Engineering properties of concrete containing natural zeolite as supplementary cementitious material: Strength, toughness, durability, and hygrothermal performance

Accepted Manuscript Engineering properties of concrete containing natural zeolite as supplementary cementitious material: Strength, toughness, durability, and hygrothermal performance Eva Vejmelková, Dana Koň áková, Tereza Kulovaná, Martin Keppert, Jaromír Žumár, Pavla Rovnaníková, Zbyněk Keršner, Martin Sedlmajer, Robert Černý PII: DOI: Reference:

S0958-9465(14)00175-9 http://dx.doi.org/10.1016/j.cemconcomp.2014.09.013 CECO 2414

To appear in:

Cement & Concrete Composites

Received Date: Revised Date: Accepted Date:

22 August 2013 18 September 2014 27 September 2014

Please cite this article as: Vejmelková, E., Koňáková, D., Kulovaná, T., Keppert, M., Žumár, J., Rovnaníková, P., Keršner, Z., Sedlmajer, M., Černý, R., Engineering properties of concrete containing natural zeolite as supplementary cementitious material: Strength, toughness, durability, and hygrothermal performance, Cement & Concrete Composites (2014), doi: http://dx.doi.org/10.1016/j.cemconcomp.2014.09.013

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Engineering properties of concrete containing natural zeolite as supplementary cementitious material: Strength, toughness, durability, and hygrothermal performance

Eva Vejmelková1, Dana Koňáková1, Tereza Kulovaná1, Martin Keppert1, Jaromír Žumár1, Pavla Rovnaníková2, Zbyněk Keršner3, Martin Sedlmajer4, and Robert Černý1,*

1

Department of Materials Engineering and Chemistry, Faculty of Civil Engineering, Czech Technical University in Prague, Thákurova 7, 166 29 Prague 6, Czech Republic 2

Institute of Chemistry, Faculty of Civil Engineering, Brno University of Technology, Žižkova 17, 602 00 Brno, Czech Republic

3

Institute of Structural Mechanics, Faculty of Civil Engineering, Brno University of Technology, Veveří 95, 602 00 Brno, Czech Republic

4

Institute of Technology of Building Materials and Components, Faculty of Civil Engineering, Brno University of Technology, Veveří 95, 602 00 Brno, Czech Republic

*Corresponding author, phone: +420 224355044, fax: +420 224354446, email: [email protected]

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Abstract A complex analysis of engineering properties of concrete containing natural zeolite as supplementary cementitious material in the blended Portland-cement based binder in an amount of up to 60% by mass is presented. The studied parameters include basic physical characteristics, mechanical and fracture-mechanics properties, durability characteristics, and hygric and thermal properties. Experimental results show that 20% zeolite content in the blended binder is the most suitable option. For this cement replacement level the compressive strength, bending strength, effective fracture toughness, effective toughness, and specific fracture energy are only slightly worse than for the reference Portland-cement concrete. The frost resistance, de-icing salt resistance, and chemical resistance to MgCl2, NH4Cl, Na2SO4, and HCl are improved. The hygrothermal performance of hardened mixes containing 20% natural zeolite, as assessed using the measured values of water absorption coefficient, water vapor diffusion coefficient, water vapor sorption isotherms, thermal conductivity, and specific heat capacity, is satisfactory.

Key words: concrete, natural zeolite, mechanical properties, fracture-mechanics properties, durability properties, hygric properties, thermal properties

1. Introduction Concrete belongs to the most frequently used materials in the civil engineering industry for many years. Its most energetically demanding component is indisputably cement. Production of cement is not only energy intensive but it presents also one of the important CO2 emission sources in the world. Depending on the production technology, the CO2 emission ranges from 2

0.73 to 0.99 t of CO2 per 1 t of cement [1]. More than one half of this amount is caused by calcination, which is integral part of the cement production. Therefore, there are a lot of attempts to use supplementary cementitious materials (SCM) which can replace at least a part of cement in concrete by more environmental friendly materials [2, 3]. The most common representatives of SCM are industrial by-products, such as fly ash [4], silica fume [5], or blast furnace slag [6]. During the last few decades, also the application of other waste materials as fine ground brick [7], rice husk ash [8], or corn cob ash [9] gains on importance. From the environmental point of view, the application of these by-products has two main advantages. The first presents the mentioned reduction of CO2 emission and energy demand. The second consists in reusing a waste material instead of landfilling it. However, the application of SCM in concrete industry is not restricted to waste materials only. There are many types of other materials which can be classified as SCM, their unifying feature being the good pozzolanic activity [10]. Natural pozzolans present a viable solution of cement replacement in concrete in the case they are widely available near to the cement production plants. After mining they do not need any special treatment except for grinding which is their main advantage. There are three main groups of natural pozzolans, namely the materials of volcanic origin, sedimentary origin and mixed origin (hybrid rocks) [10]. Zeolites, as probably the most often used natural SCM, belong to the volcanic pozzolans.

Natural zeolites contain clinoptilolite, an aluminosilicate with microporous framework structure, as the main compound. The structure of clinoptilolite is based on a 3-dimensional skeleton made of silicon tetrahedrons interconnected by oxygen atoms with a part of silicon atoms replaced by aluminum atoms. The amount of silicon oxide in zeolite is usually ~70 % and there is usually also ~ 12 % of aluminum oxide [11]. Thanks to their high specific surface, zeolites are widely used mainly in chemical engineering as catalyst support [12], 3

molecular sieves [13], or sorbents [14]. In civil engineering, their utilization as pozzolans dates back already to ancient times when the mixture of zeolites containing tuff and lime was used as hydraulic binder [15]. In today’s building industry, natural zeolites are used mostly as concrete admixtures.

