Thermal analysis testing and natural radioactivity characterization of kaolin as building material
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Kaolins are used in a multiplicity of industries because of unique physical and chemical properties. Relationships between thermal and radioactivity properties are discussed in its application as a building material. Super-fine kaolin powder with particle sizes about 30 μm was analyzed. Simultaneous TGA/DTA analysis was performed on powder samples at various heating rates in an argon atmosphere. Based on investigated thermal properties, it was concluded that dehydroxylation process can vary depending on the characteristics of starting material. The maximum degree of the dehydroxylation (DT) was obtained at the lowest rate of heating (DT = 60.79% for 10 °C min−1). With an increase in the heating rate, decline in DT value was observed. Based on comprehensive testing, it was identified that the degree of dehydroxylation does not drop below 50%. It was concluded that appointed experimental conditions seem sufficient admissible for obtaining degree of dehydroxylation (DT) higher than 50%. In order to safe use of kaolin as a building material from the standpoint of radiological safety, content of natural radionuclides was determined by gamma spectrometry.
KeywordsPowder kaolin sample Building material Degree of the dehydroxylation Natural radioactivity
Kaolin or a white clay represents the sub-group mineral which is rich in kaolinite mineral and also classified as a layered silicate mineral. Due to the endothermic process or dehydroxylation process which involves heat, kaolin particles can easily transform into the other shapes or particles [1, 2, 3]. Namely, when kaolin is heated, its dehydroxylation occurs and metakaolin is formed. The metakaolin was formed through the reaction of endothermic process. In metakaolin formation, the new formed kaolin-based product was very reactive. The metakaolin was popular in the ceramic industries and also as construction materials [4, 5], where the precursor—kaolin—belongs to a well-known building material used in the preparation procedures for the development of composite materials . Most of the researchers believe that metakaolin performs the better resistance to water and enhances mechanical properties of concrete [7, 8]. In construction and building purposes, the metakaolin in concrete reduces the water penetration and alkali silica reaction by acting as filler and increases the hydration process. The production of metakaolin from raw kaolin has an important role in building material investigations, wherein the particle size of the starting material and the particle size of the resulting products can be significant in size materials technology. In addition, the use of metakaolin in cement-based system provides, besides technical , significant environmental benefits . Metakaolin is unique in that it is not the by-product of an industrial process nor is it entirely natural; it is derived from naturally occurring mineral and is manufactured specifically for cementing applications.
Metakaolin is usually produced by the thermal treatment from kaolin clays within a definite temperature range. The heating process drives off the water from the mineral kaolinite (Al2O3·2SiO2·2H2O), the main constituent of kaolin clay, and collapses the material structure, resulting in an amorphous aluminosilicate (Al2O3·2SiO2), the metakaolin. Thermal transformation of kaolin/kaolinite [11, 12, 13, 14, 15, 16] has shown that the heating parameters, such as temperature, heating rate and time, as well as the cooling parameters (the cooling rate and ambient conditions), significantly influence the process of dehydroxylation.
Naturally occurring radionuclides comprise radionuclides associated with the 238U and 232Th decay chains as well as 40K. Building materials can contain various amounts of natural radioactive nuclides. This can be of natural origin, that is contained in raw material (natural stone), or it can be due to the addition of industrial products (zircon sand), intermediates or by-products (coal ash, phosphogypsum and furnace slags). The radioactive isotopes in building materials can increase external and internal radioactive exposures of humans. Knowledge of the level of natural radioactivity in building materials is important to assess the possible radiological hazards to human health. The worldwide average specific activity in the building materials is as follows: 226Ra (50 Bq kg−1), 232Th (50 Bq kg−1) and 40K (500 Bq kg−1) . Kaolin can be used as raw materials in building, so it should be noted that what their fraction is in the final product which is used as a construction material. Kaolin in the final product is used in percentages: 10–30%. Based on the measured activity concentration of natural radionuclides 226Ra, 232Th and 40K, hazard indices can be assumed: gamma index, radium equivalent activity, gamma dose rate, annual effective dose and internal hazard index.
Thermogravimetry/differential thermal analysis (TG/DTA) is the most commonly used technique for the thermal evaluation of clay materials  and the working process as it has a capability to measure endothermic, exothermic phenomena and mass loss change characteristics. From obtained thermoanalytical (TA) data, the corresponding thermal properties of tested material were derived.
