The modern rotary kiln consists of a steel cylinder lined with fire brick, generally 60 to 220 feet in length and from 6 to 12 feet in diam-, eter, set in a slightly inclined position upon rollers so that the raw material when fed into the upper end gradually moves toward the lower or discharge end as the kiln slowly rotates about its own axis. The output of the rotary kiln is from 150 to 2,000 barrels each per day. In these kilns the raw mix is burned or °calcined to incipient fusion* by powdered coal blown in at the lower end through a hood with 50 to 100 per cent excess air over the amount theoretically required for combustion of the coal.
The cement mixture in powder or as wet slurry is continually fed in at the upper end and as it travels downward becomes dried (if wet) and is gradually brought to the white heat of the hottest zone, issuing finally in the form of black, rounded nodules of clinker vary ing in size from birdshot to perhaps two inches. The clinker is cooled and then ground and pulverized to fine powder usually in mills of the pendulum type or in tube mills. About 2% per cent of gypsum is ground with the clinker to regulate the time of set of the cement. The fineness of grinding is such that at least 78 per cent of the product will pass a sieve of 200 meshes to the linear inch.. After the cement has been ground it is usually de posited in bulk bins holding several thousand barrels each. Samples are then taken and com plete tests made for fineness, setting, sound ness, strength, chemical analysis, etc.
Portland cement is a gray powder weighing from 80 to 110 pounds per cubic foot, depend ing upon its fineness and how much it has been compacted; in actual practice a sack of cement is ordinarily considered as containing one cubic foot and weighing 94 pounds net. Mixed with water to a paste it sets or hardens slowly, resisting pressure of the finger nail after several hours. After setting it gains rapidly in hardness during the first 30 days, the increase continuing, though more slowly, through a long period of years.
In testing portland cement, the most im portant qualities to be observed are fineness, time of setting, tensile and compressive strength and soundness. Fineness is deter
mined by sifting through standard sieves of the correct number of meshes and gauge of wire. The time of setting is determined by preparing a test piece of suitable form from neat cement and noting the time at which needles of cer tain definite size and having certain definite weights cease to make appreciable indentations on the test piece. A needle exerting a different unit pressure on the test piece is used for de termining the initial set from that used for determining the final test. For tensile strength briquettes one square inch in smallest section are made by filling standard molds with a plastic paste of cement and sand. These are kept 24 hours in moist air, then immersed in water and broken in a testing machine at the age of 7 and 28 days and if desired at longer periods. Soundness or constancy of volume is tested by making pats of cement with thin edges on plates of glass, keeping them in moist air 24 hours and then exposing in a loosely covered vessel to an atmosphere of steam above boiling water for five hours. These pats should remain firm and hard and show no signs of distortion, cracking, checking or disintegration.
The use of concrete is continually extending and being applied to many new enterprises in the structural and commercial fields. The construction of concrete roads and streets is perhaps the most prominent develop ment of the past decade and has reached such proportions that there are at the present time practically 150,000,000 square yards of such pavement in the United States, an amount sufficient to make nearly 15,000 miles of 18-foot highway. The recent war brought the prac tical development of the ocean-going concrete ship of large tonnage and the concrete barge for harbor and inland waterway transportation, also the underground concrete tank of large capacity for the storage of crude oil, fuel oil, etc. New uses for the material are discovered almost daily so that the present century may well be called the Cement Age.