Endothermal peaks gypsum

Gill (50) determined the gypsum content in a plaster of Paris-gypsum mixture from the AH of the 142°C endothermic peak. The relationship between the AH (mcal/mg) and weight percent of gypsum was a straight line. 

In normal portland cement, hydration leads to the formation of ettringite. In many instances, the formation of ettringite leads to expansion. To prevent the formation of ettringite, a gypsum-based material with 75% hemihydrate, 20% portland cement, 5% silica fume, and a superplasticizer was fabricated. The pastes were cured in water for 1 to 10 minutes and subjected to DTA. In Fig. 10, thermograms show the stepwise dehydration endothermal peaks (shown upwards) at 150° and 200°C as is typical of gypsum. There was no indication of ettringite or monosulfate, which normally are identified by peaks at 125-130° and 190-195°C. 

Reactions (1) and (2) are dehydration processes. The exothermic reaction (3) represents a lattice modification from the hexagonal to orthorhombic form. Decreases in peak temperatures (150 to 123°C and 197°C to an undetectable position) occurred when the gas pressure in the cell was reduced to 1 torr (Fig. 2). The peak for the exothermic effect was not sensitive to changes in gas pressure. The DTA curve for j8-CaS04 H2O has a sharp endothermic peak at 195°C and a small exothermic peak at 375°C (Fig. 3). The position and shape of the small exothermic peak were not significantly affected by changes in atmospheric pressures within the DTA cell. The endothermic peak has the same pressure-temperature behavior previously noted for the second endothermic curve of gypsum (a shift occurs from 195°C to 132°C for 760 torr and 1 torr respectively). 

Many accessory minerals present in a elay influence its properties in the plastic and hardened states. Some of the organic and inorganic materials that may be present in elays inelude elarin, vitrain, marcasite, pyrite, quartz, gypsum, limestone, and dolomite. Their presence is indicated by endothermal peaks caused by deearbonation or exothermic peaks caused by oxidation. DTA curves of some of the accessory materials that may be present in a clay are shown in Fig. lO.P ... 

Isa and Oruno describe a method that enables identification of intermediates (in the gypsum dehydration process) more easily. The method involves the use of simultaneous TG-DTG-DTA under various sealed atmospheres corresponding to three systems—open completely, sealed, and quasi-sealed. Endothermic DTA peaks appear earlier (129 and 133°C) than the point of decreasing TG. This technique, resembling the quasi-isothermal and isobaric thermogravimetry (Q-TG), is superior to the latter in that it needs less of the sample. 

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