Fossil meal/slaked lime. Fossil meal is an earth rich on silicon dioxide SiO2 that consists of shells of petrified silica algae. Pure fossil meal reacts with slaked lime in its natural state even in weak frost, whilst fossil meal mixed with clay needs to be fired to a temperature of 600 °C
to mix with slaked lime. Higher temperatures reduce the reactivity of the lime. Very few experiments have been undertaken with this cement.
Calcined clay/calcined lime. Most clays react with lime after they are calcined. To be used as pozzolana, clay must be calcined to sintering level, which is usually around 550-650 °C. Firing time is about half an hour, but the reactivity and viability of different types of clay varies. All ceramic clays are suitable for pozzolana, especially clays with a high kaolin content (Velosa etal., 2007).
Clay and lime cements are used today in parts of Asia. In India this cement is called 'Surkhi', and consists of lime ground with pulverized brick. It is weaker than Portland cement, but has better waterproof properties and has been used widely in dam building.
Blast furnace slag/calcined lime. The starting point for a reactive blast furnace slag is granulation. The glowing slag is tipped into a vessel filled with cold water. It is then ground into powder and mixed with calcined lime. An alternative is a mixture with dolomite calcined at 800900 °C, which also works well. The strength of slag and lime cements is good, but the mixture cannot be stored for long periods and must therefore be used shortly after production.
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