Portland Cement and Stucco

The production of commercial building materials, e.g., portland cement and stucco, utilizes calcium sulfate in the form of the dihydrate or hemihydrate.f Gypsum is added in the production of portland cement to control set. Properties of cement can be adversely affected by the formation of hemihydrate during the grinding process. In the production of stucco, the hemihydrate is the preferred form. Consequently, investigators have adapted thermal analysis methods for estimating the quantities of each phase in these products. 

The difficulty in determining the amounts of the product is associated with the separation of the two endothermic peaks which are not sufficiently resolved to allow quantification. This is generally the case when thermal analysis (e.g., DSC) is carried out in an open system (unsealed pan). The dehydration in a hermetically sealed pan improves the resolution of the DSC peaks dramatically (Fig. 26). This improved resolution results because the atmosphere of the water vapor generated by the first step retards the onset of the second dehydration and moves the peak to a 

A similar approach for the quantitative estimation of the calcium sulfate hydrates can be used with TGA analysis. 

The pinhole arrangement in the sample pan allows the initial water of dehydration to escape, but at a rate that retards the second hydration step. Using suitable reference standards, a calibration curve similar to that generated for DSC can be developed and subsequently used to determine the percent of gypsum at levels below 1 % with an accuracy of 3 to 5%. Results of this quality requires a TG with a high sensitivity and low thermal drift because the presence of, for example, 0.5 %hemihydrate in a2 mg sample, results in only a 160 pg mass loss. 

Fuj ii and co-workers have reported the important factors governing the quantitative analysis of gypsum by DTA.P These included the following recommendations  

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