An effective concrete mix contains cement, water and fine and coarse aggregates. Portland cement, developed in 1824, is the most common type in construction. A proper mixture must have enough paste to bind the aggregates, otherwise the product will be porous and rough. Concrete with too much paste will be more likely to shrink. Ideally, a concrete mix contains 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water.
Aggregates include crushed stone made from quarry rock, boulders, cobbles or gravel. Limestone and dolomite are the most common stones crushed for concrete, though granite and other rocks are also used. Limestone is sedimentary rock made mostly of the mineral calcite made by the combination of shell, coral, algal and fecal debris in shallow marine waters. It is also made by the precipitation of calcium carbonate from lake or ocean water. Dolomite is an altered form of limestone, forming when calcite is altered. Granite is a coarse igneous rock consisting mainly of the minerals quartz and feldspar. Fine aggregates, smaller than 0.2 inches, are sand, either natural or manufactured. Coarse aggregates, larger than 0.2 inches, include gravel, crushed concrete and air-cooled furnace slag.
Cement vs. Concrete
Though the words are commonly used interchangeably, cement and concrete have different compositions. Cement is a fine gray powder that binds sand and rock and is the main ingredient in concrete.