Research paperExtended and local structural description of a kaolinitic clay, its fired ceramics and intermediates: An XRD and XANES analysis
Introduction
Kaolinite (K: Al2O3·2SiO2·4H2O) is classified as a 1:1 dioctahedral phyllosilicate and it is the main component of the kaolin group of minerals. The importance of kaolinitic clays in the development of modern ceramic science can best be appreciated by considering its widespread influence on ceramic, material science, and mineralogy (Chakraborty, 2014). These clays have been widely used in different technological applications for thousands of years. The majority of the applications include a thermal treatment. The firing transformations of this family of raw materials have been widely studied (Carty and Senapati, 1998, Iqbal and Lee, 2000, Lee et al., 2008, Carbajal et al., 2007, Lisiane et al., 2012).
In kaolinitic clays the principal clay crystalline phase is kaolinite, and it is usually present with other phases like quartz, feldspars and titania. (Serra et al., 2013). The kaolinite thermal transformations are affected by heating treatments (rate, dwell and atmosphere) and presence of impurities or additives and particle size (Chakraborty, 2014).
Kaolinitic clays are utilized in a large variety of industrial applications such as ceramics, refractories, cement, filling agent in paper, plastics, rubber, cosmetics, etc. In order to reach the desired properties, thermal treatments are involved in the majority of these applications (Liu et al., 2003, She and Ohji, 2003, Lee et al., 2008).
The classic papers of Brindley and Nakahira reported for the first time a systematic study of phase transformations for the kaolinite–mullite series (Blinder and Nakahira, 1959a, Blinder and Nakahira, 1959b), and measured the transformation temperature for a model material. The actual temperature transformations can be evaluated by several thermal analysis techniques, thermogravimetry, differential thermal analysis, calorimetry, etc. (Chakraborty, 2014).
In fact, the particular properties, like chemical composition, particle size, transformation temperature, etc., of each raw natural material depend on the different geochemical constitutions of these materials including impurities, in consequence, heating rates, for example, are affected by geochemistry of each material (Chakraborty, 2014).
The kaolinite–mullite series was recently studied by means of powder neutron diffraction (De Aza et al., 2014) and the MK formation was studied by NMR (Wang et al., 2014). The mechanisms were proposed and corroborated by the employed combined structural and thermal techniques.
Kaolinite (K) dehydroxilation occurs, through a three dimensional diffusion process, with the formation of an amorphous product identified as metakaolinite (MK: Al2Si2O5); this process is completed above ∼ 650 °C. The metakaolinite remains short-range order at least to ∼ 980 °C. The formation of nanometer size and randomly oriented needle-like mullite (∼ 980–992 °C), primary mullite (Mi), side by side with a cubic phase, Si–Al spinel (SAS), and amorphous silica-rich at around ∼ 983 °C (G) was confirmed. The Mi formation is incipient. From ∼ 1136 °C growth of mullite (Mi) crystals occurs and at T > ∼ 1200 °C crystallization of high temperature cristobalite (SiO2) from a Si-rich amorphous phase takes place. Between 983 °C and 1136 °C is correct to assume that the amount of SAS will be higher than Mi. Additionally, in the Si-rich amorphous phase formed at kaolinite–muscovite interfaces occurs secondary mullite (Mii) crystallization (∼ 1300 °C). The impurities in the starting kaolin can induce a liquid phase during firing (De Aza et al., 2014). Bellotto et al. performed in situ experiments using synchrotron radiation diffraction on kaolin specimens (Bellotto et al., 1995a, Bellotto et al., 1995b). They study the kinetics of mullite formation in the 1300–1400 °C temperature range and the kinetics of dehydroxylation of two kaolin specimens in the 500–700 °C temperature range.
The stoichiometry of the metakaolin corresponds with the kaolinite one, secondly the SAS stoichiometry remains undefined, and finally mullite varies in a small range (Schneider et al., 2008). The vitreous phase (G) belongs to the alumina–silica system, with high silica concentration, accompanied by the different impurities, principally alkali (K and Na), earthen alkali (Ca and Mg), iron oxide or titania.
