TiO2 composite: Photoactive admixture for building materials based on Portland cement binder

Construction and Building Materials 35 (2012) 38–44

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Metakaolinite/TiO2 composite: Photoactive admixture for building materials based on Portland cement binder V. Mateˇjka a,⇑, P. Mateˇjková b, P. Kovárˇ c, J. Vlcˇek d, J. Prˇikryl c, P. Cˇervenka c, Z. Lacny´ a, J. Kukutschová a a

Nanotechnology Centre, VŠB – Technical University of Ostrava, 17. Listopadu 15/2172, Ostrava, Czech Republic CPIT, VŠB – Technical University of Ostrava, 17. Listopadu 15/2172, Ostrava, Czech Republic c ˇ CTCAP a.s., Nábrˇezˇí Dr. E. Beneše 24, Prˇerov, Czech Republic d Institute of Industrial Ceramics, VŠB – Technical University of Ostrava, 17. Listopadu 15/2172, Ostrava, Czech Republic b

a r t i c l e

i n f o

Article history: Received 25 August 2011 Received in revised form 14 February 2012 Accepted 25 February 2012

Keywords: Metakaoline Anatase Latent hydraulicity Cement mortar Photodegradation activity

a b s t r a c t Novel latent hydraulic and photoactive admixture metakaolinite/TiO2 was prepared, characterized and evaluated as a partial replacement for the Portland cement in mortars. The main component of this novel composite material is metakaolinite containing the anatase nanoparticles anchored on its surface. The values of the compression strength measured for the mortars containing metakaolinite/TiO2 composites reached higher values in comparison to the mortar without this admixture. Photodegradation activity of the mortars with given amount of the metakaolinite/TiO2 admixture against nitric oxide (NO) was tested using the modified ISO standard. The results showed that the conversion of NO increases with the increasing content of TiO2. The synergistic effect of the photodegradation activity and the latent hydraulicity in this material can be utilized in building industry to decrease concentration of selected air pollutants e.g. in areas affected by traffic pollution. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Building industry is predetermined sector for the application of the photocatalysts with the aim to clean the air via degradation of gaseous pollutants. Next benefits are related to self-cleaning and antimicrobial properties of the surfaces of building materials containing photocatalytically active materials. Titanium dioxide (TiO2) in the anatase form is widely studied photocatalyst showing enhanced photoactivity against number of environmentally hazardous compounds [1]. Most often, the photodegradation activity of Abbreviations: K, kaolin; OPC, ordinary Portland cement; K600, kaolin calcined at 600 °C; KATI, general assignment for kaolinite/TiO2 composite; KATI11, composite dried at 105 °C and containing 10 wt.% of TiO2; KATI16, composite dried at 105 °C and containing 60 wt.% of TiO2; KATI61, composite calcined at 600 °C and containing 10 wt.% of TiO2; KATI66, composite calcined at 600 °C and containing 60 wt.% of TiO2; P_OPC, hardened cement paste; P_OPC/KATI66, hardened paste containing Portland cement and composite KATI66 in weight ratio 1:1; STD, standard cement mortar; S5K600, mortars containing 5 wt.% of K600 as a replacement of OPC; S10K600, mortars containing 10 wt.% of K600 as a replacement of OPC; S15K600, mortars containing 15 wt.% of K600 as a replacement of OPC; S5KATI61, mortars containing 5 wt.% of KATI61 as a replacement of OPC; S10KATI61, mortars containing 10 wt.% of KATI61 as a replacement of OPC; S15KATI61, mortars containing 15 wt.% of KATI61 as a replacement of OPC; S5KATI66, mortars containing 5 wt.% of KATI66 as a replacement of OPC; S10KATI66, mortars containing 10 wt.% of KATI66 as a replacement of OPC; S15KATI66, mortars containing 15 wt.% of KATI66 as a replacement of OPC. ⇑ Corresponding author. Tel.: +420 597 321 519; fax: +420 597 321 640. E-mail address: [email protected] (V. Mateˇjka). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.02.086