In the previous studies concerning the use of zeolites in concrete production, one of the most frequent topics was their pozzolanic activity, as a fundamental condition for their utilization as SCM. Perraki et al. [16] reported a good pozzolanic reactivity of zeolite, 0.555 g of Ca(OH)2 per 1 g of zeolite according to the Chapelle test. Chan and Ji [17] in a comparative study found out that the pozzolanic activity of zeolite was lower than silica fume but higher than fly ash. Similar results were obtained by Ahmadi and Shekarchi [18] and Poon at al. [19]. Therefore, zeolite obviously has a good potential as SCM in concrete.

Mechanical properties of concrete containing natural zeolite as SCM were investigated in quite a few studies as well. Chan and Ji [17] achieved for concrete where 10% of Portland cement was replaced by zeolite the 28-days compressive strength within a range of 58 to 116 MPa, depending on the water to binder ratio. Najimi et al. [20] found out that incorporation of 15% natural zeolite in the blended binder improved compressive strength of concrete but for concrete with 30% zeolite content they observed a 25% strength decrease even with adding a superplasticizer which was not used in the reference mix. Ahmadi and Shekarchi [18] observed an increase in compressive strength for up to 20% of natural zeolite used as Portland cement replacement but this was achieved with an increasing amount of superplasticizer in the mixes containing zeolites. Uzal and Turanlı [21] reported similar compressive strength of mortar with 55% zeolite in the binder to that of 100% Portland cement mortar but once again, this could only be achieved using superplasticizers. Karakurt and Topçu [22] found 30% 4

replacement of Portland cement by zeolite in their blended cement mortars as optimum; the compressive strength was similar to the reference mortar. Valipour et al. [23] observed a fast decrease in the compressive strength of concrete with the increasing amount of zeolite (1030% of the mass of Portland cement) in the mix, even with an increasing superplasticizer dosage.

Durability properties of concretes with blended Portland cement-zeolite binders were analyzed with a much lower frequency than mechanical properties. Ahmadi and Shekarchi [18] found a positive effect of zeolite in cement mortar on the resistance to alkali-silica reaction. Janotka and Krajci [24] reported an improvement of sulfate corrosion resistance of zeolite-containing concrete. Similar effects of zeolite on alkali-silica reaction and sulfate resistance were observed by Karakurt and Topçu [22]. On the other hand, Najimi et al. [20] reported significant strength reduction of zeolite concrete after exposure to sulfuric-acid environment; ~ 20% after 356 days, as compared with ~ 5% for reference Portland-cement concrete.

The transport properties of zeolite-modified concrete were studied by Chan and Ji [17], Ahmadi and Shekarchi [18], and Najimi et al. [20], who found significant reduction of waterand chloride penetration into concrete with natural zeolite. On the other hand, Valipour et al. [23] reported water sorptivity and gas permeability to increase with the increasing amount of zeolite in the mix. Similar results were obtained for oxygen permeability by Ahmadi and Shekarchi [18], but only for zeolite dosage higher than 10%.

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In our previous investigations of concrete with natural zeolite as partial Portland-cement replacement [25] the analyzed hardened mixes exhibited very good mechanical properties but their durability properties were not satisfactory. In particular, the resistance against de-icing salts was poor; even the mix with the lowest zeolite content of 10% was much worse than the reference concrete. In this paper, we present a more successful mix design aimed at the improvement of durability properties. The hardened concrete mixes containing natural zeolite as supplementary cementitious material are investigated using a complex set of methods. A wide set of engineering properties including the basic physical characteristics, mechanical and fracture-mechanics properties, frost resistance, de-icing salts resistance, chemical resistance, hydric and thermal properties is determined and compared with the parameters of reference Portland-cement concrete.

2. Experimental methods 2.1 Basic physical characteristics The bulk density, matrix density, and open porosity were measured using the water vacuum saturation method [26]. Each sample was dried in a drier to remove majority of the physically bound water. After that the samples were placed into a desiccator with de-aired water. During three hours air was evacuated with a vacuum pump from the desiccator. The samples were then kept under water not less than 24 hours.

Characterization of pore structure was performed by mercury intrusion porosimetry. The experiments were carried out using the instruments PASCAL 140 and 440 (Thermo Scientific). The range of applied pressure corresponded to the pore radius from 10 nm to 100 μm. Since the size of the specimens is restricted to the volume of approximately 1 cm3 and the 6

studied materials contained some aggregates about the same size, the porosimetry measurements were done on samples without coarse aggregates.

2.2 Mechanical and fracture-mechanics properties The measurement of compressive strength was done by the hydraulic testing device VEB WPM Leipzig having a stiff loading frame with the capacity of 3000 kN. The tests were performed according to ČSN EN 12390-3 [27] after 28 days of standard curing. The bending strength was determined using the procedure described in ČSN EN 12390-5 [28], after 28 days of standard curing as well.

The effective fracture toughness was measured using the Effective Crack Model [29] which combines the linear elastic fracture mechanics and crack length approaches. A three-point bending test [28] of specimens having a central edge notch was used in the experiment. The loaded span was equal to 300 mm. A continuous record of the load–deflection (F–d) diagram was used for the calculation of effective fracture toughness. An estimate of fracture energy was obtained from the F–d diagram according to the RILEM method (work-of-fracture) [30].

2.3 Durability properties Frost resistance tests were carried out according to ČSN 73 1322/Z1 [31]. The samples were tested after 28 days of standard curing. The total test required 100 freezing and thawing cycles. One cycle consisted of 4 hours freezing at -20 °C and 2 hours thawing in 20 °C warm water. Frost resistance coefficient, K, was determined as the ratio of bending or compressive

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strength of specimens subjected to 100 freezing and thawing cycles to the strength of reference specimens which did not undergo the frost resistance test.