Within the temperature range (≤ 1350 °C) in which the kaolinitic materials are of technological interest, several transformations occur to kaolinite, which we intend to characterize in present study. This can be represented by the following scheme.
It is important to point out that the local crystalline nature of MK and SAS is different; MK presents a short range ordering, but no long-range order and therefore no XRD Bragg peaks are present. On the other hand SAS presents some short range order but the crystalline domains (crystallites) are nanosized and therefore are difficult to evaluate this phase using XRD analysis (Serra et al., 2013, Conconi et al., 2014).
In recent years, White et al. used the X-ray Absorption Near Edge Structure (XANES) to study the local environment in metakaolin, which became in a subject of significant debate, particularly with regard to the aluminum coordination environment determination (White et al., 2011). XANES is a powerful and versatile technique for obtaining information about the local atomic environment in materials and can be used to investigate specific elements in solids, liquids, gases or plasma (Koningsberger and Prins, 1998, Bunker, 2010).
The principal objective of the present work consists of the characterization of a series of complex aluminosilicates of technological interest, in the kaolinite–mullite series, by means of X-ray based techniques (XRD and Al K-XANES). This kaolinite–mullite series is obtained by controlled calcination of industrial kaolinitic clay (C-80 caolin PG, Argentina) accompanied by a microstructural observation by means of scanning electron microscopy.
Taking into account the low crystalline nature of some intermediates and products of the mentioned reactions, particularly MK, SAS and G; synchrotron based Al K-XANES arise to be adequate technique for characterizing these particular phases from the local point of view complementing the XRD-Rietveld analyses that evaluates the long distance order of those phases. A pure acid washed kaolinite and a sintered crystalline mullite (pure) are also evaluated for comparison.
Section snippets
Studied kaolinitic clay and structural standards
Industrial kaolinitic clay (C-80 caolin PG, Argentina) was used as model material and two high purity commercially available materials (kaolinite and mullite) were employed as standards for both structural characterizations XRD and Al K-XANES. The chemical composition is shown in Table 1. The powder presented a mean particle size (D50) of 4.0 μm. The mineralogical composition consists of kaolinite (> 70%), quartz and feldspar.
An almost pure, acid washed, kaolinite (98% kaolinite) (Fischer,
Thermal behavior (TG-DTG) and thermal treatments
Fig. 1 shows the TG and DTG analysis of the clay, the typical two mass losses can be observed in the 0–1400 °C range, the first one (≈ 2%), observed below 150 °C, corresponds to the surface water loss. The second one (≈ 7%) mass loss can be observed at the kaolinite decomposition (dehydroxylation) into MK (also a water loss). This analysis permits to identify the transformation temperature. No other mass loss (or gain) process occurs in this temperature range. Fig. 2 shows the DTA curve, and both
Conclusions
Kaolinite thermal processes present local and long distance order nature. The extended and local range order structure characterization of kaolinitic clay and its calcination products was carried out by means of two X-ray based techniques: XRD and XANES, respectively. In order to give context to these characterizations thermal analysis (TG-DTA) and electron scanning microscopy were carried out.
Both crystalline and low crystalline phases were described after thermal transformations in kaolinitic
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2022, Cement and Concrete ResearchCitation Excerpt :The peak CVI of the green spectrum at ~1550.4 eV is positioned at 2.5 eV higher energy than that of the red spectrum. Such a high peak energy represents six-fold coordinated Al (Al[VI]) [48] and the presence of a Al[IV] shoulder at ~1547.1 eV both suggest a mullite-like Al coordination environment (a mixture of Al[IV] and Al[VI]) in the green region in Fig. 1d [52,53] or a mixture of phases with both Al[IV] and Al[VI]. The green spectrum is comparable to mullite spectra in existing literature [52,53] (see Supplementary Information for the spectra of mullite and aluminosilicate glass).