Portland cement is the main component of concrete and it is made using a highly evolved recipe of calcium, silicon, aluminum, iron, gypsum and other ingredients. Modern portland cement is manufactured using giant rock crushers and industrial kilns. But the Romans, who built the Coliseum out of concrete, used much simpler technology. And Joseph Aspdin, the English bricklayer who invented portland cement, made his first batch in 1824 by burning powdered limestone and clay in his kitchen oven. He thought his cement looked like a common building stone called "portland stone" so he called his creation "portland cement."
Crush 85 pounds of limestone with a sledge hammer. Each piece of limestone should be no more than 3 inches in diameter but smaller is better. Industrial cement manufacturers use very large grinders to crush the stone.
Shovel the crushed limestone into a large wheelbarrow. Add 2 pounds of blackboard chalk to the crushed stone. In addition to chalk, cement manufacturers also add other ingredients, including sea shells, slate, sand, iron ore and blast furnace slag.
Add 15 pounds of silica clay cat litter to the wheelbarrow. Mix all the ingredients in the wheel barrow thoroughly with a shovel.
Add just enough water to make all the ingredients wet. Cement manufacturers commonly use a mixture that contains about 15 percent water by weight although some cement recipes specify that the ingredients should be dry.
Shovel the wet, but not fluid mixture of crushed stone, chalk and cat litter into a brick kiln or oven capable of reaching temperatures of at least 1,500 degrees Fahrenheit. Commercial cement manufacturers us very large, rotating kilns capable of reaching 2,700 degrees Fahrenheit.
Bake the mixture until it glows red hot. Cement manufacturers call this baked mixture of limestone, clay and chalk "clinker." Allow the clinker to cool.
Put on a dust filter face mask. Crush the clinker into a fine powder with a sledge hammer. One pound of manufactured cement contains about 150 billion grains so patience may be required.
Shovel the fine powdered clinker into the wheelbarrow. Thoroughly mix 5 pounds of plaster of Paris into the powdered clinker.
Mix one part cement, two parts sand and three parts gravel by volume and add enough water so the resulting slurry is pourable to make concrete.