The main goal of this study is to investigate the thermal properties of fine kaolin powders used as a industrial kaolin with a mean particle size of 30 μm (particles of a mean diameter about 30 μm are usually referred to as fine particles (according to some classifications as super-fine particles) whose applications in industries are vast), under the non-isothermal conditions. It should be noted that several end uses of kaolin depend upon the particle sizes. Consequently, in many applications, the coarse-particle kaolin may not work, whereas a fine particle kaolin in many cases will. In addition, the natural radionuclides content in kaolin was also determined.
The raw material used for the actual study was imported from Czech Republic as industrial kaolin, which was provided by the company Sedlecký kaolin a. s., Czech Republic (CZ). The received material was packed in special bags already in a powder form.
Characterization of materials
The chemical composition of supplied kaolin was determined by X-ray fluorescence (XRF) analyzer (Lab-X3500, Oxford Instruments, Abingdon, UK), and the results are as follows: SiO2 66.00%, Al2O3 30.98%, Fe2O3 0.80%, TiO2 0.57%, Na2O + K2O 0.50% and CaO + MgO 0.50%, LOI 0.65%. In accordance with the manufacturer’s instructions, the LOI was calculated as: LOI = [(m1 − m2)/m1] × 100 (%), where m1 and m2 are the masses of the sample before and after firing (firing temperature of 800 °C) , respectively. Namely, the firing temperature was retained during 5 h, but the increase in duration over 5 h significantly increases the LOI (so that 5 h of duration is optimum at a given firing temperature without further increasing LOI, which is already shown above in the data), since that LOI can resemble the mass loss obtained through the thermoanalytical measurements.
Generally, the water absorption decreases with the increase in firing temperature due to the formation of the liquid phase and densification at high firing temperatures. The liquid phase formed is the one which fills the pores and decreases the porosity. Water absorption is directly related to open porosity, and porosity decreases with the increase in firing temperature. Actual temperature at which the sample gives the minimum value of apparent porosity depends on the composition of raw material used. However, based on the obtained LOI, this value is much lower than the amount required for kaolin used, for example, in the porcelain industry (5.0–12.8%) . Regarding LOI, this value is required to be low because of its effect on the porosity and linear shrinkage of refractory clays. Low value also suggests the low content of volatilized components, i.e., organic matter, water and carbonates. The amount of silica in the sample (66.00%) is slightly lower than the range for silica amount, which would be considered as suitable for the production of porcelain body (66.3–79.5%) , while alumina was present in high amount (30.98%). Based on established characteristic of the sample, it seems that supplied kaolin is convenient for the cement industry usable in a high strength concretes.
The particle size analysis was performed on YX-3000A (Xiamen Yuxiang Magnetic Materials Ind. Co., Ltd. Xiamen, China) disk particle size analyzer (PSA) which adopts high-speed disk centrifugal sedimentation. The medium value of analyzed particle sizes for testing kaolin powder amounts to 30 µm which exposes brightness above 80%. The kaolin sample used for all further analyses was not subjected to any subsequent manipulations.
Thermoanalytical (TA) measurements were taken using a simultaneous non-isothermal thermogravimetric analysis (TG) and differential thermal analysis (DTA) on the SETARAM Setsys Evolution 1750 (SETARAM Instrumentation 7, rue de l’Oratoire 69300 Caluire, France) instrument. The high-purity argon (Ar) gas (99.999%) was used as the carrier gas at a flow rate of φ = 20 mL min−1. A Pt crucible was filled with about 10 mg of powder sample and heated at the different heating rates: β = 10, 15, 30 and 40 °C min−1. The tested sample at a given heating rate was controlled heated from the room temperature up to 800 °C. The differential thermal analysis (DTA) was carried out simultaneously with thermogravimetry (TG) testing under the same conditions as described above, with a 1750 °C DTA cell, and noise RMS and resolution of 20 and 0.4 µW, respectively. For each recording, the duplicate non-isothermal runs for the selected heating rate were performed under the same conditions, and it was found that the data overlap with each other, indicating the satisfactory reproducibility. The variation coefficients were below 5% for all sets of the thermoanalytical (TA) measurements.
The infrared (IR) spectra can serve as a fingerprint for mineral identification and give unique information about the mineral structure, including the family of minerals to which the specimen belongs and the degree of regularity within the structure, the nature of isomorphic substituent, the distinction of molecular water from constitutional hydroxyl and the presence of both crystalline and non-crystalline impurities.
