Hazel Oliver, age 10, of Rome., N.Y., for her question:
HOW IS CEMENT MADE?
Cement is a fine, gray powder. When it is mixed with water and sand, it becomes concrete.
Almost all cement used today is portland cement, which is a hydraulic cement. This means that it hardens under water. This cement was named portland because it has the same color as the natural stone quarried on the Isle of Portland, a peninsula on the south coast of Great Britain.
Portland cement contains about 60 percent lime, 25 percent silica and about 10 percent alumina. Iron oxide and gypsum make up the rest of the materials.
Gypsum regulates the setting, or hardening, time of cement. The lime used to make cement comes from materials such as limestone, oyster shells, chalk and a type of clay called marl.
Most cement plants are located near limestone quarries. They may also be near deposits of clay and other raw materials.
In a cement plant, the material goes through three steps: crushing and grinding, burning and fine grinding.
Primary crushers handle pieces of limestone as large as pianos.
The first crushing makes the pieces about the size of softballs. Then secondary,crushers, or hammer mills, break the rock into pieces about thee quarters of an inch wide.
The other raw materials are then added and the batch goes through a rotating ball mill.
After the raw materials have been ground, they are fed into a kiln, a huge cylindrical oven made of steel and lined with firebricks.
The kiln may be 12 feet wide and 500 feet long. The ground materials take about four hours to travel through the kiln where temperatures range from 2,600 to 3,000 degrees Fahrenheit.
A final grinding process then produces powdery portland cement that is finer than flour.
As the material comes from the kiln, it is in pieces about the size of marbles. It is now called clinker. A small amount of gypsum is added to the material before the final grinding.
Cement plants ship cement either unpackaged or packed in strong paper sacks. Unpackaged cement is shipped by railroad, truck or barge. Packaged cement is shipped in sacks containing 94 pounds or one cubic foot of cement to the sack.
The ancient Romans developed cement and concrete similar to the kinds we use today. Their cement had such great durability that some of their buildings, roads and bridges are still found today.
The Romans made cement by mixing lime to which water was added, and then they put in volcanic ash called pozzuolana. The ash produced a hydraulic cement.