Hydration of calcium chloride

The more highly complexed hydrates of calcium chloride (CaCl2 nH2 O where n > 2) may also exhibit the characteristics of a Class 2 dryiag agent, because the hydrated species can physically absorb additional water to form a saturated solution. The term absorption is used to describe the phenomenon that occurs when a gas or vapor penetrates the soHd stmcture to produce a saturated solution ... 

Self-Test 8.8A Use the data for the hydration of calcium chloride in Table 8.7 to derive the enthalpy of hydration of Ca2+. 

By definition, the desired quantity is the heat of hydration of calcium chloride hexahydrate. You cannot carry out the hydration reaction directly, so you resort to an indirect method. You first dissolve 1.00 mol of anhydrous CaCl2 in 10.0 mol of water in a calorimeter and determine that 64.85 kJ of heat must be transferred away from the calorimeter to keep the solution temperature at 25°C. You next dissolve 1.00 mol of the hexahydrate salt in 4.00 mol of water and find that 32.1 kJ of heat must be transferred to the calorimeter to keep the temperature at 25 C. 

Roozboom s researches on calcium chloride.—Another remarkable example is given by the hydrates of calcium chloride, objects of an important memoir by Roozboom. ... 

On either side of a quadruple point the solution is supersaturated with respect to one of the solid phases, which would therefore change in contact with the solution into the hydrate of lower solubility. The quadruple point is therefore also called the transition point of the two hydrates. In Fig. 22 the solubility curves of the various hydrates of calcium chloride (a, 6, and c) are plotted for temperatures up to 260°. The formulae of the solid phases are written opposite the corresponding curves. 

The solubility of calcium chloride in water is listed in Table 2.22 and plotted in Fig. 2.63. Figure 2.64 shows these same data plotted on a larger scale, and Fig. 2.65 indicates the fields of the various crystalline phases in more detail. There are at least four hydrates of calcium chloride, with 1,2,4 and 6H2O (and perhaps... 

Commercial appHcations of calcium chloride and its hydrates exploit one or more of its properties with regard to aqueous solubiUty, hygroscopic nature, the heat gained or lost when one hydrated phase changes to another, and the depressed freezing point of the eutectic solution at a composition of about 30% by weight calcium chloride. 

The properties of calcium chloride and its hydrates are summarized in Table 1. Accurate data are now available for the heats of fusion of the hexahydrate, the incongment fusion of the tetrahydrate, and the molar heat capacities of the hexahydrate, tetrahydrate, and dihydrate (1). These data are important when considering the calcium chloride hydrates as thermal storage media. A reevaluation and extension of the phase relationships of the calcium chloride hydrates, has led to new values for the heats of infinite dilution for the dihydrate, monohydrate, 0.33-hydrate, and pure calcium chloride (1). 

A study on the solubiUty of calcium chloride hydrates (3) has generated polymonials relating the weight percent of anhydrous salt in a saturated solution to temperature (°C). For 9.33 < f " C < 28.16... 

Calcium chloride is a hygroscopic salt which can exist as pure anhydrous CaClj or as a series of hydrate forms ranging from a monohydrate (CaCl2 -HjO) to a hexahydrate (CaCl j - 6 H j O). In the drying process, hydrocarbon passes up through a bed of calcium chloride and the water from the hydrocarbon forms hydrates with the calcium chloride. At the bottom of the bed the hydrate evenmally reaches a hexahydrate which turns into a brine and is drawn off. 

The water phase of oil-base mud can be freshwater, or various solutions of calcium chloride (CaCl ) or sodium chloride (NaCl). The concentration and composition of the water phase in oil-base mud determines its ability to solve the hydratable shale problem. Oil-base muds containing freshwater are very effective in most water-sensitive shales. The external phase of oil-base mud is oil and does not allow the water to contact the formation the shales are thereby prevented from becoming water wet and dispersing into the mud or caving into the hole. 

Fig. 5.9 The degree of hydration of cement pastes in the presence of calcium chloride in comparison to a plain paste measured by X-ray analysis (Young). 

Fig. 5.10 Degree of hydration of a tricalcium silicate paste in the presence of calcium chloride, measured by non-evaporable water content (Odier).

The differences in chemical composition are accompanied by differences in the morphology of the tobermorite gel. Spicular or cigar-shaped rolled sheets are formed in the normal plain hydrated cement paste, whilst in the presence of calcium chloride, thin crumpled sheets or foils are formed. It has been suggested [16] that either the high lime content or adsorbed chloride prevents rolling of the sheets. 

The peaks produced by differential thermal analysis (DTA) of hydrate C3S samples indicate a shift in the Ca(OH) 2 peaks in the presence of calcium chloride [19], It has been suggested that the Ca(OH)2 may be present in a differently bonded form when calcium chloride is present. 

The banning of calcium chloride during the last decade provided the impetus for the development of alternative materials which accelerated the hydration of cement without the potential for corrosion. A number of inorganic and organic compounds including aluminates, sulfates, formates, thiosulfates, nitrates, silicates, alkali hydroxides, carbonates, nitrites and calcium salts have been evaluated. Commercialization and field experience, however, is limited to only a few of these materials. 

To prepare the 8-hydrate, llg of calcium chloride 6-hydrate are dissolved in 5ml of water and treated with 50ml of fresh 3% hydrogen peroxide and then with 7ml of concentrated ammonia solution in 100ml of water. If this procedure is carried out at 55°C, or if only 30ml of water are used for the ammonia at 20°C, the four-tenths hydrate is obtained. Palmer may be consulted for more details. 

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