The resistance of studied concrete against de-icing salts was measured according to ČSN 731326/Z1 [32]. The tested specimens were saturated by water and put into a bath with 3% NaCl solution. Then, freeze/thaw cycles were applied. The test was finished after 100 cycles. In one cycle the tested specimen was cooled at first in an automatic conditioning device from 20 °C to -15 °C during 45 minutes, then it was left at -15 °C for 15 minutes, subsequently heated to 20 °C during 45 minutes and left 15 minutes at that temperature. After every 25 cycles the specimens were removed from the bath, their mass loss due to spalling of particles on the surface was determined, the NaCl solution was replaced and specimens put into the bath again.

The chemical resistance in various environments was tested according to the procedure developed earlier at the Brno University of Technology. The specimens were prepared in 100 × 100 × 400 mm molds and placed into a climatic chamber with 100% relative humidity environment. After 24 hours they were demolded and stored in the same environment for another 27 days. Then, the specimens were cut to 100 × 100 × 50 mm blocks and put in groups of three into the chemical environments (MgCl2 17.76 g L-1, NH4Cl 2.97 g L-1, Na2SO4 14.79 g L-1, HCl 10-3 mol L-1). One set of specimens was just after the 28-days curing subjected to the compressive-strength test to obtain control strength value. Test of concrete carbonation was performed in a desiccator where the CO2 concentration was kept at 65 ± 5 vol. % (the concentration was measured by an IR probe). The carbonation took place in an environment above saturated KNO3 solution (85 ± 5% relative humidity). The specimens denoted as “air” were stored in common laboratory conditions at 21 ± 2°C and 45 ± 5% 8

relative humidity; those marked “distilled water” were in distilled-water bath which was replaced every 10 days. The duration of the chemical resistance test was 60 days. Then, the specimens were subjected to the compressive-strength test. The coefficient of chemical resistance, Kcr, was then determined as the ratio of the compressive strength after 60 days in a specific environment and compressive strength after 60 days in drinking water.

2.4 Hygric and thermal properties The liquid water transport was characterized by the water absorption coefficient. The specimen was water- and vapor-proof insulated on four lateral sides and the face side was immersed 1-2 mm in the water. Constant water level in the tank was achieved by a Mariotte bottle with two capillary tubes. One of them, inside diameter 2 mm, was ducked under the water level. The second one, inside diameter 5 mm, was above water level. The automatic balance allowed recording the increase of mass. The water absorption coefficient A was determined from the sorptivity plot [33]. The apparent moisture diffusivity was then calculated using the method described in [34].

The wet-cup- and dry-cup methods were employed in measurements of water vapor transport parameters [26]. In the dry-cup method sealed cups containing silica gel were placed in a controlled climatic chamber with 50% relative humidity and weighed periodically. For the wet-cup method the sealed cups contained water. The measurements were done at 25 °C over a period of five weeks. The steady state values of mass gain or mass loss determined by linear regression for the last five readings were used for the determination of water vapor transport properties. The water vapor diffusion parameters were calculated from the measured data using the procedure described in [26]. 9

A DVS-Advantage device (Surface Measurement Systems Ltd.) was used for the measurement of water-vapor adsorption and desorption isotherms. The instrument measures the uptake and loss of vapor gravimetrically, using highly precise balances having a resolution of 10 µg.. The partial vapor pressure around the sample is generated by mixing the saturated and dry carrier gas streams, using electronic mass flow controllers. The humidity range of the instrument is 0 – 98% with the accuracy ± 0.5%. Values of the relative humidity are measured by the dew point calculations, using an optical sensor, i.e., by knowing the saturated vapor pressure at a given temperature under the atmospheric pressure. The dew point of water at different contents of vapor pressure is obtained by referring to a plot of the saturated vapor pressure versus temperature under the specific atmospheric pressure. In order to validate the generated relative humidity values, the DVS-Advantage instrument uses the principle that the partial vapor pressure of water above the saturated salt solution in equilibrium with its surroundings is constant at a particular temperature. The temperature is controlled in the measurement unit using a Pt100 temperature sensor. After drying the samples are put into the chamber of DVS Advantage where they hang on the bowl of automatic balance in a special steel tube. The experimental data in this paper were achieved under the temperature of 20 °C. The samples were continuously submitted to the water vapor pressures creating relative humidities 0, 20, 40, 60, 80, and 98%. The device was set on the dm dt-1 (change of mass in time) mode, with a fixed value of 0.0004 g per 5 min for all relative humidities. If this condition was satisfied, the current relative humidity stage was concluded and followed by another step.

The thermal conductivity and specific heat capacity were measured using a commercial device Isomet 2104 (Applied Precision, Ltd.). Isomet 2104 is equipped with various types of 10

optional probes, needle probes are for porous, fibrous or soft materials, and surface probes are suitable for hard materials such as concrete in this paper. The measurement is based on the analysis of the temperature response of the analyzed material to heat flow pulses. The heat flow is induced by electrical heating using a resistor heater having a direct thermal contact with the surface of the sample.

3. Materials and samples The concrete mixes were prepared with Portland cement CEM I 42.5 R (specific surface area 341 m2 kg-1, for the chemical composition see Table 1) as the main binder. A part of cement (10 - 60% by mass) was replaced by natural zeolite with a specific surface area of 227 m2 kg-1 and the chemical composition given in Table 1. The mineralogical composition of natural zeolite determined using the Rietveld method is presented in Table 2, its pozzolanic activity as measured by the Chapelle test was 0.7425 g CaO per 1 g of zeolite.

In the mix design, a different approach than in our previous work [25] was used. While in [25] the condition of constant slump was adopted, which resulted in too high values of open porosity of the mixes with higher zeolite content, here we kept constant the water to binder (w/b) ratio. We also added the aggregate 4-8 mm fraction to complete the granulometry curve; in [25] this fraction was not used because of its higher price, as compared with the 0-4 mm and 8-16 mm fractions, on the Czech market. This potential increase of price of the mix was, however, compensated by using lower amounts of cement and superplasticizer. Thus, the cost effectiveness of the previous design was maintained also in the new, improved mixes.