TiO2 in liquid phase is evaluated using photodegradation of organic dyes [2], which represent typical contaminants of wastewaters produced by textile industry. The photodegradation activity of TiO2 in gaseous phase is often verified by the elimination of nitrogen oxides (NOx) [3] or selected volatile organic compounds (VOCs) – e.g. toluene [4]. The procedure for determination of TiO2 photoactivity against NOx as well as toluene is unified by the ISO standards [5,6]. Other promising utilization of the photocatalytically active TiO2 is elimination of CO2 [7]. The photocatalytic phenomenon is closely related to the crystallite size of anatase [8], which typically falls to the maximum sizes in the order of tents nanometers. Such small particles represent several drawbacks related to manufacturing process as well as environmental hazards [9]. These negative properties can be reduced by anchoring of the nanoparticles on the surface of suitable substrates. The surfaces of silica or phyllosilicate [10–13] particles belongs to the suitable carriers for nanoparticles growing. The prepared composites show also the enhanced photodegradation activity against selected pollutants in comparison to bare nanoparticles. Kaoline is natural and abundant material with wide range of applications [14]. The main component of kaoline is kaolinite belonging to 1:1 group of phyllosilicates; other typical admixtures are quartz, illite, mica, and feldspar. Thermal treatment of kaoline above the temperature approx. 500 °C causes dehydroxylation of the kaolinite what results in metakaolinite formation. The metakaolinite is a material showing latent hydraulic properties and is

V. Mateˇjka et al. / Construction and Building Materials 35 (2012) 38–44

widely studied as an initial material for synthesis of geopolymers by using activation with different alkali substances [15,16], or used as one of the admixtures in formulation of mortars or concretes [17]. Depending on the thermal treatment conditions as well as the raw kaoline composition, different degrees of kaolinite dehydroxylation are acquired [18], whereas the degree of kaolinite dehydroxylation is important parameter, which directly influences its latent hydraulic properties [19,20]. The Chapelle test [21] and thermogravimetric analysis [22] are two most often utilized methods for the determination of the latent hydraulic properties of metakaolinite. In calcium hydroxide environment which typically originates during the Portland cement hydration, the metakaolinite undergoes pozzolanic reaction and calcium aluminum hydrates and aluminum silicon hydrates as the main products are formed [20]. As a result of the reaction, thus obtained hardened product contains lower amount of portlandite and the final material shows typically higher compressive strength as well as durability in different environments. Typical final applications of the photoactive TiO2 related to building industry were comprehensively summarized by Chen and Poon [23] and comprise exterior and interior paints, pavement blocks, and roof tiles. Pacheco-Torgal and Jalali [24] and Folli et al. [25] summarized the main advantages connected to the utilization of photocatalysts in building industry. Diamanti et al. [26] tested 9 samples of the fiber-reinforced mortars containing TiO2 in the anatase form introduced in: (i) powder form, (ii) suspension, and (iii) combination of powder form and suspension. With respect to the tests performed which comprised the photodegradation of 2-propanol and characterization of the self-cleaning properties, the authors found the best photodegradation activity for the sample containing combination of 3% of powder anatase together with 2% of anatase in form of suspension. Maggos et al. [27] performed the pilot scale experiment with NOx abatement over the mortars containing nanosized TiO2 applied on the surface of the containers stacked up in way to simulate a street canyon, other street canyon was build up with uncovered containers and served as a reference. The authors observed that the NOx concentration in the canyon build up with the containers covered with the TiO2 mortar was approx. 60% lower in comparison to the reference street canyon. Ramirez et al. [28] focused their research on the estimation of the influence of several parameters on the photocatalytic efficiency of the concretes and plasters covered by the anatase thin layer. The authors tested two methods of the TiO2 layer deposition: (i) immersion of tested samples into the suspension of TiO2 in ethanol and (ii) immersion of the tested samples into the TiO2 sol obtained by the reaction of titanium diisopropoxide bis (acetylonate) with water in the isopropanol environment. The photodegradation tests performed using toluene as a model pollutant show significant air purifying capability of the samples covered by the suspension of TiO2 in ethanol, while the activity of the samples covered by the freshly prepared TiO2 sol exhibited only negligible photodegradation activity. Poon and Cheung [29] studied the dependency of the NO removal efficiency over paving blocks containing TiO2 on several parameters, e.g. source of TiO2, porosity of the hardened blocks, curing time, and size of the aggregates. The authors showed the positive effect of the lower amount of OPC on the porosity of the hardened samples and thus higher NO removal efficiency. Meng et al. [30] discussed the role of the nano-TiO2 on the mechanical properties of cement mortars. The authors observed, that the addition of the nano-TiO2 into the composition of the mortars caused decrease of the compressive strength after the 28 days long hydration. The aim of this work is to verify the applicability of the thermally treated kaoline/TiO2 composites as a photocatalytic and latent hydraulic admixture in the cement mortars. Chemical and phase composition of the prepared kaoline/TiO2 composites were

