The Calcined Clay Way

By critically analysing these facets, it aims to provide a comprehensive guide for cement manufacturers considering the adoption of this sustainable alternative.

Suitable clay for calcination

Kaolinite-rich clays are the most suitable for producing calcined clay due to their high pozzolanic reactivity. Other clays, such as montmorillonite and illite, can also be used but typically exhibit lower reactivity. The selection of clay is crucial as it influences the calcination process and the resulting pozzolanic properties. The ideal composition of kaolinite clays includes 40 – 60% kaolinite, 20 – 40% quartz, and minor amounts of feldspar and mica. Studies have shown that clays with higher kaolinite content yield better pozzolanic reactivity, essential for enhancing cement performance.

Mineralogy and chemistry

Calcined clay is primarily comprised of dehydroxylated kaolinite, known as metakaolin, formed by heating kaolinite clay to temperatures between 600 – 800°C. The chemical composition of calcined clay includes silicon dioxide (SiO2) ranging from 45 – 55%, aluminium oxide (Al2O3) between 30 – 40%, and trace amounts of iron oxide (Fe2O3), calcium oxide (CaO), magnesium oxide (MgO), and potassium oxide (K2O).

The pozzolanic activity is attributed to the amorphous nature of metakaolin, which reacts with calcium hydroxide (Ca(OH)2) to form calcium silicate hydrate (C-S-H), enhancing the strength and durability of cement. The reactivity of metakaolin is influenced by factors such as the calcination temperature, duration, and the purity of the raw clay.

Production technologies

Several technologies are employed in the production of calcined clay. This includes the following:

Rotary kilns

Widely used for their ability to control temperature and residence time, rotary kilns ensure uniform calcination. They can process large quantities of clay and are suitable for continuous operations. They are particularly effective for kaolinite-rich clays and can achieve consistent calcination results.

Flash calciners

Suitable for fine and powdery clays, flash calciners offer rapid heating and efficient energy use. They operate at high temperatures for short durations, which minimises energy consumption and maximises throughput. They are ideal for processing clays with a particle size less than 75 μm.

Fluidised bed calciners

Fluidised bed calciners provide uniform heating and are ideal for large-scale production, but require a fine particle feed. They ensure efficient heat transfer and uniform calcination, making them suitable for producing high-reactivity metakaolin. These systems can be integrated with preheaters and waste heat recovery units to improve energy efficiency.

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