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The consistence was monitored by the slump test according to ČSN EN 12350-2 [35], using a conical mold (upper diameter 100 mm, lower diameter 200 mm, height 300 mm). The result of the slump test (in mm) was the difference between the height of the mold and the uppermost point of the specimen after the test. At the highest content (60%) of natural zeolite in the mix, the fine pore structure of natural zeolite absorbed water already in such extent that higher amounts of water and superplasticizer had to be added to achieve adequate workability.

The composition of the analyzed concrete mixes (denoted as CZ ref, CZ 10, CZ 20, CZ 40, and CZ 60), is summarized in Table 3, together with the slump values.

The measurement of material parameters of hardened concrete mixtures was done (unless stated otherwise) after 28 days of standard curing. It took place in a conditioned laboratory at the temperature of 22 ± 1°C and 25 - 30% relative humidity. The following specimens were used in the experiments: basic physical properties - 6 specimens 50 x 50 x 50 mm, pore structure – 2 specimens ~ 40 x 40 x 10 mm, compressive strength – 3 specimens 150 x 150 x 150 mm, bending strength – 3 specimens 100 x 100 x 400 mm, fracture-mechanics properties – 3 specimens 100 x 100 x 400 mm, frost resistance – 3 specimens 100 x 100 x 400 mm, deicing salts resistance - 3 specimens 100 x 100 x 400 mm, chemical resistance – 3 specimens 100 x 100 x 50 mm, liquid water transport properties - 5 specimens 100 x 100 x 20 mm, water vapor transport properties - 6 specimens 150 x 150 x 20 mm, sorption isotherms - 2 specimens 40 x 40 x 10 mm, thermal properties - 3 specimens 70 x 70 x 70 mm.

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4. Results and discussion 4.1 Basic physical characteristics Table 4 shows that application of zeolite did not have any substantial influence on the matrix density which was almost equal for all mixtures; the difference was less than 1.5 %. On the other hand, the values of bulk density varied widely. With the increasing amount of zeolite the bulk density decreased by up to 9%. A reverse trend could be seen for the open porosity, which was for the reference material 36% lower than for the material with 60% zeolite replacement. In a comparison with the corresponding mixes analyzed in [25], the open porosity was 15-30% lower and the difference increased with the increasing dosage of natural zeolite in the binder. This was the first important indicator of the successfulness of the mix design presented in this paper.

The total pore volume indicated by mercury intrusion porosimetry (MIP) increased with the amount of added zeolite (Fig. 1) which agreed with the measurement of basic physical properties by water vacuum saturation (Table 4). The increase of pore volume was mainly due to fine pores of diameter below 0.1 µm (Fig. 2). Although incorporation of pozzolanic admixtures themselves into the binder system often causes refinement of the pore structure, in this particular case the most probable reason was the preservation of microcrystalline structure of zeolite in the hardened mixes. As it was reported by Ghourchian et al. [36], MIP specific pore volume of pure clinoptilolite is about 0.08 cm3 g-1 and dominantly pores smaller than 0.1 µm are present. Thus, a presence of unreacted zeolite in the mix should be accompanied by an increase of pore volume just in the range below 0.1 µm as it was observed in Fig. 2. The measured pore size distribution presents then an important indicator of the effectiveness of zeolite as a part of the blended binder. Fig. 2 shows only a moderate increase of pore volume in the 0.01 - 0.1 µm range for the mixes containing up to 20% natural zeolite. 13

However, for concretes with 40% and 60% zeolite content in the binder the increase of pore volume in that range was rather fast. This indicated that an only up to 20% replacement of Portland cement by natural zeolite might be an effective solution, as for the incorporation of zeolite into the hydration process.

The volume of capillary pores in the 0.1 - 10 µm range increased with the increasing amount of zeolite in the mix, though not so remarkably as in the 0.01 - 0.1 µm range (Figs. 1, 2). A substantial difference could be observed between the mixes with the zeolite content up to 20% and the mixes with higher zeolite dosage (Fig. 1). This indicated a more compact microstructure of the hardened mixes with the 20% and lower amount of zeolite in the binder. Regarding the quantitative aspect, the amount of pores in the range of 0.1 - 10 µm was for all analyzed mixes relatively low, approximately one third of the total pore volume (Fig. 1).

The pore distribution in the 0.1 - 10 µm range differed significantly from the corresponding concretes studied in [25] where the mix with 20% zeolite content exhibited a distinct peak at approximately 700 nm and the concrete with 40% of natural zeolite as Portland cement replacement had a relatively high volume of capillary pores within the range of 1 - 10 μm. These differences confirmed the positive effect of the new mix design which was supposed to be reflected in the improvement of other engineering properties.

4.2 Mechanical and fracture-mechanics properties Basic mechanical properties are presented in Table 5. Both compressive and bending strength after 28 days decreased with increasing zeolite content in the mix but the strength values of CZ 10 and CZ 20 with 10, resp. 20% natural zeolite in the blended binder could still be considered as acceptable. The difference in compressive strength, fc, between the reference 14

mix CZ ref and CZ 10 and CZ 20 decreased with time up to 360 days which was, apparently, due to the pozzolanic activity of zeolite. The pozzolanic reaction was most intensive in the time period of 28 to 90 days where the fastest increase in fc was observed. While fc of CZ 20 after 28 days was ~ 25% lower than CZ ref, after 90 days it was only ~ 10%.

The 28-days values of all three investigated fracture-mechanics parameters (Table 6), i.e., the effective fracture toughness, effective toughness, and specific fracture energy exhibited similar features as the basic mechanical properties, in a qualitative sense. The 20% dosage of zeolite in the binder was found, once again, limiting for the achievement of satisfactory mechanical performance.

