Air-cooled blast furnace slag can be added to the inlet of the cement kiln. The effect of dust recirculating from the cooler in the secondary air is potentially disastrous. I have seen occasions where the dust keeps increasing until the kiln is full of dust and the firing has to stopped while the kiln is emptied of dust. The dust affects the combustion of the fuel in the flame and the problems tends to intensify in a vicious circle. Potential causes are poor combustion in the flame or elevated sulphate content in the clinker. (See the April 2003 issue of International Cement Review for an article about dust formation with pet coke firing in cement kilns).