Enthalpy changes in clinker formation

The enthalpy change on formation of Portland cement clinker cannot be calculated with high precision, mainly because of uncertainties associated with the clay minerals in the raw material. Table 3.1 gives data for the main thermochemical components of the reaction, almost all of which have been calculated from a self-consistent set of standard enthalpies of formation, and which are therefore likely to be more reliable than other values in the literature. The conversion of the clay minerals into oxides is an imaginary reaction, but valid as a component in a Hess s law calculation. Few reliable thermochemical data exist for clay minerals those for pyrophyllite and kaolinite can probably be used with sufficient accuracy, on a weight basis. 

AS4H (pyrophyllite) - a-AljOj + 4Si02 (quartz) + HjO(g) + 224 AS4H 

AS2H2 (kaolinite) - a-Al20j + 2Si02 (quartz) + 2Fl20(g) + 21 

The overall enthalpy change in forming clinker is dominated by the strongly endothermic decomposition of calcite. The component reactions for the replacement of clay minerals by oxides are endothermic, because the heat required for dehydroxylation exceeds that liberated on forming the products. 

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