The effects of temperature on the swelling properties of smectites are important for a variety of different geological conditions, but studies on this topic have been rather limited. The purpose of this study was to investigate the swelling behavior of Na- and Ca-montmorillonite at various temperatures greater than room temperature, up to 150°C, using in situ X-ray diffraction (XRD) analysis. A sample chamber was designed, the temperature and humidity of which were controlled precisely, for environmental in situ measurements. The XRD measurements were performed at small relative humidity (RH) intervals for precise observation of the swelling behavior.

The swelling behavior of Na-montmorillonite showed distinct zero-, one-, and two-layer hydration states. The basal spacings of Na-montmorillonite changed continuously with RH for various temperatures in the transition region between the zero- and one-layer hydration states, and the swelling curves of the transition region moved to greater degrees of RH with increasing temperature. The basal spacings jumped from the one- to two-layer hydration states for all temperatures at almost the same RH.

The basal spacings of Ca-montmorillonite changed continuously from the zero- to the two-layer hydration states at all temperatures. This behavior is remarkably different from that of Na-montmorillonite. At low-RH conditions, the d001 value of Ca-montmorillonite decreased with increasing temperature. The swelling curves of Ca-montmorillonite did not show a plateau at any temperature for the one-layer hydration state. The swelling curves of Ca-montmorillonite moved to greater RH with temperature, similar to the transformation region between the zero- and one-layer hydration states in Na-montmorillonite. These differences between Na- and Ca-montmorillonite are related to the hydration powers of exchangeable cations.

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