When gypsum is totally dehydrated, what is the percentage weight loss?
Cement consists primarily of calcium silicates, calcium aluminates, and calcium aluminoferrites. Except for these main constituents, other additives are used in small quantities.
One important additive is gypsum in the form of the dihydrate (CaSO4⋅2H20) or the hemihydrate (CaSO4⋅½H20). The gypsum is ground with the clinker and serves to retard the setting and hardening of the cement. Without gypsum, hydrated calcium aluminate crystallizes within about 10 min; gypsum retards this process by several hours or even days depending on the gypsum content. In the quality control of cement, it is therefore important to know the exact content of the CaSO4 dihydrate or hemihydrate.
Thermogravimetry is a good analytical method for determining the gypsum content. The dihydrate and the hemihydrate lose their water of crystallization on heating. In the case of the dihydrate, the elimination takes place in two steps (via the hemihydrate), and with the hemihydrate in just one step. The experimental difficulty is that both reactions occur close together in the same temperature range and, depending on the conditions, overlap. This makes quantification of the two types of gypsum difficult. To achieve a separation of the two types of gypsum, a sample must be heated in an almost completely sealed crucible. This shifts the dehydration of the hemihydrate to higher temperatures thereby improving the separation of the two processes. The gypsum content in cement is usually in the percent range. From stoichiometric considerations, dehydration of the dihydrate is expected to give a total mass loss of 20.9%, made up of steps of 15.7% (dihydrate → hemihydrate) and 5.2% (hemihydrate → anhydrous form, based on the mass of the dihydrate). The dehydration of the hemihydrate is expected to give a mass loss of 6.2%.
Cement consists primarily of calcium silicates, calcium aluminates, and calcium aluminoferrites. Except for these main constituents, other additives are used in small quantities.
One important additive is gypsum in the form of the dihydrate (CaSO4⋅2H20) or the hemihydrate (CaSO4⋅½H20). The gypsum is ground with the clinker and serves to retard the setting and hardening of the cement. Without gypsum, hydrated calcium aluminate crystallizes within about 10 min; gypsum retards this process by several hours or even days depending on the gypsum content. In the quality control of cement, it is therefore important to know the exact content of the CaSO4 dihydrate or hemihydrate.
Thermogravimetry is a good analytical method for determining the gypsum content. The dihydrate and the hemihydrate lose their water of crystallization on heating. In the case of the dihydrate, the elimination takes place in two steps (via the hemihydrate), and with the hemihydrate in just one step. The experimental difficulty is that both reactions occur close together in the same temperature range and, depending on the conditions, overlap. This makes quantification of the two types of gypsum difficult. To achieve a separation of the two types of gypsum, a sample must be heated in an almost completely sealed crucible. This shifts the dehydration of the hemihydrate to higher temperatures thereby improving the separation of the two processes. The gypsum content in cement is usually in the percent range. From stoichiometric considerations, dehydration of the dihydrate is expected to give a total mass loss of 20.9%, made up of steps of 15.7% (dihydrate → hemihydrate) and 5.2% (hemihydrate → anhydrous form, based on the mass of the dihydrate). The dehydration of the hemihydrate is expected to give a mass loss of 6.2%.
