption and CO 2 emission, partial replacement of Portland cement (PC) with supplementary cementitious materials (which will be referred to as SCMs) has been used.The advantages of their use are economic, environmental and technical.SCMs comprise a wide variety of natural pozzolans and by-products of industries.Natural zeolites belong to the group of natural pozzolans, and the group of SCMs.
Natural zeolites (NZs) are microporous, crystalline, hydrated aluminosilicate minerals with an open, threedimensional framework of silicon-oxygen [SiO 4 ] 4-and aluminium-oxygen [AlO 4 ] 5-tetrahedra [2,3].Crystals are characterized by a honeycomb-like structure with small pores and channels that provide large internal and external surface areas [4,5].The high specific surface area of NZ represents the base of its high reactivity [5].
When water is added to dry mixture of cement and SCM it initiates cement hydration and formation of different hydrated products like C-S-H and C-A-H gels, AFm, AFt phases and calcium hydroxide (CH), etc.The pozzolanic reactions of SCMs represent a complex process which starts in the presence of calcium hydroxide (Ca(OH) 2 ) or Ca 2+ ion and water.The decomposition of SCMs begins on the surface of their grains under the attack of hydroxyl ion (OH ¯).The driving force is the difference in Gibbs energy between the reactants and hydrated products.The release or dissolution primarily of silica from the SCMs will lead to saturation of pore solution at some point.Consequently, heterogeneous nucleation starts and it is followed by the growth of C-S-H products at the SCMs grain surface.The formation of the C-S-H layer, which encircles the surface of the SCM grain, will influence further the rate of reaction.Namely, the growth and densification of the layer made of hydrated products would hamper the diffusion of hydroxyl ions towards the unreacted phases and cause the decrease of the rate of reaction.Hydration products of binder that contain ordinary Portland cement (PC) and a SCM differ in comparison with hydration products of PC.The hydration products of the binders consisting of PC and SCM generally contain C-(A)-S-H, AFm-type phases and ettringite.Those SCMs abounding in MgO could have hydrotalcite-type hydrated phases.The C-S-H phase is significantly different than in pastes/mortar/cement based on PC.The ratio of Ca/Si is lower due to the decrease of the amount of portlandite and due to the larger availability of Si and Al ions originating from dissolving of SCMs.The C-S-H phase with lower Ca/Si ratio usually has tobermorite-like structure [6].The average chain length increases when the Ca/Si ratio decreases.In such case the uptake of aluminium is increased and C-(A)-S-H is formed.The formation of C-(A)-S-H phase leads to an increased uptake of alkalis due to negatively charged structure and, accordingly, to the decrease of pore solution pH value.A significant drop in pore solution pH below 10 could affect a decrease in pozzolanic reaction rate, furthermore it can also lead to the destabilization of AFm and AFt reaction products.Also, the morphology of C-S-H phase is changed from fibrillar particles to sheet-like two dimensional foils when the Ca/Si ratio decreases [7].
When zeolite is used as SCM it should be expected that pozzolanic reaction is going to be more specific due to the following reasons: sane kristalne rešetke zeolita; • izmenljivi katjoni koji se nalaze u strukturi zeolita mogu da utiču na pucolansku reakciju zeolita i odnos Ca/Si u C-S-H fazi.
• zeolite structure absorbs significant amount of water, • diffusion of negatively charged hydroxyl ions into the zeolite pore network is hindered by negatively charged zeolite framework, • exchangeable cations from zeolite structure could affect the pozzolanic reaction of zeolite and Ca/Si ratio in C-S-H phase.
Zeolite can absorb a great amount of water in pore structure.While zeolite structure framework is being degraded, this water is released and it could have double function in the pozzolanic reaction.The first one is contribution to hydration process in case the amount of water in the pore system of "binder" paste has decreased, and second one could be a decrease of pH value as dilution effect.
The pozzolanic reaction of zeolite under the action of hydroxyl ions proceeds by dissociation of Si-OH and Al-OH into Si-O and Al-O groups.Ca 2+ cations from the portlandite saturated solution are adsorbed on the surface.The presence of Si-O and Al-O groups is assumed to influence the further degradation of Si-O-Si and Al-O-Si bonds underlying in zeolite framework.The negative zeolite framework charge impedes the diffusion of hydroxyl ions deep into zeolite framework.This process mostly depends on the concentration of Si-O and Al-O groups on the surface, which makes the dissolution process dependent on specific surface area.
The role of the type of exchangeable cations in pozzolanic reaction of zeolite is very important.They may influence the rate of hydration, the degree of polimerization as well as morphology of C-S-H phase [8].For instance Na + ion is faster released into the pore solution and therefore increases pH value.Accordingly, zeolite dissolution and reactivity will increase.Also the presence of Na + ion can influence the formation of foil like morphology of C-S-H phase, with a higher degree of ordering and larger crystals.This will lead to the formation of a structure with a higher permeability and consequently a higher zeolite reactivity.
Zeolite with Ca 2+ as an exchangeable cation will have lower consumption of portlandite produced in the process of cement hydration.Unreacted portlandite may crystallize and form massive crystals which could decrease mechanical properties of paste/mortar/ concrete [9].
All the mentioned facts indicate the complexity of the mechanism of zeolite pozzolanic reaction and the necessity to investigate this process in order to understand and control it.
The significance of understanding the process of zeolite pozzolanic reaction and gaining more fundamental insight into the reaction mechanisms arises in the first place from the increased durability.The increased durability of mortar/concrete made of zeolite used as a SCM is recognised through their reduced porosity, increased chemical resistance and higher content of C-S-H phase.Additionally, their environmental impact (Cr 6+ adsorption, anti-bacterial and humidityconditioning material), catalytic (high specific area, cation exchange capacity and porosity), economical (low cost of production), as well as widespread presence are very important for their application.The main zeolite deposits in Serbia are of high quality containing more than 70% of zeolites (e.g.Beočin, Igroš, Jablanica 1, Toponica, Zlatokop) [10].
The aim of this paper is to present the results of textural, mineralogical and mechanical properties obtained for cement based pastes prepared with different amounts of zeolite (0%, 10%, 20% and 30%) as SCM.The progress of hydration process has been followed for paste samples 28 and 60 days old.

