Relationship between granulometric distribution and compressive strength of heat-treated Toarcian-Bajocian gypsum, Tunisia: implications for casting teeth in molds

Toarcian-Bajocian gypsum deposits of the Mestaoua formation (southern part of Tunisia) correspond to very thick gypsum layers. The natural gypsum and their heat-treated products were investigated to assess their utilization in dental field (cast plaster into molds for dental prosthesis). XRD data show that the Mestaoua sample (GMes) constituted of gypsum. However, the commercial product (PXETD) is made up of bassanite. Powder X-ray diffraction combined with chemical and infrared analyses reveals that heat-treated GMes samples at different temperatures (200 °C, 220 °C, 240 °C, 260 °C) are dominantly bassanite with minor amounts of anhydrite, particularly, in the samples heated at 240 °C and 260 °C. The data obtained revealed that the sample heated at 220 °C and 240 °C are distinguished by satisfactory values of compressive strength, which are close to the value observed for a commercial product, and are suitable for use in dentistry. The slight decrease in the compressive strength is commonly due to the less abundant water molecules and the two divergent modes of particle distribution, which enhance pore space. Bulk properties of the sample heated at 240 °C are generally appropriate for use as a material to prepare molds in dental fields, as confirmed by the casting of plaster into mold teeth, where teeth revealed clearly steaks and showed even small details.