Quality Control of Commercial Plasters

Storage conditions of commercial plasters often result in partial hydration of the hemihydrate material. Dynamic or isothermal thermogravimetry can be used to estimate the primary phases and impurities present in commercial plaster. In dynamic TG experiments at low heating rates, the hemihydrate and dihydrate contents can be estimated from separate mass change steps of the respective dehydration reactions shown in the resulting TG curves. 

A typical TG curve of a commercial plaster (Fig. 29) shows two steps for mass loss. The first step is caused by the dehydration of calcium sulfate dihydrate. The second step is due to the dehydration of calcium sulfate hemihydrate. The second step is attributed to the hemihydrate present in the original sample and also to that formed during the first step of decomposition. 

A typical calculation utilizing a TG trace (dynamic method) can be described as follows. The mass losses in steps 1 and 2 are designated MLl and ML2, respectively. Further, H is the mass of the hemihydrate in the sample, D is the mass of dihydrate in the sample, and / is the mass of the impurities in the sample. The values ofH, D, and/can be calculated by the following equations  

The isothermal TG method is described as follows The dihydrate and hemihydrate are decomposed simultaneously during isothermal tests when the temperature of the experiments is maintained at 250°C or higher. The isothermal TG curves show the cumulative mass loss of these reactions (Fig. 30). Two different analyses are required, one for the original sample 

It is apparent that calculation of D, H, and I depends on the complementary data obtained from analysis of the modified sample. The modified sample is mixed with water (70% by mass). The fully hydrated sample is then dried at 45°C prior to being subjected to isothermal analysis. The apparent impurity content, / can be calculated using the total mass loss, MT, from the following equation  

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Thermogravimetry can be used for quality control of commercial plasters either by d5mamic or isothermal methods. The partial hydration of the plasters, which have not been well stored, can be subjected to identification and estimation. 

Dweck, J., Lasota, E. I. P., Quality Control of Commercial Plasters by Thermogravimetry, Thermochimica Acta, 318 137-142 (1998)... 

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