Tom Nelson, age 13, of Pocatello, Ida., for his question:
HOW IS CEMENT MADE?
Cement is a fine, gray powder. 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 can harden under water. It 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 10 percent alumina. Iron oxide and gypsum make up the rest.
Gypsum regulates the setting, or hardening, time of cement. The lime used to make cement comes from 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 materials goes through three steps: crushing and grinding, burning and fine grinding.
Primary crushers handle pieces of limestone that may be as large as household refrigerators. Then secondary crushers, or hammer mills, break the rock into pieces about three quarters of an inch across.
The other raw materials are added and the batch goes through a rotating ball mill.
After mixing, the raw materials 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 up ingredients may take about four hours to travel through the kiln, where temperatures range as high as 3,000 degrees Fahrenheit.
As the material emerges from the kiln, it is in pieces about the size of marbles. It is now called clinker.
A final grinding process then produces powdery portland cement that is finer than flour.
Before the marble size pieces called clinker are put into a mill for a final grinding, a small amount of gypsum is added.
Cement is shipped unpackaged by railroad, truck or barge, or in 94 pound sacks each containing one cubic foot of the material.
The ancient Romans developed cement similar to the we use today. Theirs had such great durability that some Roman buildings, roads and bridges are still found in good shape today.
The Romans made cement by mixing lime and water, then put in volcanic ash called pozzuolana. The ash produced hydraulic cement.