39

determined using X-ray fluorescence spectroscopy and X-ray diffraction method, respectively. The Chapelle test was used for evaluation of the latent hydraulicity of the calcined kaoline/TiO2 composites. The selected kaoline/TiO2 composites were used for partial replacement of the OPC in the formulation of the cement mortars. The compressive strength test was used for the effect evaluation of the composite presence on the mechanical properties of the hardened cement mortars. The photodegradation activity of the prepared cement mortars was evaluated by the photodegradation of NO. 2. Experimental 2.1. Sample preparation Kaoline/TiO2 composites assigned as KATITX (where T means temperature of thermal treatment, and X means TiO2 content – 1–10 wt.%, 2–20 wt.%, . . .) were prepared using hydrolysis of the kaoline SAK47 (LB Minerals) assigned as K and titanyl sulfate (TiOSO4, Precheza a.s. Prˇerov Czech Republic) suspension. The appropriate kaoline: TiOSO4 ratio gives desired content of TiO2 in the final composite. After the hydrolysis the resulting white slurry was filtered and dried at 100 °C and the KATI1X composites were obtained. After the calcinations of KATI1X for 2 h at 600 °C the KATI6X composites were obtained. Detailed information about the preparation of this composite can be found in work by Mamulová Kutláková et al. [11]. The K sample calcined for 2 h at 600 °C was assigned as K600. With the aim to compare the hydration process of samples with and without the KATI66 composite the cement pastes based on the ordinary Portland cement (OPC) 42.5R supplied by Cement Hranice a.s. (Czech Republic) were prepared. The weight ratio of KATI66: OPC was set 1:1, water–cement + KATI66 (w/c + p) ratio was set to reach the value 0.3 in this sample. The standard cement paste containing only OPC and water (w/c = 0.3) was also prepared. Prepared mixtures were formed into the molds (25  25  25 mm) and stored in moist environment. After 24 h the samples were taken out of the molds and then stored in moist environment (90% rel. humidity and 20 °C) for 28 days. The hardened pastes assigned as P_OPC/KATI and P_OPC, were crushed, milled and gently mixed in agate mortar with acetone to stop the hydration process. Four groups of the cement mortars differing in the presence of given admixture were prepared. The first group represents the standard cement mortar (STD); the second group represents cement mortars containing 5, 10 and 15 wt.% of K600 as a replacement of OPC, assigned as S5K600, S10K600, S15K600, respectively; the third group represent cement mortars containing 5, 10 and 15 wt.% KATI61 as a replacement of the OPC assigned as S5KATI61, S10KATI61, S15KATI61, respectively; and the fourth group represents cement mortars containing 5, 10 and 15 wt.% KATI66 as a replacement of the OPC assigned as S5KATI66, S10KATI66, S15KATI66, respectively. The composition of the samples is given in Table 1. The prepared mixtures were formed into the molds (40  40  160 mm) to obtain the samples required for the compressive strength test and into the molds (50  100  10 mm) to obtain the samples designed for the NO photodegradation experiment. After the forming, the samples were stored in the moist environment. After 24 h the samples were taken out of the molds and then stored in the moist environment (90% rel. Humidity and 20 °C) for 28 days. 2.2. Sample characterization 2.2.1. X-ray powder diffraction (XRPD) The XRPD patterns were recorded under CoKa irradiation (k = 1.789 nm) using Bruker D8 Advance diffractometer (Bruker AXS. Germany) equipped with the fast position sensitive detector VÅNTEC 1. The measurements were carried out in the reflection mode, powder samples were pressed in the rotational holder. Phase composition was evaluated using the database PDF 2 Release 2004 (International Centre for Diffraction Data). 2.2.2. Determination of latent hydraulicity The latent hydraulicity of the selected samples was determined using the Chapelle test [21]. In this test 1 g of sample is mixed with 1 g of Ca(OH)2 and 100 ml of boiling water. The suspension is boiled for 16 h under reflux condenser, after this time period the suspension is cooled and free Ca(OH)2 is separated the by means of sucrose extraction followed with filtration. The amount of Ca(OH)2 is determined in supernatant liquor by titration with HCl solution. 2.2.3. Scanning electron microscopy (SEM) The morphology of the hardened mortar samples was observed by the scanning electron microscope eXPLORER (ASPEX). The samples were coated with an Au/Pd film and the SEM images were obtained using a backscattered secondary electron (BSE) detector. Map of elemental composition at the surface of the hardened mortar S15KATI66 was determined using energy dispersive (EDX) analysis.