The procedure used for FTIR spectroscopy analysis was conducted in accordance with the FTIR procedure proposed for clay minerals . The Fourier transform infrared (FTIR) spectrum was acquired using a Thermo Nicolet 380 FTIR (Thermo Fisher Scientific Ltd., Waltham, MA, USA) instrument equipped with a Smart Orbit™ ATR attachment, which contains a single-reflection diamond crystal. The FTIR spectrum was taken in the attenuated total reflectance (ATR) mode where typically 64 scans are performed for each spectrum at a resolution of 4 cm−1, in the wave number range of 400–4000 cm−1. About the 1 mg of the sample (the dried sample, which was previously dried for 2 h at a temperature of T = 105 °C) accompanied with 100 mg KBr (the dried powdered sample was homogenized in spectrophotometric grade KBr in an agate mortar and pressed at 3-mm pellets with a instrument distributed hand press) was done. The FTIR peaks were reported based on the % transmittance to a given wave number (cm−1).
The determination of the natural radioactivity content was performed by gamma spectrometry analysis. The sample was counted using a high-purity germanium detector (HPGe) with the relative efficiency of 20% and energy resolution of 1.8 keV for the 1332-keV 60Co peak. Calibration of detector was performed using silicone resin matrix spiked with a series of radionuclides (241Am, 109Cd, 139Ce, 57Co, 60Co, 203Hg, 88Y, 113Sn, 85Sr and 137Cs) with the total activity of 41.48 kBq on August 31, 2012 (Czech Metrological Institute, Praha, 9031-OL-420/12, type CBSS 2). The calibration was performed in the 500-cm3 Marinelli beaker geometry, too. In order to safe use kaolin in final product as a building material, calculated gamma index (I), based on the concentration of 226Ra, 232Th and 40K, must be less than 1 , and then kaolin can be used in high construction for interior. For the same order, radium equivalent activity  must be less than 370 Bq kg−1 and internal hazard index must be less than 1 . Absorbed gamma dose rate in indoor air  and annual effective dose also can be calculated.
Results and discussion
It can be seen from Fig. 1 that experimental sample undergoes certain physical and chemical variations with an increasing temperature, and these variations are as follows: The mass loss of 2.73% on TG curve includes liberation of adsorbed water (namely, which is the first to be lost by heating) up to 150 °C followed by deep endothermic peak at 93.01 °C within the first (I) decomposition region.
However, it can be observed within the same mass loss region (I) a certain change in the form of small endothermic effect, approximately at 300 °C (~ 296.21 °C) (Fig. 1). Namely, between 200 and 450 °C, the mass loss can be attributed to the pre-dehydration process, which results in a reorganization of octahedral layer. Above changes can be generally attributed to the evaporation of adhesion water and thermal expansion effect. In temperature range 450–650 °C, dehydroxylation process and the formation of metakaolin take place through major endothermic effect. This is the reason for the mass loss of 5.63% in the second (II) region of TG curve, followed by endothermic peak at 492.90 °C (Fig. 1). Quantity of produced amorphous material which can be estimated by difference to 100% of kaolin content depends on the aging time, the temperature and Si/Al ratio. Therefore, the position of DTA peak attributable to dehydroxylation process may vary depending on characteristics of starting material (such as particle size of powdered material, taking into account the way of packing as oriented or random (disoriented) powder forms through the bulk sample and also through particle size fractions changing, the heating rate, rate of gas flows, water partial pressure above sample, etc.).
To quantify the mass loss with temperature, a degree of the dehydroxylation DT  was determined after each recording. The DT quantity obtained by TG may be expressed as DT = 1 − (m/mmax), where m and mmax are residual and maximum sample mass loss, respectively. We focused on the conversion, which depends on the concentration of constituent water in kaolinite crystal , because residual mass loss measured by TG gives an information on how much ‘water’ is retained in the partially dehydroxylated system. The dehydroxylation of pure kaolin (46.50% SiO2, 39.50% Al2O3 and 14.00% H2O) in an ambient temperature results in the mass loss of about 14.00% and DT = 100%, which corresponds to mass in bound hydroxyl ions in kaolin. For our sample, which contains a higher SiO2 content (66.00%) and Al2O3 = 30.98%, at the various heating rates, the following values of DT were obtained: 60.79% (10 °C min−1), 59.79% (15 °C min−1), 56.29% (30 °C min−1) and 52.43% (40 °C min−1). Maximum degree of dehydroxylation DT was obtained at the lowest rate of heating (60.79% for 10 °C min−1). With an increase in the heating rate, there is a decline in DT value. However, the degree of the dehydroxylation does not drop below 50%.
