Aggregates can include sand, crushed stone, gravel, slag, ashes, burned shale, and burned clay. Fine aggregate (fine refers to the size of the aggregate particulates) is used in making concrete slabs and smooth surfaces.
Coarse aggregate is used for massive structures or sections of cement.
Cement has been around a lot longer than the building material we recognize as concrete.
CEMENT IN ANTIQUITY
Cement is thought to be older than humanity itself, having formed naturally 12 million years ago, when burnt limestone reacted with oil shale. Concrete dates back to at least 6500 BC, when the Nabatea of what we know now as Syria and Jordan used a precursor of modern-day concrete to build structures that survive to this day. The Assyrians and Babylonians used clay as the bonding substance or cement. The Egyptians used lime and gypsum cement. The Nabateau are thought to have invented an early form of hydraulic concrete—which hardens when exposed to water—using lime.
The adoption of concrete as a building material transformed architecture throughout the Roman Empire, making possible structures and designs that could not have been built using just the stone that had been a staple of early Roman architecture.
Suddenly, arches and aesthetically ambitious architecture became much easier to build. The Romans used concrete to build still-standing landmarks such as the Baths, the Colosseum, and the Pantheon.
The arrival of the Dark Ages, however, saw such artistic ambition dwindle alongside scientific progress.
In fact, the Dark Ages saw many developed techniques for making and using concrete lost. Concrete would not take its next serious steps forward until long after the Dark Ages had passed.
THE AGE OF ENLIGHTENMENT
In 1756, the British engineer John Smeaton made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into the cement. Smeaton developed his new formula for concrete in order to build the third Eddystone Lighthouse, but his innovation drove a huge surge in the use of concrete in modern structures. In 1824, the English inventor Joseph Aspdin invented Portland Cement, which has remained the dominant form of cement used in concrete production. Aspdin created the first true artificial cement by burning ground limestone and clay together. The burning process changed the chemical properties of the materials and allowed Aspdin to create a stronger cement than plain crushed limestone would produce.
THE INDUSTRIAL REVOLUTION
Concrete took a historic step forward with the inclusion of imbedded metal (usually steel) to form what’s now called reinforced concrete or ferroconcrete. Reinforced concrete was invented (1849) by Joseph Monier, who received a patent in 1867.
Monier was a Parisian gardener who made garden pots and tubs of concrete reinforced with an iron mesh. Reinforced concrete combines the tensile or bendable strength of metal and the compressional strength of concrete to withstand heavy loads. Monier exhibited his invention at the Paris Exposition of 1867. Besides his pots and tubs, Monier promoted reinforced concrete for use in railway ties, pipes, floors, and arches.
But its uses also ended up including the first concrete-reinforced bridge and massive structures such as the Hoover and Grand Coulee dams.