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Table 1 Composition related to the total amount of testing samples (wt.%).

a b

Sample

CEMa

Mineral admixtures

STD S5K600 S10K600 S15K600 S5KATI61 S10KATI61 S15KATI61 S5KATI66 S10KATI66 S15KATI66

22.2 21.1 20.0 18.9 21.1 20.0 18.9 21.1 20.0 18.9

– 1.1 2.2 3.3 1.1 2.2 3.3 1.1 2.2 3.3

Silica sand PR30 D50 0.75 mm

PR31 D50 0.38 mm

PR33 D50 0.30 mm

22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2

22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2

22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2 22.2

Water

– – – – 0.1 0.2 0.3 0.7 1.3 2.0

11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1 11.1

CEM I. 42.5R Cement Hranice a.s. Calculated TiO2 content in the mixture without water.

2.2.4. Compressive strength test The values of compressive strength were obtained experimentally by means of the destructive compressive test. Hydraulic press ZD-10 with uniaxial tensile load was used for this experiment. The samples were gripped by means of thrust plates with dimensions 40  40 mm. 2.3. Tests of NO photodegradation The apparatus and the conditions used for NO photodegradation are in agreement with the requirements given by the international standard ISO 221971:2007 [5]. NO was used as a model pollutant during the experiment and its initial stream dosed using mass flow controllers was mixed with air to give the final concentration approx. 1.2 ppm. HORIBA APNA-370 analyzer was used for detection of NO concentration. The flow rate of inlet gas mixture through the reactor was 3 l/ min. OSRAM EVERSUN L40 W/79 K tube served as a source of UV irradiation during all of the photodegradation tests. Distance of the UV lamp from the sample surface was set to give the intensity (measured at 356 nm) 10 W/m2. The relative humidity was kept at the value 49.7 ± 0.3% during the measurement. The experiments run in shortened irradiation period for 68 min.

3. Results and discussion 3.1. Chemical and phase composition of the KATI composites Chemical composition of the sample K, K600, KATI61 and KATI66 composites is shown in Table 2. The loss on ignition (LOI) was determined after 1 h long annealing the samples at 1000 °C. The original K sample consists of SiO2 and Al2O3 in weight ratio 1.61, this value is higher in comparison to the value 1.18 calculated for the ideal pure kaolinite, what implies the presence of admixtures in the raw K sample. Sample K consists of small amount of TiO2 as shown in Table 2, whereas the TiO2 content in KATI61 and KATI66 composites reaches approx. 11 and 59 wt.%, respectively. The presence of sulfur expressed as SO3 determined in these composites is associated with the preparation procedure, when TiOSO4 is used as a source of TiO2. The LOI value determined for sample K600 reaches 2.5% and thus reveals presence of volatile constituent stable up to 600 °C and unstable at 1000 °C at this sample. Although the presence of sulfur in KATI61 contributes to its LOI value, this value is slightly lower in comparison to the LOI determined for K600. Obviously higher LOI value was even determined