Materials and mixture proportions
The materials used in this study are Portland cement, CEM I 42.5 (Lafarge, Beočin, Serbia) and natural zeolite (d< 125 µm) from Igroš quarry (Brus, Serbia).CEM I 42.5 meets the criteria of standard SRPS EN 197-1.

Chemical analysis
The chemical composition of NZ has been determined according to SRPS EN 196-2.
spectra have been recorded with a spectral resolution of 4 cm -1 in the range of 400 -4000 cm -1 , with 32 averaged scans per one measurement.The spectra of all investigated samples have been measured using the KBr pellet technique.One of the advantages of using FTIR spectroscopy is a possibility to study vibration properties of both amorphous and crystalline samples [12].

Textural characterization
Low temperature nitrogen adsorption, Model 2000, (ASAP Micrometrics, USA) has been used for the determination of textural properties (specific surface area, pore volume and average pore diameter).Powder samples (natural zeolite, ordinary Portland cement-CEM I) have been oven-dried at 100 o C. The pore size distribution and the cumulative volume of pores have been determined by mercury intrusion porosimetry (Hg Porosimeter Carlo Erba 2000 WS, Italia), diameter range of 0.002-200 µm.Paste samples (C, CZ10, CZ20, CZ30, 28 and 60 days old) were ovendried and degassed at 50 o C. At a given temperature there is no destruction of the structure of C-S-H, ettringite and other products of hydration.

Compressive strength
The compressive strength of pastes has been determined on 10 samples per mixture.All the samples have been cured in accordance with standard SRPS EN196-1 [13], except for the condition related to the quality of curing water (deionized water has been used instead of tap water).

Chemical analyses
Chemical composition of NZ is shown in Table 2.The stability of zeolite structure depends on several factors.One of them is ratio Si/Al and for the used NZ the value of this ratio is 4.95.The smaller the ratio, the greater is the extent of isomorphous substitution of Al for Si in the tetrahedral framework [14].

