Material for portland cement must be com paratively free from magnesia since the specifi cations limit the magnesia content to 5 per cent. The clay or shale must be siliceous, preferably containing two and one-half to three times as much silica as alumina plus iron oxide. In preparing the mixture the proportions must be correct and uniform. It is found that the best results are obtained with mixtures in which the percentage of lime is 2.8 silica + 1.1 alumina.
In the manufacture of portland cement, the first essential is to mix the raw materials in the correct proportions and grind the mixture to a high degree of fineness. Failure to produce cement of good quality results more often from neglect of the latter precaution than from all other causes combined. In England, the mix ture of chalk and clay was by the wet process, in which the materials were stirred with water in wash-mills and the °slurry° allowed to settle in large reservoirs until firm enough to be cut into blocks and dried by the waste heat of the burning kilns. The dry process is generally used in the case of lime stone and cement rock, both in Europe and America, although a number of plants using limestone and clay are operating on a semi-wet process. In the dry process, the materials are crushed, dried and finally ground together in tube mills which consist of revolving cylinders partly filled with rounded pebbles. Marl and clay are usually treated by a wet process, in which the materials are mixed as taken from the beds and ground in tube mills with sufficient water to make a thick slurry.
The burning of portland cement was for merly done in simple, intermittent vertical kilns similar to lime kilns or in ring kilns consisting of a number of chambers connected to a central stack into which the fuel was introduced through openings in the roof. Later, continu ous kilns were adopted in which the bricks of dry slurry and the fuel were fed in at the top and the clinker periodically withdrawn at the bottom. These three classes of kilns required the materials to be molded into bricks and dried, an operation involving much labor.
In 1885 Ransome patented in England a process of burning cement material in the form of powder in revolving kilns heated by a flame of gas. The invention was a failure in Europe but was taken• up and developed by American engineers and found to be especially adapted to conditions in this country. Petroleum was
first used as fuel and later this gave place to powdered coal blown in by a blast of air. The rotary process was found to yield cement superior in several respects to the product of the older types of kiln and to be adapted to the burning of cement mixture in the form of dry powder or wet slurry without previous molding into bricks. With the advantage of the rotary !din, the American portland cement in dustry began, about 1890, a career of wonderful expansion and development which is still in active progress. At the present time there are more than 600 kilns of this type in operation in the United States and many have been built after American designs in the leading factories of Europe.
There are three distinct and separate opera tions from the preparation of the raw material to the finished product. The first is mechanical and includes the assembling, preparing, grinding and amalgamating of the raw materials; the second is chemical, during which the material prepared by the first process is calcined or roasted at a high temperature, bringing about chemical composition of the various ingredients. The third and final process is partly mechanical and partly chemical, in which the clinker result ing from the calcination together with a small percentage of retarder is reduced to a fine powder.
In the dry process the raw materials are quarried, crushed, dried, pulverized, and mixed in the proper proportions. The powdering or pulverizing of the raw materials is one of the most important steps in cement manufacture and decided advances have been made in the last few years in grinding and pulverizing machines. Grinding machines nearly all work on the principle of striking or pounding the material between a hammer in some form and a solid metal mass.
After the raw material has been prepared for mixing this operation is accomplished by means of automatic weighing machines which weigh out just the right quantity of each of the two materials. These machines are always un der the direct control of the chemist in charge of the operation day and night. There is then a perfect grinding of the raw materials to fine powder called the °raw mix.* In the wet process used by some plants, a slurry is made containing the proper amount of each raw material which is then put in storage tanks and from there pumped into the rotary kilns.