c acid of the
carbonate of lime, and the combined water of the clay are driven off,
and the resulting lime begins to act chemically on the dehydrated
clay. The material is then in a partially burnt and slightly sintered
state, but it is not fully clinkered and would not make Portland
cement. The material continues to descend by the rotation of the kiln
and reaches the lower end nearest the burner where the temperature is
highest, and is there heated so highly that the union of the lime,
silica and alumina is complete, and fully burnt clinker falls out of
the kiln. It is extremely hot, and is cooled usually by being passed
down one or more rotating cylinders, similar to the first, but
smaller, and acting as coolers instead of kilns. On its way down the
cylinders the clinker meets a current of cold air and is cooled, the
air being correspondingly warmed and passing on to aid in the
combustion of the fuel used in heating the kiln. This regenerative
heating is similar in principle and effect to that obtained by means
of the shaft and ring kilns described above. The output of these kilns
varies from 200 to 400 tons per kiln per week according to their size
and the nature of the raw materials burned, as against 30 tons per
week for an ordinary chamber kiln. A large saving in labour is also
secured. The rotatory system presents many advantages and is rapidly
replacing the older methods of cement making. Fig. 3 represents
diagrammatically a rotatory cement plant on the Hurry & Seaman system,
which was one of the first to make cement by the rotatory process
successfully on a large scale, using powdered coal as fuel. Rotatory
kilns of various other makes are now in use, but the same principles
are embodied, namely, the employment of a rotating inclined cylinder
for burning the raw materials, a burner fed with powdered coal and a
blast of air, and some device such as a cooling cylinder or cooling
tower by which the clinker may be cooled and the air correspondingly
heated on its way to the burner.

[Illustration: FIG. 3.]

Another method of making Portland cement which has been proposed and
tried with some success consists in fusing the raw materials together
in an apparatus of the type of a blast furnace. The high temperature
necessary to fuse cement clinker makes this process difficult to
accomplish commercially, but it has many inherent merits and may be