Abstract
This report was drafted in the course of work undertaken by RILEM technical committee 23-GP and constitutes quite an exhaustive review of methods used for analysis (determination of chemical and mineralogical composition) of natural, synthetic or residual calcium sulphates and commercial plasters before and after hardening. The different points examined include: - the preparations of samples: the sampling itself, method of drying and determination of saturation water (or free water) procedure for forming a solution in order to determine chemical composition; - the determination of basic constituents: SO3, CaO, crystallization water; - the determination of impurities: alkalines, alkaline clay, iron, aluminium, chlorides, CO2 and carbonates, silica and clays, phosphorus, fluorine, boron, organic matter. Apart from conventional physical methods used in analytical chemistry (nephelometry, colorimetry, potentiometry, atomic absorption, flame photometry), information is given concerning results obtained with certain particular techniques (emission spectrography, fluorescence and spectrometry, X-ray emission and microprobe) but these techniques call for rather costly equipment; - the determination of mineralogical composition (dihydrate, hemihydrate, anhydrate III or II contents) which can be carried out by computation using elementary chemical analysis data or by using physico-chemical techniques (gravimetry and thermogravimetry, differential thermal analysis, calorimetry, thermodilatometry) to bring about phase changes (dehydration, rehydration) of the different constituents present in the sample. The minnralogical composition is then determined by computation using the results obtained or from prior calibration on samples of known mineralogical composition. Other physico-chemical methods (spectrography of infrared absorption, X-ray diffraction, optical microscopy) are used to determine contents at different phases and to identify them (gypsum, hemihydrate, anhydrite, impurities such as silica, carbonates, clay minerals). The characterization of hemihydrates α and β (materials which differ through their state of crystallization) can be carried out by X-ray diffraction, DTA, thermodilatometry or optical microscopy).
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