Gypsum is conveyed to furnace – like lime – after excavation, crushing, and grinding, where it is heated up to 180°C, so that it loses some of the water molecules and can be used in for of hemihydrate gypsum as a construction or casting material. Gypsum is naturally white; yet it might appear in gray, bluish gray, pink, or yellow colors due to the presence of impurities such as organic materials, clay, and iron oxide. Gyprock has a monoclinic crystal structure with specific weight of 2.32 and hardness of 1.5 to 2. Gypsum is mixed with coarse and fine crystalized gyprocks and solved in hydrochloric acid and hot water.
Pure gyprock contains 20.9% water, 46.6% SO3, and 32.5% CaO. Purity of gyprock is determined by its mineral gypsum content. According to the American Society for Testing and Materials (ASTM) minimum purity of gyprock required for industrial application is 70%, whereas purity of the majority of commercial gypsums is 75-90%. The major gyprock impurities are shale, clays, and calcium carbonate. Remains of biological interactions and activities are also observed occasionally.
Initial and final setting times of gypsum plasters vary depending on application and consumption culture. Ordinary and micronized construction gypsums available as 40-200 mesh gypsum powder have a short setting time. For example, the initial setting time of 40 mesh construction gypsum produced in accordance with the national Iranian standards is 5-7 minutes, whereas the setting time of 200 mesh construction gypsums is usually 6-20 minutes.
Calculating Gypsum Setting Time Using Trowel Method
This method is used to determine the setting time of gypsums without additives. The mix contains 100 gr of water and enough gypsum to saturate the mix. The mix is steered well by a spoon and poured on a glass plate, so that diameter of the mix spread on the glass is 10-12 cm with a thickness of 5 mm. The time is measured from beginning of adding gypsum to the water and then the spread is cut by a blade. Thickness of the blade should not exceed 1.5mm. If the two tips of the cut adhere again, the initial setting had not taken place. The test is repeated until the two tips no more adhere, which shows that the gypsum is set.