Textural characterization of powder components
The results of true density, specific surface area and the volume of pores are presented in Table 4.They confirmed the porous structure of zeolite.Tabela 4. Specifična masa, BET specifična površina, zapremina pora [14] Table 4. True densities, BET surface area, Volumes of pores [14] Specifična

Paste samples
All paste samples have been examined by the means of XRD in order to identify hydration products as well as the degradation of mineral clinoptilolite, zeolite main component, in relation to the age of hydration (28 and 60 days), Figs. 1, 2. The progress of pozzolanic reactions has been coupled with the formation of new hydrated compounds and reduction of the amount of Ca(OH) 2 in the paste.The formed hydration products are the same for both 28-day and 60-day old samples and they are specified as: portlandite, ettringite, C-S-H and C-A-H phases.
The reduction of the peak intensity related to portlandite (Ca(OH) 2 ) has been expected to gradually decrease as the amount of zeolite has increased.This statement is not valid for the results of XRD analysis of samples aged 28 days, Fig 1 .Exactly, the greatest intensities have the peaks of portlandite (d∼4.9, 2.6 Å) for paste CZ20 28 days old.Most probably this result is a consequence of the zeolite property known as extraframework cation content.The exchange of the zeolite extraframework cations with the cations present in the pore solution can either increase or lower the alkali concentration in the pore solution.This depends on zeolite ion exchange capacity.In this case, a reasonable explanation is that after zeolite structure has PZ i njegovog ulaska u pucolansku reakciju došlo do otpuštanja extraframework katjona u porni rastvor.Kao been partly broken down and taken part in pozzolanic reaction, the extraframework cations have been released posledica navedenog javila se povećana koncentracija alkalija i zbog jonskog efekta zaustavlja se rastvaranje Ca(OH) 2 .To je moglo da utiče na strukturu C-S-H, što je i potvrđeno FTIR analizom -polimerizacija SiO 4 4-grupa manje je zastupljena u formiranoj C-S-H fazi (sl.3).
into the pore solution.Consequently, alkali concentrations have increased and, due to ionic effect, they have suppressed Ca(OH) 2 dissolution.This also might have influenced the structure of C-S-H, which has been confirmed by FTIR analysis (polymerization of the SiO 4 4units is less pronounced in formed C-S-H phase), Figs. 3, 5a.
As seen from Figures 1 and 2 the peaks corresponding to clinoptilolite, the main component of zeolite, have not disappeared during the hydration process.It clearly implies that the process of zeolite degradation is still unfinished even after 60 days.
The formation of calcium silicate hydrate (C-S-H) has been evident for the samples at both ages and is attributed to the existence of the gradual increase of background in the range 2θ: 25-35 o , Figs.1, 2.

FTIR analyses
The main objective of the FTIR study has been to analyze the hydration products of the prepared pastes after 28 and 60 days of hydration.The comparison of the obtained FTIR spectra of the pastes is presented in Figs 3 and 4.
According to the obtained FTIR spectra presented in Figures 3 and 4, the appearance of the absorption bands at ~3640 cm -1 , ~3435 cm -1 and ~1646 cm -1 has been noted.These bands are ascribed to the existence of the vibration of different modes of O-H molecular group.Namely, the absorption bands at ~3640 cm -1 indicate the presence of stretching vibrations of Ca-OH from portlandite, while the absorption bands at ~3435 cm -1 and ~1646 cm -1 are associated with the presence of hydrogen bonded OH group in molecules of H 2 O which are adsorbed on the surfaces of the tested samples.The occurrence of the absorption bands at ~1421 cm -1 , ~876 cm -1 and ~711 cm -1 is associated with the presence of the carbonates in the structure of the tested samples [12].The presence of the absorption band at the ~1111 cm -1 could be attributed to the presence of the SO 4 2molecular vibration which could be assigned to the presence of the ettringite mineral phase or gypsum.The existence of the OH absorption band at ~3435 cm -1 is attributed to hydrogen bonded OH group and could be related to the water present in ettringite structure which additionally supports the fact that ettringite is present.
The presence of the hydration products such as calcium silicate hydrates (C-S-H) could be related to occurrence of absorption bands at ∼970 cm -1 due to the existence of the Si-O stretching vibrations of Q 2 tetrahedra of the SiO 2 .Additionally, the absorption bands at ~660-670 cm -1 and at ~450-500 cm -1 , which are designated to the presence of Si-O-Si bending vibration and deformation of SiO 4 tetrahedra vibrations, respectively, could be ascribed to the presence of the C-S-H, as well [16].
Comparing the FTIR spectra of the reference sample (cement pastes without zeolite) and pastes with different amount of zeolite in both cases (after 28 and 60 days of hydration), certain changes of the obtained FTIR spectra have been noticed, which could indicate the existence of certain alteration of the structure.Namely, there has been a notable gradual decrease of absorption bands intensities related to the presence of Ca(OH) 2, CaCO 3 and SO 4 2-with the increase of the wt% of zeolite in the structure, Figs. 3, 4, both after 28 and 60 days of hydration.Moreover, there is a difference regarding the shift of the absorption band at ~ 970 cm -1 related to the existence of Si-O stretching vibrations of Q 2 tetrahedra of the C-S-H, Figs. 3 and 5.
This points out that the increase of zeolite amount thoroughly influences pore solution chemistry, generally increasing alkali, Si and Al contents and at the same time decreasing Ca concentrations in the pore solution.The alkali concentration in the contact fluid has been affected by the extra-framework cation content and cation exchange characteristics of zeolite.Due to the ion effect the increasing alkali content decreases Ca content while concentrations of Si and Al generally increase through time.Accordingly, the decrease of Ca/Si ratio leads to lengthening of silicate chains (increase of polimerization).That could be a reasonable explanation for the obtained results, Figs 3-5.According to the performed FTIR analysis it can be undoubtedly concluded that introduction of the zeolite phase has influenced the composition and structure of the CSH phase during hydration process.

