Lime mortar (made of lime, sand, and water) was not forgotten, and was used as cement throughout the middle ages.
In 1756 it was improved by John Smeaton, a British engineer, when he was given the job of constructing the Eddystone Lighthouse off the coast of Plymouth. He added powdered brick to the cement mix and made the first modern concrete by using pebbles as the aggregate.
Joseph Aspdin, a Yorkshireman, invented the first modern artificial cement in 1824 by burning limestone and clay together. When the result was mixed with water the result was stronger than the limestone variety. He called it Portland cement, because it looked like Portland stone when it dried. Portland cement is still the main cement used in concrete production.
In the 1830s a Isaac Charles Johnson, a cement manufacturer in the south of England, improved it by burning it at a much higher temperature.
The next step forward was reinforced concrete: concrete with an imbedded metal frame. It was invented by Joseph Monier, a Parisian gardener, who made garden pots and tubs of concrete reinforced with iron mesh.
Concrete is strong under compression, but it lacks tensile strength which is provided by the insertion of metal mesh or rods. The reinforcing steel not only added strength but also gave the material the tensile strength to survive stresses such as wind and earthquakes, where concrete on its own would crumble. It is also more resistant to fire than bare metal would be, which makes it particularly attractive as a material for skyscrapers and shell constructions.
The next innovation was pre-stressed concrete. Eugène Freyssinet, a French engineer, invented it in the 1920s. Steel cables are laid in the empty mould and stretched. After the concrete has been poured and allowed to set, the cables are released. The steel tries to return to its original length and compresses the concrete. If ordinary concrete is put under pressure it stretches and breaks. Pre-stressed concrete counteracts any stretching force by the steel’s compression force. Since the strength is achieved without big external supports, pre-stressed concrete allows more elegant structures to be built. In 1938 Freyssinet developed a practical system for applying tension to steel. From then on, pre-stressed concrete came into general use worldwide.
In addition to its potential for immense strength and its initial ability to adapt to virtually any form, concrete is fire resistant and has become one of the most common building materials in the world.