Table 2 Chemical composition of the original K sample, K600, KATI61 and KATI66 composites (in wt.%).

a

TiO2b

Sample

Al2O3

SiO2

SO3

K2O

TiO2

Fe2O3

LOIa

K K600 KATI61 KATI66

32.7 35.9 31.1 12.2

52.7 57.7 52.2 20.0

<0.005 <0.005 0.595 2.45

1.58 1.61 1.06 0.465

1.15 1.27 11.4 59.0

0.65 0.72 0.456 0.148

11.2 2.5 2.2 4.6

LOI – loss on ignition.

for the KATI66 composite (see Table 2). The LOI value determined for the composites KATI61 and KATI66 is given by the sum of the amount of water and sulfur. The identification of volatile components in sample KATI66 was performed by the means of TG-MS method, and registered TG curve and mass spectra for H2O and SO2 are pictured in Fig. 1. TG-MS curves registered for sample KATI66 reveal releasing of the water as well as sulfur dioxide. Considering the MS spectra of water, only the presence of water absorbed at the surface of composite KATI66 which releases in temperature interval 100–200 °C was identified. The MS spectra registered for KATI66 did not verify the presence of any water releasing at temperatures higher than 475 °C and thus confirm complete dehydroxylation of kaolinite in composite KATI66. At the temperature higher than 500 °C SO2 starts slowly release from the KATI66 composite as evident from the mass spectra of SO2. The amount of TiO2 in composite KATI61 is six times lower in comparison to composite KATI66 and thus the presence of sulfur is also lower what is reflected in lower value of LOI determined for this composite (Table 2). Phase composition of the original kaolinite and the prepared composites before and after 2 h long calcination at 600 °C reveals their XRPD patterns pictured in Fig. 2. The raw kaoline (sample K) consists of kaolinite as a main phase and also consists of quartz and mica which represent the typical admixtures of kaoline. After the calcination at 600 °C the (0 0 1) basal diffraction peak of kaolinite disappears, what is typical feature of the dehydroxylation of the kaolinite structure and confirms the metakaolinite formation. In general, the process of kaolinite dehydroxylation up to approx. 950 °C is described by the following reaction scheme (1) [31]: Al2 O3  2SiO2  2H2 OðkaoliniteÞ ! Al2 O3  2SiO2 ðmetakaoliniteÞ þ 2H2 O ð1Þ

After the synthesis of TiO2 (KATI11 and KATI61) the intensities of the diffraction peaks belonging to kaolinite, quartz and mica proportionally decreased (Fig. 2). The presence of any form of TiO2 is hardly evident on the diffraction patterns of KATI11 and KATI16 samples and became evident on the diffraction patterns of these composites after their calcination. Disappearing of the kaolinite (0 0 1) diffraction peak on the diffraction patterns of KATI61 and KATI66 composites again clearly verifies the dehydroxylation of the kaolinite structure which is connected with the metakolinite formation. Together with dehydroxylation of the kaolinite structure the anatase structure which is poorly crystalline after the synthesis become well defined what is reflected by the sharpening of the anatase diffraction peaks as is evident mainly in the case of the KATI66 composite (see Fig. 2).

V. Mateˇjka et al. / Construction and Building Materials 35 (2012) 38–44

TG

102

(a)

100

-9.5

TG (%)

96

-10.5

94

-11.0

92 -11.5

SO2

90

-10.0

98 TG

96

-11.0

92

-11.5

SO2

-12.0

88

100 200 300 400 500 600 700 800 900 1000

-10.5

94

90

-12.0

88

-9.5

Ion current (A)

H2O

Ion current (A)

-10.0

98

(b)

H2O

100

TG (%)

102

41

100 200 300 400 500 600 700 800 900 1000

Temperature (°C)

Temperature (°C)

1

1

Fig. 1. TG curve and corresponding mass spectra belonging to H2O and SO2 registered for sample: (a) K and (b) KATI66 composite.