The 28-days compressive strength of concrete containing natural zeolite as supplementary cementitious material was the parameter most frequently studied by other investigators. Therefore, there is a lot of research material for possible comparisons. Chan and Ji [17] reported for concrete with 10% zeolite and the same w/b as in this paper fc = 57.9 MPa. As they achieved it with a much higher amount of binder in the mix (500 kg/m3 while we had only 350 kg/m3), our CZ 10 mix with fc = 47.8 MPa can be considered as comparable. Najimi et al. [20] measured fc = 36.6 MPa for 15% zeolite content in the mix, w/b = 0.50 and the same amount of binder as in this paper. Our measurement for CZ 20 gave fc = 44.7 MPa which was, once again, a comparable result, taking into account that Najimi et al. [20] used superplasticizer in an amount of 0.4% of mass of cement while we had 1.1% of the mass of binder. Ahmadi and Shekarchi [18] achieved for their concrete mix containing 20% natural zeolite (with w/b = 0.40, total binder amount of 400 kg/m3, superplasticizer 7 L/m3) fc = 42 MPa after 28 days and fc = 50 MPa after 90 days. Valipour et al. [23] for a very similar mix reported fc = 40 MPa after 28 days. This was in both cases slightly worse than our CZ 20 mix 15

with fc = 44.7 MPa after 28 days and fc = 58.8 MPa after 90 days. In a comparison with our previous research on zeolites fc was for CZ 20 after 28 days ~ 10% lower but after 360 days ~ 10% higher than the corresponding mix in [25] which was a good result considering the differences in the amount of binder (484 kg/m3 in [25] against 350 kg/m3 in this paper).

Although it may seem surprising, the bending strength of zeolite-containing concrete was studied very rarely in the past. In fact, we did not find any measurements presented in relevant literature sources (Web of Science, Scopus). In a comparison with other SCM, our CZ 10 mix achieved ~ 40-45% lower bending strength than the corresponding mixes containing fly ash [4], metakaolin [37], or slag [38]. However, this was mainly due to the lower content of cement in our mixes (350 kg/m3 against 484 kg/m3 in [4], [37], and [38]).

The fracture-mechanics parameters, on the other hand, are not commonly reported in the investigations of concrete properties. Therefore, in this case the lack of research material was to be expected. A comparison of our CZ 10 mix with the corresponding mixes containing fly ash [4], fine-ground ceramics [7], metakaolin [37], or slag [38] showed that its effective fracture toughness was lower than for slag and ceramics, the same as for metakaolin, and higher than for fly ash. The effective toughness of CZ 10 was the highest, as compared with [4], [37] and [38], but the specific fracture energy the lowest. In particular, the results achieved for toughness parameters were remarkable, considering the significantly lower cement content in CZ 10.

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4.3 Durability properties The frost resistance of the reference mixture and the concrete with 10% and 20% content of natural zeolites (Table 7) met the standard criterion of K > 0.75 which guaranteed sufficient performance. The other materials were not successful in that respect. The best results were achieved for both compressive and bending strength with CZ 10 and CZ 20 which performed better than CZ ref.

The de-icing salt resistance of the analyzed mixes was, from a qualitative point of view, similar to the frost resistance. The best performance exhibited CZ 20 and CZ 10 which were better than the reference concrete (Fig. 3). On the other hand, the behavior of materials with higher zeolite content appeared as quite unsatisfactory.

The chemical resistance of all concretes was excellent in all studied environments (Table 8). The standard criterion of Kcr ≥ 0.75 was safely met in all cases. The use of zeolite in the blended binders exhibited a clear positive effect on the resistance against Na2SO4, MgCl2, and NH4Cl. The hydrochloric-acid resistance was approximately the same for all analyzed mixes. The resistance to CO2 environment increased up to 20% zeolite content, then it slightly decreased but the differences were low.

In the durability studies presented by other investigators, Karakurt and Topçu [22] and Janotka and Krajci [24] reported an improvement of sulfate resistance of concrete containing 17

zeolites. This was in accordance with our results. In a comparison with our previous research [25] where the resistance of zeolite concrete against de-icing salts was poor for all zeolite contents in the blended binder, the mixes designed in this paper exhibited significantly better performance. This was an apparent consequence of the significant improvements in both total pore volume and pore distribution which were analyzed in Section 4.1 in detail.

4.4 Hygric and thermal properties Liquid water transport parameters increased with the increasing dosage of zeolite in the blended binder (Table 9). This corresponded with the pore distribution data, namely the volume of capillary pores (Fig. 2) which is the most important factor for the ability of a porous medium to transport water in liquid form. For lower zeolite contents, up to 20%, the values of water absorption coefficient and apparent moisture diffusivity were still acceptable. However, for higher cement replacement levels the acceleration of water transport was already so high that it could present a risk for concrete durability.

The water vapor diffusion coefficient increased with the increasing cement replacement level by natural zeolite (Tables 10, 11). This was in accordance with the increasing open porosity (Table 4). Contrary to the liquid water transport, the transport of water vapor can be realized also in smaller pores [39], so that the total pore volume presents the most important indicator of its intensity. In the dry-cup arrangement (Table 10) the water vapor transport was always slower than in the wet-cup conditions (Table 11). This was in accordance with the results obtained for other porous materials where a significant acceleration of water vapor transport with increasing relative humidity was observed [40]. The main reason for this finding was,

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apparently, the incorporation of transport of capillary condensed water in smaller pores into the water vapor transport parameters at higher relative humidities [39].

The water vapor adsorption capability of analyzed concretes was for lower zeolite content in the mix, up to 20%, very similar, within the limits of experimental uncertainty (Fig. 4). For CZ 40 the water vapor adsorption increased by ~ 10% which was in accordance with its higher volume of small pores (Fig. 2) but for CZ 60 it was again on the level of CZ ref which did not exactly conform to the porosity data. The adsorption and desorption branches of sorption isotherms exhibited very significant differences. This sorption hysteresis can be explained by a considerable amount of “bottleneck” pores [39].

