Concrete History

Cement has been around for at least 12 million years. When the earth itself was undergoing intense geologic changes natural, cement was being created.
It was this natural cement that humans first put to use. Eventually, they discovered how to make cement from other materials. A concrete history timeline is here.

The naturally formed concrete chemical is called portlandite. portlandite is a mineral, calcium hydroxide (Ca(OH)2), which occurs naturally in Ireland. It is equivalent to a product of hydration of portland cement.

The word concrete comes from the Latin word "concretus" which means "to harden".
In Serbia, remains of a hut dating from 5600 BC have been found, with a floor made of red lime, sand, and gravel. The pyramids of Shaanxi in China, built thousands of years ago, contain a mixture of lime and volcanic ash or clay.
The Assyrians and Babylonians used clay as cement in their concrete. The Egyptians used lime and gypsum cement.

The Romans used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to build many famous Roman structures including the Appian Way, the Roman Baths of Caracalla, the Basilica of Maxentius, the Coliseum and Pantheon in Rome, and the Pont du Gard aqueduct in south France. They used broken brick aggregate embedded in a mixture of lime putty with brick dust or volcanic ash by the Romans. Many structures that used stone.
They built ~5,300 miles of roads. The current U.S. Interstate Highway System has 4,200 miles.

pozzolan is a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but that will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties; there are both natural and artificial pozzolans.
artificial pozzolan materials include fly ash and silica fume. Natural pozzolan is raw or calcined natural material that has pozzolanic properties (for example, volcanic tuffs or pumicites, opaline cherts and shales, clays, and diatomaceous earths).
Fly ash especially is a useful additive aggregate to concrete mixtures used today. It can sometimes be obtained cheaply because it is a waste by-product of coal power plants. Fly ash and silica fume are also very light while providing good strength.
Silica fume is very fine noncrystalline silica produced in electric arc furnaces as a byproduct of the production of elemental silicon or alloys containing silicon.

In 1756, 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. In 1824, English inventor, Joseph Aspdin invented Portland Cement, which has remained the dominant cement used in concrete production.
Joseph 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 Joseph Aspdin created a stronger cement than what using plain crushed limestone would produce.

Concrete that includes imbedded metal (usually steel) is called reinforced concrete or ferroconcrete. Reinforced concrete was invented (1849) by Joseph Monier, who received a patent in 1867. Joseph 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. Joseph Monier exhibited his invention at the Paris Exposition of 1867. Besides his pots and tubs, Joseph Monier promoted reinforced concrete for use in railway ties, pipes, floors, arches, and bridges.

Concrete Science

Modern concrete is composed of a mixture of portland cement, sand, and pea gravel-sized to palm-sized stone. The key ingredient of this mixture is the cement. The components
of cement when mixed with water undergo chemical reactions which bond the individual components of the mixture together. The raw components necessary to
create cement are as follows:

• 61-67% Calcium oxide (lime)
• 20-25% Silicon oxide (quartz is a silicon oxide)
• 3-7% Aluminum oxide (bauxite)
• 2-5% Iron oxide (rust)
• 1-3% Magnesium oxide

The raw, powdered mixture is heated in a sloped, cylindrical, rotating kiln with temperatures increasing along the kiln to 1480 degrees Celsius.In the high temperatures the calcium and silicon oxides react to form dicalcium and tricalcium silicates. Small amounts of tricalcium aluminate and tetracalcium aluminoferrite are also formed.The resulting minerals are crushed to a fine powder and mixed with gypsum and is called cement.

• 45-65% Tricalcium silicate
• 15-30% Dicalcium silicate
• 1-8% Tricalcium aluminate
• 8-15% Tetracalcium aluminoferrite
• 1-3% Magnesium oxide
• 1-3% Gypsum

When water is mixed with cement, the mixture sets in a few hours and hardens over a period of weeks. The initial setting is caused by a reaction between the water, gypsum, and tricalcium aluminate(Ca3Al), forming the crystalline hydration products calcium-alumino-hydrate(CAH), ettringite (Aft), and monosulfate (Afm). The later hardening and the development of cohesive strength is due to the reaction of water and tricalcium silicate (Ca3Si), forming an amorphous hydrate product called calcium-silicate-hydrate(CSH gel).

Cement mixtures which contain a higher percentage of tricalcium silicate will hydrate more rapidly and lead to higher early strength but leads to lower long-term strength.

In each case the hydration products surround and cement together the individual aggregate grains. The hydration of dicalcium silicate (Ca2Si) proceeds more slowly than that of the above compounds slowly increasing later age strength. Higher percentages of Ca2SI in a cement mix leads to a denser ultimate structure and higher long term strength.

The ultimate cementing agent is probably gelatinous silica(SiO2). All three reactions mentioned above release heat.