3

4

4

3

4 K K600 KATI11

2

KATI16 KATI61 KATI66

0

20

30

40

2-Theta - Scale Fig. 2. XRPD diffraction patterns of samples K, K600, KATI11, KATI16, KATI61 and KATI66. 1. kaolinite, 2. anatase, 3. quartz, 4. mica.

3.2. Determination of the latent hydraulic activity of the composites The formation of metakaolinite after kaolinite dehydroxylation imparts latent hydraulic properties to the prepared composites, what can be measured e.g. using the Chapelle test [21]. The

100

CaO consumption (%)

90 80

86.0 78.5

84.4

80.3

76.1

70

69.7

65.9

60 50 40 30 20 10 0

0

10

20

30

40

50

60

w (TiO 2) (%) Fig. 3. The relation between the extent of CaO consumption and the amount of TiO2 in KATI composites calcined 2 h at 600 °C.

relation between anatase content in the KATI composites calcined for 2 h at 600 °C and CaO consumption is pictured in Fig. 3. CaO consumption determined for K600 sample is 79%. The CaO consumption determined for KATI61 and KATI62 (composites containing 10 and 20 wt.% of anatase and calcined 2 h at 600 °C) is higher in comparison to K600 sample. This fact can be attributed to the higher extent of kaolinite dehydroxylation promoted by sulfuric acid (main component of TiOSO4 colloid suspension) during the KATI composite preparation. This presumption is in good agreement with the observed effect of sulfuric acid on activation of the kaolinite surface described by Panda et al. [32]. Decrease of the CaO consumption observed for the composites with higher anatase content is related to decrease in the amount of the metakaolinite ready for the reaction with Ca(OH)2. The effect of the KATI66 composite on the phase composition of hydrated cement paste was studied using X-ray powder diffraction method. The X-ray diffraction patterns of the hardened cement pastes P_OPC and P_OPC/KATI are shown in Fig. 4. Portlandite is the main crystalline compound originating during the hydration of the materials containing the OPC binder and was identified on the diffraction pattern of the hardened paste P_OPC (Fig. 4). The X-ray diffraction analysis also revealed the presence of tricalcium silicate (C3S) at this sample, what signalizes only partial hydration of this mineral. The intensities of the portlandite diffraction lines significantly decreased in the case of the cement

4

2

3

1

(a)

4 4 1

V. Mateˇjka et al. / Construction and Building Materials 35 (2012) 38–44

42

10

1

1

1

20

30

40

50

60

5

5

7

7

7

4

5

7

5

5

4

6

2 7

2 2 1

5

6

2 7

(b)

2

5

3

3

1

2

2

2

1

4

2

70

80

2-Theta - Scale Fig. 4. XRPD patterns of the hardened pastes after their 28 days long storage in moist environment: (a) P_OPC and (b) P_OPC/KATI66. 1. portlandite, 2. ettringite, 3. calcium carbonate, 4. tricalcium silicate, 5. anatase, 6. quartz, 7. calcium aluminum carbonate hydroxide hydrate.

paste containing the KATI composite (sample P_OPC/KATI), see Fig. 4. This fact is attributed to the pozzolanic reaction of metakaolinite with calcium hydroxide during the hydration process. The elimination of portlandite in the hardened cement mortars containing metakaolinite is the main benefit leading to formation of more ductile materials with lower tendency for corrosion in aqueous environment. 3.3. Determination of the compressive strength The values of compressive strength obtained for the reference cement mortar (STD) and cement mortars with the calcined kaoline K600, KATI61 and KATI66 composites are graphically compared in Fig. 5. It is evident, that the values of compressive strength measured for the mortars containing K600, KATI61 and KATI66 composites are higher in comparison to the values measured for the sample STD. The values of the compressive strength determined for the mortars containing 5 and 10 wt.% of K600 sample as an OPC replacement (S5K600 and S10K600 samples) are slightly higher in comparison to these values measured for mortars with the same amount of KATI61 and KATI66 (samples S5KATI61, S10KATI61, S5KATI66 and S10KATI66) composites. The increase of the value of the compressive strength of the cement mortars due to the presence of metakaolinite is well known and is ascribed to the reaction of metakaolinite with calcium hydroxide (pozzolanic reaction)