Thermal conductivity in dry state (Table 12) decreased with increasing zeolite dosage which was a consequence of increase in porosity (Table 4). Thus, the addition of zeolite improved the thermal insulating properties of concretes. The differences were substantial; thermal conductivity of the material with 60% replacement level was ~ 20% lower than for reference concrete. The specific heat capacity in dry state (Table 12) showed a slight decrease with the increasing amount of zeolite but the differences were very low, up to ~ 2%, which was within the error range of the measurement method.

The dependence of thermal conductivity of all analyzed hardened concrete mixtures on moisture content was very important, its values in water saturated state were almost two times higher than in the dry state (Fig. 5). The presence of water was thus a significant factor causing a substantial worsening of the thermal insulation capability of studied concretes. The specific heat capacity of all mixes increased fast with increasing moisture content (Fig. 6) 19

which was due to the very high specific heat capacity of water. This feature was, however, not negative because it increased their heat accumulation function.

The results of measurements of liquid water transport parameters obtained by other investigators were not unambiguous. Chan and Ji [17] found water initial surface absorption decreasing up to 15% cement replacement level and increasing above that limit. Najimi et al. [20] reported a decrease in water penetration depth up to 30% of zeolite content in the blended binder. In the measurements performed by Valipour et al. [23] the water sorptivity increased for the mixes with up to 30% natural zeolite used as cement replacement. The main reason for the observed discrepancies were, apparently, the different amounts of superplasticizer used in [17], [20], and [23] which, in addition, increased with the increasing zeolite content. This, unfortunately, did not allow any direct comparison with the results obtained in this paper where the superplasticizer content in zeolite concrete mixes was constant. In a qualitative sense, our results were similar to those reported by Valipour et al. [23].

The measurements of water vapor transport parameters of zeolite containing concrete were not found in common literature sources, except for our previous experiments on other mixes [25]. Therefore, only indirect comparisons could be done. Ahmadi and Shekarchi [18] found the oxygen permeability to increase with increasing zeolite content in the mix. Similar results were reported by Valipour et al. [23] for gas permeability. This was in a qualitative agreement with the results obtained in this paper.

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The thermal conductivity and specific heat capacity of concrete with natural zeolite were also not investigated before, with the only exception of our previous measurements [25] which were performed for the mixes designed in a different way. From a qualitative point of view, the results presented in this paper agreed with those reported for fly ash [4], fine-ground ceramics [7], metakaolin [37], and slag [38].

5. Conclusions The experimental analysis of basic physical characteristics, mechanical and fracturemechanics properties, durability characteristics, and hydric and thermal properties of concrete containing natural zeolite as partial replacement of Portland cement in an amount of up to 60% by mass showed a good potential of natural zeolite as supplementary cementitious material. The main results can be summarized as follows: •

The pore size distribution was identified as an important indicator of the effectiveness of zeolite as a part of the blended binder. While only a moderate increase of pore volume in the 0.01 - 0.1 µm range, which is characteristic for zeolites in general, was observed for up to 20% replacement level, for concretes with 40% and 60% zeolite content in the binder the increase of pore volume in that range was remarkable. This indicated that an only up to 20% replacement of Portland cement by natural zeolite was an effective solution, as for the incorporation of zeolite into the hydration process.

•

The compressive strength, bending strength, effective fracture toughness, effective toughness, and specific fracture energy after 28 days decreased with the increasing zeolite content in the mix but their values for concretes with up to 20% natural zeolite in the blended binder could still be considered as acceptable. In addition, within the

21

time period of 28 to 360 days the compressive strength of mixes containing zeolite increased faster than the reference mix, as a result of the pozzolanic activity of zeolite. •

The frost resistance and de-icing salt resistance of zeolite containing mixes were for an up to 20% cement replacement level better than the reference mix but higher zeolite dosage appeared as quite unsatisfactory.

•

The chemical resistance was excellent in all studied environments up to the 60% cement replacement level. The use of zeolite in the blended binders exhibited a clear positive effect on the resistance against Na2SO4, MgCl2, and NH4Cl. The hydrochloric-acid resistance was approximately the same for all analyzed mixes. The resistance to CO2 environment increased up to 20% zeolite content, then it slightly decreased but the differences were low.

•

The hygrothermal performance was satisfactory for the hardened mixes with up to 20% natural zeolite. Water absorption coefficient and apparent moisture diffusivity increased with the increasing zeolite dosage but up to 20% cement replacement level they were still acceptable. The increased water vapor diffusion coefficient and thermal conductivity, on the other hand, did not present any problem from a hygrothermal or durability point of view. The water vapor adsorption and specific heat capacity of concretes with natural zeolite exhibited only minor changes in a comparison with the reference mix.

•

The mix design presented in this paper proved to be more successful than the previous one reported in [25]. Adopting a higher water/binder ratio in a combination with lower superplasticizer dosage, decreasing the amount of binder and adding the aggregate 4-8 mm fraction for completing the granulometry curve led to a significant decrease of volume of pores in the 0.1 - 10 µm range which reduced also the total open porosity. The changes in the pore structure were then reflected in the improvement of 22

engineering properties of the analyzed hardened mixes. These findings well illustrated the peculiarities of using zeolite-blended cements in composite mix design, where the zeolite content is supposed to be well balanced with the water/binder ratio, superplasticizer dosage, the binder/aggregate ratio and granulometry of the applied aggregates.

The experimental results summarized above showed that although from both environmental and economic points of view the zeolite amount in the concrete mixes should be as high as possible, there were technological and physico-chemical limits for its dosage. Among the mixes analyzed in this paper, 20% zeolite content in the blended Portland-cement based binder was the best option. However, it should be noted in that respect that for zeolite contents greater than 20% further improvements could possibly be achieved by increasing the superplasticizer dosage. This will be subject of future investigations.

