Cement Mill - Temperature Control

Temperature Control

Heat generated in the grinding process causes gypsum (CaSO₄.2H₂O) to lose water, forming bassanite (CaSO₄.0.2-0.7H₂O) or γ-anhydrite (CaSO₄.~0.05H₂O). The latter minerals are rapidly soluble, and about 2% of these in cement is needed to control tricalcium aluminate hydration. If more than this amount forms, crystallization of gypsum on their re-hydration causes "false set" - a sudden thickening of the cement mix a few minutes after mixing, which thins out on re-mixing. High milling temperature causes this. On the other hand, if milling temperature is too low, insufficient rapidly soluble sulfate is available and this causes "flash set" - an irreversible stiffening of the mix. Obtaining the optimum amount of rapidly soluble sulfate requires milling with a mill exit temperature within a few degrees of 115 °C. Where the milling system is too hot, some manufacturers use 2.5% gypsum and the remaining calcium sulfate as natural α-anhydrite (CaSO₄). Complete dehydration of this mixture yields the optimum 2% γ-anhydrite. In the case of some efficient modern mills, insufficient heat is generated. This is corrected by recirculating part of the hot exhaust air to the mill inlet.