Physico-chemical Aspects of Hydration

At temperatures below 350 °C, the ealcium oxide component of high-caleium quicklimes generally reacts readily with water, liberating heat (276kcal/kg CaO — equation 20.1) [20.1]. At higher temperatures, the reverse reaction occurs (see section 19.2). 

The MgO component of high-calcium quicklimes and of dolomitic limes is relatively unreactive to water. Generally less than 25 % of it reacts under normal hydration conditions (equation 20.2) [20.2]. 

Highly hydrated dolomitic lime is generally produced by reacting the quicklime with water under hydrothermal conditions (i.e. under steam pressure at temperatures in excess of 100 °C in an autoclave) for the requisite time. The heat of hydration is 211 kcal/kg CaO. 

High-calcium and partially hydrated dolomitic limes are hydrated at temperatures approaching 100 °C. This is achieved by adding a sufficient excess of water to moderate the temperature by boiling. In practice, with high-calcium quicklime, approximately double the stoichiometric quantity of water is added, with most of the excess being vented to atmosphere as steam, after absorbing much of the heat of reaction. 

The addition of water to the lime should be done under well agitated conditions. This helps to avoid localised overheating. 

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