Acknowledgement This research has been supported by the Czech Science Foundation. under project No P104/12/0308.

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[19] Poon CS. Lam L. Kou SC. Lin ZS. A study on the hydration rate of natural zeolite blended cement pastes. Construction and Building Materials 1999;13:427-432 [20] Najimi M. Sobhani J. Ahmadi B. Shekarchi M. An experimental study on durability properties of concrete containing zeolite as a highly reactive natural pozzolan. Construction and Building Materials 2012;35:1023-1033 [21] Uzal B. Turanlı L. Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure. Cement and Concrete Composites 2012;34:101-109 [22] Karakurt C, Topçu IB. Effect of blended cements with natural zeolite and industrial byproducts on alkali-silica reaction and sulfate resistance of concrete. Construction and Building Materials 2011;25:1789-1795. [23] Valipour M, Pargar F, Shekarchi M, Khani S. Comparing a natural pozzolan, zeolite, to metakaolin and silica fume in terms of their effect on the durability characteristics of concrete: A laboratory study Construction and Building Materials 2013;41:879–888 [24] Janotka I, Krajci L. Sulfate resistance and passivation ability of the mortar made from pozzolan cement with zeolite. Journal of Thermal Analysis and Calorimetry 2008;94:714. [25] Vejmelková E. Keppert M. Ondráček M. Černý R. Effect of Natural Zeolite on the Properties of High Performance Concrete. Cement Wapno Beton 2013;18/80:150 -159 [26] Roels S, Carmeliet J, Hens H, Adan O, Brocken H, Černý R, Pavlík Z, Hall C, Kumaran K, Pel L, Plagge R. Interlaboratory Comparison of Hygric Properties of Porous Building Materials. Journal of Thermal Envelope and Building Science 2004;27:307-325 [27] ČSN EN 12390-3 Testing of hardened concrete – Part 3: Compressive strength. Czech Standardization Institute. Prague 2002 26

[28] ČSN EN 12390-5 Testing of hardened concrete – Part 5: Bending strength. Czech Standardization Institute. Prague 2007 [29] Karihaloo BL. Fracture Mechanics of Concrete. New York: Longman Scientific & Technical, 1995 [30] RILEM TC 50-FMC Fracture Mechanics of Concrete. Determination of the Fracture Energy of Mortar and Concrete by Means of Three-Point Bend Tests on Notched Beams. Materials and Structures 1985;18:285-290 [31] ČSN 73 1322/Z1:1968 Concrete testing – Hardened concrete – Frost resistance. Prague: Czech Standardization Institute, 2003 [32] ČSN 731326/Z1:1984. Determination of the resistance of the surface of concrete against water and de-icing salts. Prague: Czech Standardization Institute, 2003 [33] Vejmelková E. Pavlíková M. Jerman M. Černý R. Free Water Intake as Means of Material Characterization. Journal of Building Physics 2009;33:29-44 [34] Kumaran MK. Moisture Diffusivity of Building Materials from Water Absorption Measurements. Journal of Thermal Envelope and Building Science 1999;22:349-355 [35] ČSN EN 12350-2 Testing fresh concrete: Slump test. Prague: Czech Standardization Institute, 2000 [36] Ghourchian S. Wyrzykowski M. Lura P. Shekarchi M. Ahmadi B. An investigation on the use of zeolite aggregates for internal curing of concrete. Construction and Building Materials 2013;40:135-144 [37] Vejmelková E. Pavlíková M. Keppert M. Keršner Z. Rovnaníková P. Ondráček M. Sedlmajer M. Černý R. High performance concrete with Czech metakaolin:

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Experimental analysis of strength, toughness and durability characteristics. Construction and Building Materials 2010;24:1404-1411 [38] Vejmelková E. Pavlíková M. Keršner Z. Rovnaníková P. Ondráček M. Sedlmajer M. Černý R. High performance concrete containing lower slag amount: A complex view of mechanical and durability properties. Construction and Building Materials 2009;23:2237-2245 [39] Černý R. Rovnaníková P. Transport processes in concrete. London: Spon Press, 2002 [40] Kočí J, Žumár J, Pavlík Z, Černý R. Application of Genetic Algorithm for Determination of Water Vapor Diffusion Parameters of Building Materials. Journal of Building Physics 2012;35:238-250

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Table 1 Chemical composition

Amount [% by mass] Component Cement

Natural zeolite

SiO2

21.89

74.69

Al2O3

5.60

14.99

Fe2O3

3.75

1.53

CaO

62.33

3.28

MgO

1.04

0.653

K2 O

0.92

3.60

Na2O

0.11

0.834

TiO2

0.30

0.204

P2O5

0.17

0.021

SO3

2.88

-

Not identified

1.01

0.02

Loss on ignition

-

5.32

Equilibrium moisture content

-

11.44

29

Table 2 Mineralogical composition

Mineral

Amount [% by mass]

Clinoptilolite

44.5

Cristobalite

9.2

Quartz

3.5

Illite + Mica

5.6

Feldspar (albite)

2.6

Amorphous phase

34.5

30

Table 3 Composition of concrete mixtures

Composition [kg m-3] Component CZ ref

CZ 10

CZ 20

CZ 40

CZ 60

CEM I 42.5 R - Mokrá

350

315

280

210

140

Natural zeolites ZEOBAU 200

-

35

70

140

210

Aggregates 0-4 mm

748

748

748

748

748

Aggregates 4-8 mm

240

240

240

240

240

Aggregates 8-16 mm

882

882

882

882

882

Plasticizer Mapei N200

3.85

3.85

3.85

3.85

4.55

Plasticizer Dynamon SX 14

-

-

-

-

1

Water

160

160

160

160

190

Slump

130

90

60

20

10

31

Table 4 Basic physical properties

Bulk

Matrix

Open

density

density

porosity

[kg m-3]

[kg m-3]

[%]