Textural characterization of pastes
Mercury intrusion porosimetry (MIP) has been used to characterize pore structure.In general, determination of the pore structure has become important in understanding chemical, physical and mechanical properties.The following analysis has been based on a general classification of the pores that has been given by the IUPAC (micropore-up to 2 nm, mesopore 2-50 nm, macropore> 50 nm).If one takes into account that MIP method is inappropriate to measure the pores with d<2 nm, then it is logical that the results of the study relate only to meso-and macroporosity.In this study, it has been expected that capillary porosity (IUPAC nomenclature, 10 nm-10 µm) is dependent only on the amount of hydrated products, particularly due to the precipitation of C-S-H phase since its formation leads to the reduction of the pore diameter and capillary porosity.
The capillary pores with diameters in the range from 10 nm to 50 nm (mesopores) influence drying shrinkage and creep, while macropores, with diameters greater than 50 nm, influence the strength and impermeable characteristics of paste, mortar and concrete [17].
It is commonly observed that the addition of SCMs results in an increase of total porosity as well as a decrease in mean pore size.U tabeli 5 prikazane su vrednosti ukupne poroznosti, srednjeg prečnika pora, makroporoznost i mezoporoznost.Količine makropora izračunate su u odnosu na vrednost ukupne poroznosti i izražene u procentima.
Uzorci pastā starosti 60 dana, koji sadrže cement i zeolit, imaju manju ukupnu poroznost u odnosu na referentnu pastu, tab. 5. Kod uzoraka C, CZ10 i CZ30 In Table 5, the results of total porosity, average pore radius, macro-and mesoporosity are presented.The quantities of macropores were calculated in relation to the total porosity and are expressed in percentages.
In this study, the influence of zeolite as a SCM for the samples 28 days old is in accordance with the mentioned course just regarding total porosity, Table 5.For the pastes with zeolite, 28 days old, opposite to the expected decrease in the average diameter, there has been an increase in relation to reference paste.Macroporosity has been increased when the amount of zeolite has been increased (comparison among samples CZ10, CZ20 and CZ30).Dominant pore diameter interval has been in the range, up to 0.06 µm for cement pastes with zeolite, while it has been particularly prevailing with pores up to 0.03 µm for reference paste, Fig. 6.
dominant pore interval has been up to 0.03 µm, Fig 7.
The values obtained for sample CZ20 have been completely different from the others.In this case the total porosity has been very low (2.87 %), average pore radius extremely high (237 µm) and the ratio of macropores extremely high (2.71/2.87=94.42%).These uncommon results need further clarification.To be precise, for this sample it has been difficult to accept that the total porosity is so low, especially if we take into account the obtained values for compressive strength, Table 6.It seems that in this case some pores are not registered by MIP, like hollow-shell pores [18].Usually, hollow-shell pores are formed during hydration process within the clinker grains, while hydrated product (C-S-H) is usually formed on the surface.They are often called Hadley grains and are in the size range from 1 to 15 µm.Furthermore, they are embedded in cement gel resulting that they are connected to the continuous capillary pore system through smaller gel pores.Because of their "inkbottle" effect, they may not easily be detected by MIP.A similar situation may appear when reacted zeolite grains have been in question.Namely, the "ink-bottle" effect may be the result of formation of C-S-H phase on the surface of zeolite, while the inside of zeolite grain is hollow.If the structure of the formed hydrated products is too dense, pores in the interior of the grain can be difficult to detect.Slika 6. Raspodela veličine pora za paste starosti 28 dana Figure 6.Pore size distribution for the pastes 28 days old Slika 7. Raspodela veličine pora za paste starosti 60 dana Figure 7. Pore size distribution for the pastes 60 days old Generalno, uzorci pastā CZ20 i CZ30 pri starosti od 60 dana imaju značajno manju poroznost u poređenju sa uzorcima C i CZ10, kao i u poređenju sa svim uzorcima starim 28 dana (tab.5).Na osnovu rezultata ŽP za uzorke ispitane nakon 28 dana hidratacije, može se zaključiti da su mezopore dominante.Kod uzoraka C, CZ10 i CZ30 koji su ispitani nakon 60 dana hidratacije, može se zaključiti da preovlađuju mezopore dok su kod paste CZ20 makropore dominantne.

