Heats ofhydration

This method sounds simple at first. However, there are certain difficulties. One has to decide on a value of n in the plot of /r and this may not always be unity or simple. Various terms that affect the calculation of the heat ofhydration of ions depend on r, r, andr". Against which one should one plot ... 

Once one has thus obtained the experimental absolute heat of hydration of the proton, one can use it in combination with experimental values of Hhcontaining electrolytes (e.g., HCl) to obtain absolute values of the heat ofhydration of other ions, (Table 2.13). Such an approach has abetter basis diaii an older method in which the salt considered had equal radii and the absolute heal of solvation of each was equal to half that of the whole. However, this assumption of the equality of the heats of hydration of ions with the same radius is just not true (Fig. 2.32). 

Observation of the hydration process indicated the following. In the first few minutes of the dissolution of free lime, the hydration of anhydrite and hemihydrate and the formation of calcium aluminate hydrate and monosulfate hydrate occur. Ettringite is formed within one hour, monosulfate hydrate within 2-6 hours and C-S-H gel within 1-16 hours, with the maximum heat ofhydration of C3S at about 10 hours. Measurements of the non-evaporable water indicate that the amount of combined water is 60-80% of the theoretically determined total amount of combined water at complete hydration. (The total amount of combined water was estimated to be 36%.) The amount of ettringite in the paste is estimated to be about 18-25% for the period one hourto seven days. The monosulfate content increases from about 10% at six hours to about 15-25%inone day. The alite in Reg Set cement paste is approximately 65-70% hydrated in one day and 8 0-95% hydrated in seven days. It is suggested that fluoride is tied up as A1(0H)2F. The possibility of fluoride-substituted ettringite and the formation of halo-aluminate hydrates of the form ( A CaXj wHjO is conceivable. 

Finally, this section ends with a reminder that heats, entropies, and free energies ofhydration depend on concentration (Fig. 2.14) and that there are significant changes in values at very low concentrations. It is the latter values that are the desired quantities because at high concentrations the heats and free energies are influenced not only by ion-solvent interactions (which is the objective of the venture) but also by interionic forces, which are much in evidence (Chapter 3) at finite concentrations. 

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