CZ ref

2320

2620

11.6

CZ 10

2270

2630

13.7

CZ 20

2240

2605

14.1

CZ 40

2200

2610

15.7

CZ 60

2120

2590

18.4

Material

32

Table 5 Basic mechanical properties

Material

Compressive strength

Bending strength

[MPa]

[MPa]

28 days

90 days

360 days

28 days

CZ ref

59.6

65.3

67.0

7.4

CZ 10

47.8

58.8

61.2

6.6

CZ 20

44.7

58.5

60.4

6.3

CZ 40

32.4

39.7

46.2

5.4

CZ 60

26.0

30.2

35.8

5.3

33

Table 6 Fracture-mechanics properties

Specific fracture

Effective fracture Effective toughness Material

energy

toughness [MPa m1/2]

[N m-1]

[J m-2]

CZ ref

1.71

76.3

228.6

CZ 10

1.47

73.4

229.0

CZ 20

1.30

66.1

214.6

CZ 40

1.11

44.5

168.0

CZ 60

0.96

31.9

161.9

34

Table 7 Frost resistance

Frost resistance coefficient K Material as the ratio of compressive strengths as the ratio of bending strengths CZ ref

1.01

0.88

CZ 10

1.15

1.14

CZ 20

1.08

0.99

CZ 40

0.73

0.56

CZ 60

0.51

0.06

35

Table 8 Corrosion resistance in various environments

Coefficient of corrosion resistance Kcr Environment CZ ref

CZ 10

CZ 20

CZ 40

CZ 60

1.00

1.00

1.00

1.00

1.00

0.92

0.93

0.93

0.93

0.98

0.96

1.07

1.07

1.07

1.07

MgCl2

0.94

0.94

0.97

1.05

1.01

NH4Cl

0.94

0.98

1.05

1.03

1.13

Na2SO4

0.94

1.09

1.05

1.08

1.20

HCl

1.02

1.00

1.00

1.01

1.05

CO2

1.01

1.04

1.07

1.03

0.98

Drinking water Air Distilled water

36

Table 9 Liquid water transport properties

Water absorption coefficient

Apparent moisture diffusivity

[kg m-2s-1/2]

[m2 s-1]

CZ ref

0.008

4.39E-09

CZ 10

0.011

6.44E-09

CZ 20

0.025

3.33E-08

CZ 40

0.043

1.05E-07

CZ 60

0.067

1.36E-07

Material

37

Table 10 Water vapor transport properties – dry-cup arrangement

Water vapor

Water vapor

diffusion

diffusion

permeability

coefficient

[s]

[m2s-1]

[-]

CZ ref

2.02E-12

2.78E-07

82.8

CZ 10

2.57E-12

3.53E-07

70.3

CZ 20

2.68E-12

3.68E-07

62.6

CZ 40

2.95E-12

4.05E-07

57.2

CZ 60

3.34E-12

4.60E-07

50.1

Water vapor diffusion resistance factor

Material

38

Table 11 Water vapor transport properties – wet-cup arrangement

Water vapor

Water vapor

diffusion

diffusion

permeability

coefficient

[s]

[m2s-1]

[-]

CZ ref

2.45E-12

3.37E-07

69.3

CZ 10

2.77E-12

3.81E-07

60.7

CZ 20

3.10E-12

4.26E-07

55.1

CZ 40

4.55E-12

6.26E-07

38.2

CZ 60

6.44E-12

8.85E-07

27.3

Water vapor diffusion resistance factor

Material

39

Table 12 Thermal properties in dry state

Thermal conductivity

Specific heat capacity

[W m-1K-1]

[J kg-1K-1]

CZ ref

1.830

728

CZ 10

1.690

726

CZ 20

1.560

723

CZ 40

1.490

718

CZ 60

1.077

712

Material

40

Figure captions Figure 1 Cumulative pore volume Figure 2 Pore size distribution Figure 3 Loss of mass due to the de-icing salts action Figure 4 Sorption isotherms Figure 5 Thermal conductivity Figure 6 Specific heat capacity

41

0.10

Pore volume [cm3g-1]

0.09

CZ-ref CZ10 CZ20 CZ40 CZ60

0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.01

0.1

1

Pore diameter [µm]

Figure 1 Cumulative pore volume

42

10

100

0.012 CZ ref CZ 10 CZ 20 CZ 40 CZ 60

Pore volume [cm3g-1]

0.010 0.008 0.006 0.004 0.002 0.000 0.01

0.1

1

Pore diameter [μm]

Figure 2 Pore size distribution

43

10

100

Loss of mass [gm-2]

16000 CZ ref CZ 10 CZ 20 CZ 40 CZ 60

14000 12000 10000 8000 6000 4000 2000 0 25

50

75

Number of cycles

Figure 3 Loss of mass due to the de-icing salts action

44

100

4.0

Moisture content [% kg/kg]

3.5 3.0 2.5

CZ ref adsorption CZ ref desorption CZ 10 adsorption CZ 10 desorption CZ 20 adsorption CZ 20 desorption CZ 40 adsorption CZ 40 desorption CZ 60 adsorption CZ 60 desorption

2.0 1.5 1.0 0.5 0.0 0

20

40

60

80

Relative humidity [%]

Figure 4 Sorption isotherms

45

100

Thermal conductivity [W m-1 K-1 ]

2.6 2.4 2.2 2.0 1.8 CZ ref

1.6

CZ 10

1.4

CZ 20

1.2

CZ 40 CZ 60

1.0 0

5

10 15 3 -3 Moisture [% m m ]

Figure 5 Thermal conductivity

46

20

Specific heat capacity [J kg-1 K-1 ]

1050 1000 950

CZ ref

900

CZ 10

850

CZ 20

800

CZ 40

750

CZ 60

700 0

5

10 Moisture [% m3 m-3]

Figure 6 Specific heat capacity

47

15

20