Thermal Behavior of Calcium Sulfate Dihydrate by STA

Calcium sulfate dihydrate (CaSO₄∙2H₂O) or gypsum is a natural mineral used in various industrial application. Gypsum is utilized as a retarding agent in cement, construction material for walls and ceilings, surgical splints in medical applications, casting molds and modeling etc. Therefore it is important to understand the thermal behavior of gypsum. Here, we confirm the behavior of calcium sulfate dihydrate using STA (TG-DTA).

Measurement and results  

A 10 mg sample amount weighed in a Pt crucible was heated at 20℃/min from RT~1450℃ in air atmosphere flowing at 300ml/min.  

Measurement results of CaSO₄∙2H₂O revealed an overlapping two-stage mass loss of 21% with an endothermic reaction due to dehydration of crystal water (CaSO₄∙2H₂O→CaSO₄∙0.5H₂O +1.5 H₂O↑→CaSO₄ + 0.5H₂O↑). Then at 362℃, an exothermic peak can be observed due to phase transition from α to β form. As heating continues, an endothermic peak at 1220℃ is detected due to phase transition from β to α form. On the TG curve, a mass loss is gradually observed from 1250℃ due to partial decomposition of CaSO₄ (CaSO₄→CaO + SO₃↑). From 1280℃, a eutectic mixture of CaO and unreacted CaSO₄ as reaction product was formed, followed by further decomposition as indicated by the mass loss on the TG curve. At the end of the measurement, CaO was remained as a residue. (CaSO₄∙CaO) → CaO + SO₃↑).

Figure 1: Measurement result of CaSO₄∙2H₂O by STA (TG-DTA)