Calcium sulfate.hemihydrate CaSO

It combines with sulfur dioxide to form calcium sulfite hemihydrate, CaSOs MiH20 which can oxidize in air in the presence of moisture to give calcium sulfate dihydrate, CaS04 2H2O. However, when SO2 is passed through a solution of calcium hydroxide, calcium bisulfite, Ca(HS03)2 is obtained. The solution is yellowish when it contains bisulfite in aqueous SO2. 

Two main categories of the wet process exist, depending on whether the calcium sulfate is precipitated as the dihydrate or the hemihydrate. Operation at 70—80°C and 30% P20 in the Hquid phase results in the precipitation of CaSO 2 filterable form 80—90°C and 40% P20 provide a filterable CaSO O.5H2O. Operation outside these conditions generally results in poor filtration rates. A typical analysis of wet-process acid is given in Table 4. For more detailed discussion of the wet-process acid, see Fertilizers. 

In order for normal set to occur in Portland cement paste, mortar, or concrete, calcium sulfate must be present in the cement-water system. In today s cements, most of the calcium sulfate introduced into the system as a component of the cement, can be present in one or more forms gypsum (CaSO, 2H O), hemihydrate (CaSO. 1/2H O), soluble anhydrite (or natural anhydrite4 (CaSO ). It is very... 

It is well known that chemically, gypsum is calcium sulfate dihydrate "CaS0. 2H20" which on heating to a temperature of about 130°C loses three fourths of its water and becomes the hemihydrate CaSO,. HpO (Plaster of Paris). 

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