Čvrstoća pri pritisku
Smatra se da su čvrstoće cementnih pastā određene brzinom hidratacije cementa, dok se u cementnozeolitnim pastama uticaj zeolita ogleda kroz efekat razređivanja (smanjena količina cementa u odnosu na Generally, samples CZ20 and CZ30 at age of 60 days have a significantly lower porosity compared to C and CZ10 samples as well as in comparison to all the samples at age of 28 days, Table 5.According to the obtained MIP results for the samples tested after 28 days of hydration, it was concluded that mesopores have been dominant.In samples C, CZ10 and CZ30 which were tested after 60 days of hydration, mesopores have been prevailing, while macropores has been dominant for CZ20 sample.
Na slici 8. i u tabeli 6. prikazane su srednje vrednosti čvrstoće pri pritisku (f cm,28 , f cm,60 ) određene na 10 uzoraka za svaku pastu, starosti 28 i 60 dana.Zbog rasipanja rezultata određene su i standardne devijacije (σ 28 , σ 60 ), tabela 6. content decreases in comparison with pastes containing ordinary cement), the filler effect, the hydration acceleration effect and pozzolanic reaction.The first three parameters are regarded to govern the strength development up to 28 days, while the last one is assumed to influence the strength after 28 days.According to literature [17] it is possible to correlate the strength value of pastes with the macroporosity of the samples.This correlation is based on the fact that the pores larger than 50 nm, referred to as macropores, are most likely to be rather significant for the strength and impermeability properties [17].Therefore, the results of compressive strength and macroporosity have been presented, analysed and compared in this article.
The compressive strength values have also been expressed as relative percentages of the 28-day compressive strength of the reference paste, Tab. 7.
After 28 days of curing the results of compressive strength values have shown that pastes CZ10 and CZ20 by 8.5% and 12.40%, respectively, have had higher compressive strength than the reference sample.However, after 60 days a slight decline is shown, Tab. 7, Fig 8.
With an increase of zeolite content, the compressive strength of paste samples CZ20 and CZ30 at the age of 60 days has been reduced compared to the strength of the reference paste.In contrast, the sample CZ 10 has been the only one which has almost reached the compressive strength value of the reference sample.Slika 8. Čvrstoće pri pritisku pastā C, CZ10, CZ20 i CZ30 starosti 28 i 60 dana Figure 8. 28-day and 60-day compressive strengths of C, CZ10, CZ20 and CZ30 pastes S ciljem utvrđivanja u kojoj meri je makroporoznost uticala na vrednosti čvrstoća, analiziran je redosled po kom se smanjuju vrednosti čvrstoća s redosledom po kom se povećava količina makropora za uzorke starosti 28 i 60 dana (tab.5, 7).Dobijeni rezultati su prikazani u tabeli 8.
• Rendgenostrukturna analiza potvrdila je prisustvo minerala portlandita, etringita, C-S-H, C-A-H.Osim toga, u pastama s prirodnim zeolitom indentifikovan je klinoptilolit za uzorke obe starosti.Semikvantitativna analiza intenziteta pikova portlandita, za uzorke pastā starih 28 dana, ukazuje na nedostatak korelacije između pravca u kom opada intenzitet pikova portlandita i pravca u kom raste sadržaj zeolita.Sličan odnos je The analysis of the results presented in Table 8 has shown that there is a certain degree of correlation for 60 days aged samples.Namely, sample CZ20 has had a medial position in both analysed sequences, while samples C and CZ10 have similar values of compressive strength as well as microporosity.Therefore, they have taken the same position in both analysed sequences, Tab. 8.