Important factor which may affect on the degree of the dehydroxylation represents the stoichiometric ratio between SiO2 and Al2O3 regarding to the fractional conversion of the ratios between the AlO6 and AlO4 as well as ≡Si–O–Al= and ≡Si–O–Si≡ . Therefore, the increased SiO2 content could affect on the degree of dehydroxylation during the course of this process. Also, crystallization that occurs during metakaolin formation may decrease with an increasing SiO2/Al2O3 ratio, which can also affect the DT value . It was found that based on dehydroxylation temperature peak position, it may be judged whether it was a well-ordered, slightly disordered, strongly disordered or an extremely disordered metakaolin structure. In our case, the peak occurs at the temperature of 492.90 °C (Fig. 1) [at other heating rates, the peak temperatures are as follows: 470.72 °C (10 °C min−1), 514.40 °C (30 °C min−1) and 522.21 °C (40 °C min−1)], which is below 530.00 °C, and from standpoints of the above classification, we have the presence of extremely disordered metakaolin structure . Dehydroxylation continued to finally identify mass loss up to approximately 800 °C. Since the main constituent of kaolin is kaolinite, Al2Si2O5(OH)4, in this regard, structural hydroxyl groups were removed from kaolinite by a reorganization of octahedral layer of kaolinite to metakaolin (Al2O3·2SiO2) when temperature raised up to the range of ΔT ~ 450–560 °C, resulting in greatest mass loss (Fig. 1).
The bands at 3700, 3660 and 3620 cm−1 show the frequency peaks with the medium absorbance strength of OH stretching. The peak at position 3700 cm−1 is assigned to the inner-surface OH stretching. The peak at position 3660 cm−1 is attributed to the inner-cage OH stretching of kaolinite . The inner-hydroxyl group is obtained at 3620 cm−1 which is typical for a high amount of Al–OH in the octahedral sheet . Absorption band at about 1116 cm−1 can be assigned to Si–O normal to the plane stretching of kaolinite which is close to the 1125 cm−1 reported by Davarcioglu and Ciftci .
Band at 1050 cm−1 can be assigned to Si–O planar stretching, which is common for kaolinite. In addition, the absorption band at 916 cm−1 is due to the OH deformation band . The band at 795 cm−1 is an indication of Al–O–Si inner-surface vibration. Observed band at 696 cm−1 belongs to quartz interference (theoretical value for quartz interference occurs at 695 cm−1) . It should be noted that bands at 530 and 462 cm−1 can be attributed to Fe–O stretching vibration (theoretical value is 535 cm−1 for kaolin sample with 46.81% SiO2, 32.59% Al2O3 and 4.18% Fe2O3 , while current kaolin pattern has higher SiO2 content, approximately the same content of Al2O3 (~ 30.98%) and lower content of Fe2O3 (~ 0.80%) and Si–O–Si bending) , respectively. It can be seen that the Si–O, Al–O and OH play main functional roles in kaolin for its identification and characterization. Experimental conditions seem sufficiently admissible for obtaining DT > 50% for fine kaolin powders.
The natural radioactivity results
The obtained value for gamma index for low construction in case of kaolin is 0.20. All obtained results are less than 1, how it is required for safe use.
The estimated indoor gamma dose rate values for kaolin sample was 0.12 µGy h−1. Considering the fact that the average gamma dose rate indoors in Europe is 0.07 μGy h−1 , gamma dose rate calculated for investigated sample exceeds the average value in Europe.
The calculated value for internal hazard index was 0.57, so this value is less than 1 and radiation hazard is insignificant.
Relationships between thermal and radioactivity properties of kaolin as industrial building material are studied and discussed in this paper. The simultaneous TGA/DTA analysis was performed on powder prepared samples (which were heated from room temperature up to 800 °C) at the various heating rates in non-isothermal conditions under argon atmosphere. It was concluded that dehydroxylation process can vary depending on the characteristics of starting material. Maximum degree of dehydroxylation DT was obtained at the lowest heating rate (DT = 60.79% for 10 °C min−1). With an increase in the heating rate, decline in the DT value was observed. It has been shown that experimental conditions applied are sufficient to obtain degree of dehydroxylation higher than 50%. Based on the studied characteristics of the sample, it was derived that it is more suitable for the cement industry. Results of activity concentration measurements in building material (kaolin) used in construction industry in Serbia allow to conclude that measured sample could be safely used in building constructions.
Authors would like to acknowledge the financial support of the Ministry of Education, Science and Technological Development of the Republic of Serbia under the Projects 172015, III43009 and III45020.
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