Gypsum content determination in Portland cements by thermogravimetry

摘要

Gypsum quantitative determination in cements may be done from their thermogravimetric analysis (TG) data, which uses the mass of the water released from its gypsum dehydration during cement analysis, to determinate its content stoichiometrically. Depending on the operational conditions used to mill the gypsum with clinker, its partial dehydration may occur, which leads to unreal results when only unhydrated cement TG data are used. To solve this problem, TG analysis of the 1-h paste of respective cement must be done on respective initial cement mass basis. The method was applied to four different classes of Portland cements, showing that for all of them, the application of the method was needed to avoid erroneous determinations. As mineral composition data show that the cements have other sulfated phases, different from gypsum and its dehydration phases, the total cement SO3 content cannot be used directly to quantify gypsum. However, using the present suggested thermogravimetric method, the original gypsum content can be determined, which also estimates and includes the amount which was consumed to form ettringite during the first hour of hydration.