Calcium sulphate phases

Two calcium sulphate phases coexist In the crystallizer, the hemlhydrate fed In and the gypsum being formed. Figure 4 shows the solubility of these two phases as a function of temperature for one particular acid liquor. The two solubility lines cross at the transition tenq[>erature (75 C in this case). Above this temperature, the hemlhydrate Is the stable phase (has the lower solubility) and below, gypsum. 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

Nitro compounds. Aliphatic nitro compounds are acidic. They are freed from alcohols or alkyl halides by standing for a day with concentrated sulphuric acid, then washed with water, dried with magnesium sulphate followed by calcium sulphate and distilled. The principal impurities are isomeric or homologous nitro compounds. In cases where the nitro compound was originally prepared by vapour phase nitration of the aliphatic hydrocarbon, fractional distillation should separate the nitro compound from the corresponding hydrocarbon. Fractional crystallisation is more effective than fractional distillation if the melting point of the compound is not too low. 

In this technique it is usual to employ glass plates coated with layers of the solid stationary phase, which adhere to the plates, generally by virtue of a binding agent, such as calcium sulphate, which is incorporated. The prepared thin layer on glass is often called a chromaplate. 

Dissolve 37.5 g (0.25 mol) of dry sodium iodide (1) in 250 ml of dry acetone in a 500-ml flask fitted with a reflux condenser protected by a calcium chloride guard-tube, and add 30.2 g (25 ml, 0.2 mol) of l-bromo-3-methylbutane. A precipitate of sodium bromide soon begins to form leave the reaction mixture at room temperature for 30 minutes, and then boil under reflux for 45 minutes to complete the reaction. Allow to cool and filter off the sodium bromide, washing the residue with a little acetone. Remove the acetone from the filtrate on a rotary evaporator, and shake the residual organic halide with 100 ml of water. Separate the lower dark-coloured layer and wash it twice more with 50 ml portions of water incorporate sufficient crystals of sodium thiosulphate into the first portion of wash-water to decolourise the organic phase. Dry the product over anhydrous calcium sulphate, filter and distil, collecting the l-iodo-3-methylbutane at 145-147 °C. The yield is 26 g (66%). 

Dissolve 5.3 g (0.05 mol) of benzaldehyde (previously shaken with sodium hydrogen carbonate solution) and 0.25 g (0.67 mmol) of tetrabutylammonium iodide in 50 ml of dichloromethane. Place this solution in a 250-ml, three-necked round-bottomed flask equipped with an efficient sealed stirrer unit, a reflux condenser and a thermometer sited in a screw-capped adapter, and supported in an oil bath mounted on an electric hot plate. Introduce 50 ml of a 50 per cent (w/v) aqueous solution of sodium hydroxide, and then 10.2 g (0.05 mol) of finely powdered trimethylsulphonium iodide. Adjust the electric hot plate so that the oil bath is maintained at a constant temperature of 55 °C for 60 hours and during this period stir the reaction mixture rapidly (1). Pour the reaction mixture on to ice, separate the organic phase and extract the aqueous solution with one 20 ml portion of dichloromethane. Wash the combined organic phases successively with four 20 ml portions of water, two 10 ml portions of a saturated solution of sodium metabisulphite and finally two 20 ml portions of water. Dry the organic phase over anhydrous calcium sulphate, remove the dichloromethane on a rotary evaporator and distil the residue. Collect the phenyloxirane as a fraction having b.p. 191— 192 °C the yield is 4.7 g (78%). 

Portland cement clinkers contain small amounts of alkalis and sulphates derived from the raw materials and fuel. Both alkalis and SO3 can be present in the major clinker phases, but tend to combine preferentially with each other to form alkali or potassium calcium sulphates, and it is necessary to consider these components together. In addition, silicate and aluminate phases containing sulphate can form either as intermediates or in undesirable deposits in eement making, and a calcium aluminate sulphate is a major constituent of some expansive and other speeial cements. 

Much of the SO3 is present at the clinkering temperature in a separate liquid phase, immiscible with the main clinker liquid. The alkali cations are distributed between the two liquids and the alite and belite. During cooling, some redistribution of alkali cations and sulphate ions between the liquids may be expected to occur, the sulphate liquid finally solidifying below 900 C to yield alkali or potassium calcium sulphates. 

Syngenite, if present, decomposes at 250-300°C. Small amounts of CH can occur due to hydration, and of CaCOj due to carbonation during i milling or storage, impurity in the gypsum, or, where specification allows it, deliberate addition. The increasing tendency to allow additions of sub-1 stances other than calcium sulphate may lead to the presence of other phases ... 

If the contents of water-soluble K2O and Na20 are known, it is necessary only to calculate AT, and TV, from them, and R, and then to carry out stages 2 and 4. From AT, TV, and C the quantities of individual sulphate phases may be estimated as described in Section 3.5.6. In the example given, the calculation indicates that the sulphate phases account for all the SO3, 0.07% of Na20, 0.28% of KjO and 0.04% of CaO. These probably occur as aphthitalite (0.6%) and calcium langbeinite (0.1%). 

A thin layer of the appropriate stationary phase is bonded to a suitable plate made of glass, aluminium foil, or plastic (polyethylene terephthalate). Adherence to the plate is usually assured by mixing a binding agent such as calcium sulphate with the stationary phase. Glass is the most popular plate material, but the others have the advantage that they... 

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