Throughout history, cementing materials have played a vital role. They were used widely in the ancient world. The Egyptians used calcined gypsum as a cement. The Greeks and Romans used lime made by heating limestone and added sand to make mortar, with coarser stones for concrete.
The Romans found that a cement could be made which set under water and this was used for the construction of harbours. The cement was made by adding crushed volcanic ash to lime and was later called a "pozzolanic" cement, named after the village of Pozzuoli near Vesuvius.
In places such as Britain, where volcanic ash was scarce, crushed brick or tile was used instead. The Romans were therefore probably the first to manipulate the properties of cementitious materials for specific applications and situations.
Hadrian's Wall, England, a few miles east of Housesteads.
Marcus Vitruvius Pollio, a Roman architect and engineer in the 1st century BC wrote his "Ten books of Architecture" - a revealing historical insight into ancient technology. Writing about concrete floors, for example:
"First I shall begin with the concrete flooring, which is the most important of the polished finishings, observing that great pains and the utmost precaution must be taken to ensure its durability".
"On this, lay the nucleus, consisting of pounded tile mixed with lime in the proportions of three parts to one, and forming a layer not less than six digits thick."
And on pozzolana:
"There is also a kind of powder from which natural causes produces astonishing results. This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers are constructed of it in the sea, they set hard under water."
(Vitruvius, "The Ten Books of Architecture," Dover Publications, 1960.)
His "Ten books of Architecture" are a real historical gem bringing together history and technology. Anyone wishing to follow his instructions might first need to find a thousand or so slaves to dig, saw, pound and polish...
After the Romans, there was a general loss in building skills in Europe, particularly with regard to cement. Mortars hardened mainly by carbonation of lime, a slow process. The use of pozzolana was rediscovered in the late Middle Ages.
The great mediaeval cathedrals, such as Durham, Lincoln and Rochester in England and Chartres and Rheims in France, were clearly built by highly skilled masons. Despite this, it would probably be fair to say they did not have the technology to manipulate the properties of cementitious materials in the way the Romans had done a thousand years earlier.
The Renaissance and Age of Enlightenment brought new ways of thinking, which for better or worse, led to the industrial revolution. In eighteenth century Britain, the interests of industry and empire coincided, with the need to build lighthouses on exposed rocks to prevent shipping losses. The constant loss of merchant ships and warships drove cement technology forwards.
Smeaton, building the third Eddystone lighthouse (1759) off the coast of Cornwall in Southwestern England, found that a mix of lime, clay and crushed slag from iron-making produced a mortar which hardened under water. Joseph Aspdin took out a patent in 1824 for "Portland Cement," a material he produced by firing finely-ground clay and limestone until the limestone was calcined. He called it Portland Cement because the concrete made from it looked like Portland stone, a widely-used building stone in England.
While Aspdin is usually regarded as the inventor of Portland cement, Aspdin's cement was not produced at a high-enough temperature to be the real forerunner of modern Portland Cement. Nevertheless, his was a major innovation and subsequent progress could be viewed as mere development.
A ship carrying barrels of Aspdin's cement sank off the Isle of Sheppey in Kent, England, and the barrels of set cement, minus the wooden staves, were later incorporated into a pub in Sheerness and are still there now.
A few years later, in 1845, Isaac Johnson made the first modern Portland Cement by firing a mixture of chalk and clay at much higher temperatures, similar to those used today. At these temperatures (1400C-1500C), clinkering occurs and minerals form which are very reactive and more strongly cementitious.
While Johnson used the same materials to make Portland cement as we use now, three important developments in the manufacturing process lead to modern Portland cement:
- Development of rotary kilns
- Addition of gypsum to control setting
- Use of ball mills to grind clinker and raw materials
Rotary kilns gradually replaced the original vertical shaft kilns used for making lime from the 1890s. Rotary kilns heat the clinker mainly by radiative heat transfer and this is more efficient at higher temperatures, enabling higher burning temperatures to be achieved. Also, because the clinker is constantly moving within the kiln, a fairly uniform clinkering temperature is achieved in the hottest part of the kiln, the burning zone.
The two other principal technical developments, gypsum addition to control setting and the use of ball mills to grind the clinker, were also introduced at around the end of the 19th century.