The use of the Empyrean diffractometer in combination with the Anton Paar MHC-trans chamber enables us to characterize in situ the swelling of clay minerals at variable temperature and relative humidity conditions.
By definition X-ray diffraction is the most comprehensive tool to study clay swelling. The measurements can be performed both ex situ and in situ, with the latter undoubtedly being a more accurate probe. Here we present an in situ comparative study of the swelling of montmorillonite and corrensite clays in a wide range of temperature and relative humidity.
Water swelling is an intrinsic property of clay materials (e.g. smectite, vermiculite-like clays, some hydrous micas) and is associated with a hydration process. Water molecules enter the space between the silicate layers thus expanding the unit cell (or crystal structure) along the c-axis. The degree of swelling depends on the structure of a layered silicate and its chemical composition. A quantitative measure of the clay swelling is the spacing between (001) lattice planes referred to as basal spacing. By definition X-ray diffraction is the most comprehensive tool to study clay swelling. The measurements can be performed both ex situ and in situ, with the latter undoubtedly being a more accurate probe. Here we present an in situ comparative study of the swelling of montmorillonite and corrensite clays in a wide range of temperature and relative humidity.
X-ray diffraction measurements in transmission geometry were performed using the Empyrean X-ray diffraction platform configured with a Cu LFF tube, an X-ray focusing mirror and the PIXcel3D used as a line detector. In transmission geometry not only a better low-angle performance over a wide 2theta range can be achieved, but also the sample displacement error, due to water absorption by the clay materials, is eliminated. Thus, usage of the transmission geometry enables the measurement of the true unit cell change as a function of relative humidity. The Anton Paar Multi-Sample Humidity Chamber (MHC-trans) was used to create a sample environment with variable temperature (T) and relative humidity (rH) conditions. The chamber has an internal sample changer enabling automatic measurements on up to eight samples at a time[1]. A series of XRD patterns were measured in the interval from 15 to 90 %rH at 25, 50 and 70 ºC. To ensure the same starting conditions for the measurement series at each temperature, the samples were dried at 15 %rH and 90 ºC inside the MHC-trans chamber for 2 hours.
The use of the Empyrean diffractometer in combination with the Anton Paar MHC-trans chamber enables us to characterize in situ the swelling of clay minerals at variable temperature and relative humidity conditions.
Figure 1. Clay minerals are utilized in a wide range of industrial processes, e.g. construction, production of ceramics, pesticides, textile and paper. Clays are used as drilling muds in the gas and oil industry and for disposal of radioactive waste.
The degree of swelling in a clay is to a large extent determined by the stacking of tetrahedral layers (Si, Al)O4 and octahedral layers (Mg, Al)O6 within the crystal structure. Montmorillonite has a smectite structure formed by a regular stack of the octahedral (O) and tetrahedral (T) layers forming T-O-T sheets. Large cations as well as molecular water can easily migrate into the space between the sheets which results in its highly expansive nature. Corrensite is a 1:1 regular interstratification of chlorite and smectite and appears as a regular stack of chlorite and smectite sheets.
As chlorite does not absorb water, corrensite is expected to show a lower degree of swelling compared to montmorillonite under the same conditions. This is indeed confirmed by the experimental data. The low-angle part of the XRD patterns of montmorillonite and corrensite measured at 25 ºC and a relative humidity from 15 to 90 %rH is presented in Figure 2. Compared to corrensite, montmorillonite shows a significantly larger shift of the (001) reflection (= increase of the basal spacing) with increasing relative humidity.
By applying an automatic sequential profile fit[2,3] to the low-angle part (2-10 deg 2theta) of the entire XRD data set (28 patterns) the position of the (001) reflection as a function of temperature and relative humidity is obtained (Figure 3). As seen in Figure 2 in the entire investigated range of temperature and relative humidity montmorillonite exhibits a higher degree of swelling, with the highest value of Δ = 3.10(2) Å at 95 %rH and 25 ºC. At the same conditions the basal spacing of corrensite is increased by 0.64(6) Å only. At higher temperatures the degree of swelling also increases with the increasing relative humidity. However, the absolute values of the basal spacing are reduced above 55 %rH and below 35 %rH for montmorillonite and corrensite, respectively. The negative temperature slope of the basal spacing is related to the desorption of water molecules triggered by the temperature increase.
Figure 2. XRD patterns of montmorillonite and corrensite measured at 25 ºC and relative humidity from 15 % to 90 %rH. Patterns of montmorillonite are shifted along the “intensity’ axis for clarity.
Figure 3. Basal spacing of montmorillonite (top) and corrensite (bottom) as a function of temperature and relative humidity
Using an Empyrean diffractometer equipped with a MHC-trans chamber we characterized in situ the degree of swelling of montmorillonite and corrensite in a wide range of relative humidity (rH) and temperature (T). As expected the swelling of montmorillonite is higher than that of corrensite at the same T-rH conditions. Both clay minerals show a negative temperature slope of the basal spacing above 50 ºC.