CONCLUSION
The following conclusions can be drawn from the results of this study: • The main mineral of natural zeolite from Igroš, Serbia, is clinoptilolite.Ratio Si/Al is 4.95.Specific surface area is 78.1586 m 2 /g.
• XRD analysis of the pastes has confirmed the presence of minerals portlandite, ettringite, C-S-H, C-A-H.Additionally, clinoptilolite has been indentified in pastes with zeolite, for both ages.Semiquantitative analysis of portlandite peak intensities, for paste samples 28 days old, has indicated the absence of correlation between the sequence of decrease of portlandite peak intensities and the sequence of dobijen za uzorke starosti 60 dana.Postojanje extraframework katjona kao i jonsko-izmenjivačka sposobnost smatraju se odgovornim za sprečavanje rastvaranja portlandita.
• According to the performed FTIR analysis, it can be concluded that introduction of the zeolite phase in cement pastes has influenced composition and structure of the C-S-H phase during the process of hydration.The pastes with zeolite after 28 days of hydration have had the shift towards the lower wavenumber values for CZ10 and CZ20 pastes (974.05cm -1 , 981.09 cm -1 , respectively) in comparison with the reference paste, which could be attributed to the decrease of polymerization level of the C-S-H phase.After 60 days of hydration, there has been the shifting of the absorption band related to the existence of the Si-O stretching vibrations of Q 2 tetrahedra of the C-S-H phase towards the higher wavenumber values in comparison with the reference paste.This indicates better polymerization of the C-S-H structure with the addition of zeolite with prolonged hydration time.
• Samples CZ20 and CZ30 at the age of 60 days have had a significantly lower total porosity compared to C and CZ10 samples.Samples CZ10, CZ20, CZ30 at the age of 60 days have had lower total porosity compared to the samples cured for 28 days.For all 28 days old samples mesopores are dominant.For C, CZ10 and CZ30 60 days old samples mesopores are dominant, while for CZ20 macropores are dominant.
• After 28 days of curing, the results of compressive strength values have shown that pastes CZ10 and CZ20 by 8.5 % and 12.40%, respectively, have had higher compressive strength than the reference sample.
With an increase of zeolite content, the compressive strength of paste samples CZ20 and CZ30 at the age of 60 days has been reduced compared to the strength of the reference paste.In contrast, the sample CZ10 has been the only one which has almost reached the compressive strength value of the reference sample.

ACKNOWLEDGEMENTS
The research work reported in this paper is a part of the investigation within the research project TR 36017 "Utilization of by-products and recycled waste materials in concrete composites in the scope of sustainable construction development in Serbia: investigation and environmental assessment of possible applications", supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia.This support is gratefully acknowledged.

Miroslava RADEKA Tiana MILOVIC Mirjana MALESEV Vlastimir RADONJANIN Mirjana LABAN
This paper presents the results of the study examining the effect of natural zeolite from Igroš, Serbia, on the hydration and compressive strength of Portland cement based pastes, in which a part of cement has been replaced by natural zeolite.The experimental part comprises three stages.In the first stage, the chemical composition of natural zeolite and the textural properties of both natural zeolite and Portland cement have been examined.In the second stage, Portland cement based pastes incorporating 0%, 10%, 20% and 30% per weight of natural zeolite have been prepared and cured for 28 and 60 days.Finally, mineralogical properties and compressive strength of pastes have been examined.The products of hydration and relative changes of portlandite consumption throughout hydration process (28 and 60 days) have been studied by the means of Xray diffraction and Fourier transform infrared spectroscopy.Additionally, mercury intrusion porosimetry has been used to characterise pore structure.

Table 3 .
%] The highest value has had calcium cation.

Table 5 .
The results of textural analysis

Table 8 .
Correlation between compressive strength and macroporosity