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Kinetics of Thermal Dehydration of Gypsiferous Soils¹

ABSTRACT

Recording the decrease in weight with time upon heating by electrically reproducible infrared (IR) radiation offers a fast method for studying the kinetics of the dehydration of gypsiferous soils. Plotting the weight loss rate against time gives a dehydration curve made of two peaks. The first peak is due to soil moisture and the second to the crystal water of gypsum. The product of dehydration is best regarded as soluble anhydrite (CaSO₄ · H₂O) rather than hemihydrate (CaSO₄ · H₂O) or insoluble anhydrite (CaSO₄). This is indicated by the loss in weight and the IR spectra. The kinetic curves for the dehydration of gypsum at 156, 123, and 95°C are of sigmoidal forms. Beyond the inflexion point, most of the data fits the "diminishing sphere" kinetics formula. The rate constants and activation energies for the interface advance process shows that the soil matrix acts as a catalyst and that Na montmorillonite is a more effective catalyst than the soil matrix. A rate-limiting step for the interface advance is the necessity for crystal water to pass through an adsorbed state. Soil minerals may offer effective adsorption sites. In contrast to the growth process soil matrix is a more effective catalyst for nucleation than Na montmorillonite.

¹ Contribution from the Dep. of Soils and Water, King Faisal University, Al-Hassa, Saudi Arabia. Supported by the Saudi Arabian National Center for Science and Technology (SANCST) under contract AR-1-018.

² Associate Professor of Soil Chemistry and Teaching Assistants, respectively, King Faisal University.

Received for publication May 22, 1981. Accepted for publication January 6, 1982.