Concrete has been used for ages as a building material. Concrete, when freshly mixed, can take nearly any form and shape, but dries and cures to rock-like hardness. Concrete is a mix of coarse rock, fine sand and a water-activated substance that binds the rock and sand together. In the modern era, that binder is Portland cement.
A good general purpose concrete mix combines one part Portland cement, 2.5 parts sand, three parts gravel and 0.5 parts clean water. For more strength and water resistance, reduce the sand to two parts. Mix dry ingredients well, then add the water and keep mixing until everything is thoroughly wet. To test for readiness, smooth a sample with the back of a shovel, then cut a groove in it with the side of the shovel. If the shovel cuts easily and the sides of the groove hold their shape, the concrete is ready. If you cannot cut a distinct groove, add more water. If the groove caves in, add more dry ingredients.
The ratios between cement, sand and gravel vary depending on the purpose of your concrete. For instance, a concrete mix for foundation walls or a base for paving slabs consists of one part Portland cement, 2.5 parts sand, 3.5 parts gravel and 0.5 parts water. For foundation footings, mix one part Portland cement, three parts sand and five parts gravel plus enough water to make it pour easily. For outdoor paving of walks or driveways, mix one part Portland cement, 1.5 parts sand, 2.5 parts gravel and 0.5 parts water. Mortar is masonry concrete used to bond stone, brick and blocks. It consists of one part Portland Cement, one part lime, six parts sand and enough water to make a mix that mounds and sticks on a trowel.
Portland cement is the heart of concrete. It is made from calcium compounds such as limestone, marl, chalk and sea shells, plus a mixture of sand, shale, clay and iron ore that contribute silicon, aluminum and iron compounds. The raw materials are mined from open-pit quarries and fed through rock crushers that grind them down to pebble size. Portland cement makers may also incorporate industrial-process byproducts such as ground-up fly ash, mill scale and metal refining slag.
The crushed mineral ingredients are fed into a furnace called a kiln for conversion into Portland cement. In the kiln, the crushed mineral mix is tumbled and heated to 2,750 degrees Fahrenheit. The extreme heat triggers a series of chemical reactions that convert the mineral mix into dry pellets of Portland cement called clinkers. These pellets are cooled to ambient temperature by forced air, then fed into a ball or tube mill that crushes the clinkers into a fine powder. At this stage, crushed gypsum is added to the cement mix to control the setting time. The finished cement is bagged for shipment.