which is favorable to form the denser structure [33]. The values of compressive strength measured for all of the samples containing the KATI composites are slightly lower in comparison to samples with K600. The increasing content of TiO2 in the KATI composites for given load of these composites led to slight decrease of the compressive strength values (see Fig. 5). The explanation of this phenomenon arises from the composition of the KATI composites. The higher the TiO2 content at these composites the lower the amount of the metakaolinite is and thus lowers extent of pozzolanic reaction which is favorable for the formation of harder structures. 3.4. Distribution of TiO2 in the hardened samples The character of the hardened structure of the S15KATI66 sample was studied on the polished cross section of this sample using SEM operating in BSE mode and is pictured in Fig. 6. The distribution of TiO2 on the imaged surface was revealed using the EDX mapping of Ti. Maps of Si, Al and Ca as the main elements of phases occurring in hardened sample are also shown for better visualization of the resulting structure. The EDX map of Ti revealed relatively homogenous distribution of this element and also verifies homogenous distribution of TiO2. Intensity of the EDX signal registered for Si is strong in the area of the occurrence of particles of silica sand (marked by 1 in Fig. 6a). Except the clearly visible edge of silica sand in the BSE micrograph,

Fig. 5. Dependency of the compressive strength values on the amount of an admixture replacing the equivalent amount of OPC in prepared cement mortars.

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3.5. NO removal efficiency test The mortars containing 5, 10, and 15 wt.% of K600, KATI61 and KATI66, respectively were used for determination of their photodegradation activity against NO. The values of NO conversion were calculated using the following equation:

X NO ¼

out C in NO  C NO

C in NO

!  100

ð2Þ

where cNO is the concentration of NO (ppmv), superscripts: in the inlet, out the outlet, XNO is the NO conversion (%). The dependency of the NO conversion during irradiation is shown in Fig. 7. The average NO conversion was calculated for each mortar and the values are compared in Table 3 together with the standard deviation. When comparing the values of the NO conversion it is evident, that the highest photodegradation was achieved in the case of the mortars containing the KATI66 (SXKATI66) composite and the negligible conversion was achieved for the samples containing calcined kaoline (SXK600) due to the absence of the photoactive anatase particles. Comparing the values of the NO conversion for the samples containing the same amount of KATI66 (SXKATI66) and KATI61 (SXKATI61) it is evident that the ratio reaches 6:1 what is in direct relation to the TiO2 amount, remember that KATI61 contain 11 wt.% of TiO2 and KATI66 contain 59 wt.% of TiO2. Hassan et al. [35] studied the NO conversion over the top layer of paving

11 10 9 8 7 6 5 4

10% 5%

1.5

15%

1.0

10% 5%

0.5

15% 10%

0.0

5% 0

10

20

30

40

50

60

SXKATI66

15%

SXK600 SXKATI61

another bright grain (marked by 2 in Fig. 6a) is also clearly observable. With respect to the high intensity of Ca and also Si in this region this particle probably represents partially unreacted C3S [34], the presence of unreacted C3S phase was previously revealed in the hardened samples P_OPC and P_OPC/KATI using XRPD method (Fig. 4).

NO conversion (%)

Fig. 6. Distribution of the elements on the surface of hardened S15KATI66: (a) SEM micrograph in BSE mode, the distribution of selected elements (Ti, Al, Si, Ca) obtained using EDX mapping analysis and (b) detail of the selected region. EDX map showing Ti distribution and EDX spectrum of selected point.

