Effect of mechanical activation on the thermal behavior of gypsum (CaSO4.2H2O)

Gypsum (CaSO4.2H2O) is a naturally occurring calcium sulphate mineral, which contains two moles of water in crystal structure. By heating to leave a half mole of water in gypsum structure, plaster (CaSO4.1/2H2O) is manufactured. When powdered plaster is mixed with water, it solidifies again, and transforms to a building material with binding feature. If the heating temperature of gypsum increases, anhydrite (CaSO4) is obtained.In this thesis, with the help of mechanical activation, decrease in the calcination temperatures of gypsum for conversion to plaster and anhydrite was the aim. For this purpose, gypsum was exposed to intensive milling using planetary ball mill with tungsten carbide milling media. Unmilled gypsum sample and samples obtained by milling for various periods was characterised intensively by means of BETsurface area analysis, SEM (scanning electron microscopy), XRD (X-ray diffractometry), PSD (particlesize distribution), IR (infraredspectroscopy), TG (thermogravimetry) and DSC (differential scanning calorimetry).The powder characterization studies revealed the following results: particle size of samples, determined by PSD and SEM analysis, decreased at first up to 15 min of milling and then increased due to aggregation of fine particles. BET Surface area of the milled samples increased in parallel with milling time. Several XRD crystal structural disorders in the mineral gypsum, which may cause to mechanical activation was encountered. The TGA results show that temperature regions where gypsum converts to plaster and where plaster converts to anhydrite shift to low temperature regions. According to DSC analysis, as a result of milling for 15 and 18 min, it was determined that there was a significant decrease in the total specific energy required fort he complete conversion of gypsum to anhydrite.It is concluded that mechanical activation of gypsum is achievable but very challenging, given its difficulty of miling for prolonged periods mainly due to low Mohs hardness value and its structural water content.