Since 1890 great progress has been made in the United States in the application of theoretical and scientific principles to the technology of Portland cements, and the result has been an enormous expansion of the business with an improved quality of the product.
The original method pursued in England, and largely adopted elsewhere, was to grind the materials very wet, floating off the fine particles to a large tank where they were allowed to settle. The settling and drying required a great deal of time. This method was followed by a dry process in which the materials were ground together dry and then moistened sufficiently to be molded into briquettes. The briquettes were then dried and stacked in a kiln and burned. The introduction of rotary kilns rendered the molding and drying unnecessary as either dry or wet materials, as a dry powder or wet mud are fed directly to the kilns. The composition of the materials, whether it
be cement rock or mixed materials, must be maintained by constant chemical analysis, as the percentage of carbonate of lime should not vary by more than 1⁄2 per cent, from that found correct for the other materials used.
From the foregoing statements it will be observed that two quite different methods of manufacture are followed. In the first the cement 'mix' is molded into briquettes, which are dried, stacked in a kiln and burned. For the burning by this method three different forms of kiln