70

Time (min) Fig. 7. The dependency of NO conversion on the time of illumination.

blocks and acquired 18% of NO removal efficiency over layer containing 3 wt.% of TiO2 photocatalyst. In our work, the highest value of the NO conversion is 9% achieved for the S15KATI66 sample containing in sum 2 wt.% of TiO2 carried by the KATI66 composite. This is one-half of the value reached by Hassan and her co-workers over the surface containing 3 wt.% of TiO2. Our samples containing 3 wt.% of TiO2 carried by the KATI composite should reach approx. 14% efficiency in NO removal as estimated using the extrapolation method. The reasons for the lower NO photodegradation efficiency using composites KATI in comparison to the efficiency reached by Hassan et al. could be attributed to the formation of denser and thus less porous structure. Higher efficiency of NO removal can be achieved by increasing of the TiO2 content in the hardened samples while there are generally two approaches how to increase the amount of TiO2 on the surface of the hardened samples with KATI

V. Mateˇjka et al. / Construction and Building Materials 35 (2012) 38–44

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Table 3 Average values of NO conversion (%) with standard deviation. Sample

NO conversion

Sample

NO conversion

Sample

NO conversion (%)

S5K600 S10K600 S15K600

0.174 ± 0.144 0.474 ± 0.216 0.457 ± 0.045

S5KATI61 S10KATI61 S15KATI61

0.925 ± 0.210 0.945 ± 0.190 1.44 ± 0.30

S5KATI66 S10KATI66 S15KATI66

5.01 ± 0.33 6.13 ± 0.18 8.70 ± 0.04

composites: (i) utilizing the higher amount of the KATI66 composites in the formulation of mortars or (ii) increase of the TiO2 content in the KATI composite which is subsequently used as a partial replacement of OPC (e.g. composite KATI67). The calcined KATI composites represent valuable material which extends the portfolio of admixtures which can be utilized in connection with OPC. Further research should be addressed to the detailed description of the hydration process occurring in the mixtures of KATI with OPC as well as the possible utilization of the KATI composites for the preparation of geopolymers.

4. Conclusion The calcined KATI composite represents the novel valuable latent hydraulic admixture for building materials based on the Portland cement. The latent hydraulicity of this composite is assured by the presence of metakaolinite. Thermally treated KATI composites undergo the pozzolanic reaction in the alkaline environment which originates during the hydration of the Portland cement. After the hardening period the resulting product contains lower amount of portlandite, whereas the TiO2 particles remains in its anatase form and keep its photodegradation activity. The degree of NO conversion depends on the TiO2 content and it was observed that the higher content of TiO2 carried by KATI composite means the higher NO removal efficiency. The increase of the NO removal efficiency of the resulting materials can be influenced also by other factors e.g. fineness and porosity of the photocatalytically active surface. Acknowledgments This paper was created in the Project No. CZ.1.05/2.1.00/01.0040 ‘‘Regional Materials Science and Technology Centre’’ within the frame of the operation programme ‘‘Research and Development for Innovations’’ financed by the Structural Funds and from the state budget of the Czech Republic. Financial support of Technology Agency of the Czech Republic within the Project TA01011030 is also greatly acknowledged. References [1] Carp O, Huishman CL, Keller A. Photoinduced reactivity of titanium dioxide. Prog Solid State Chem 2004;32:33–177. [2] Han F, Rao Kambala VS, Srinivasan M, Rajarathnam D, Naidu R. Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment: a review. Appl Catal A 2009;359:25–40. [3] Wu Z, Wang H, Liu, Gu Z. Photocatalytic oxidation of nitric oxide with immobilized titanium dioxide films synthesized by hydrothermal method. J Hazard Mater 2008;151:17–25. [4] Colón G, Maicu M, Hidalgo MC, Navío JA, Kubacka A, Fernández-García M. Gas phase photocatalytic oxidation of toluene using highly active Pt doped TiO2. J Mol Catal A: Chem 2010;320:14–8. [5] ISO 22197-1:2007: Fine ceramics (advanced ceramics. Advanced technical ceramics) – Test method for air purification performance of semiconducting photocatalytic materials – Part 1: Removal of Nitric Oxide. [6] ISO 22197-1:2007: Fine ceramics (advanced ceramics. Advanced technical ceramics) – Test method for air purification performance of semiconducting photocatalytic materials – Part 3: Removal of Toluene.

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