The History of Concrete
One of our blog posts last week covered the exciting new development in Jeddah (Saudi Arabia’s second largest city after the capital of Riyadh). The Kingdom Tower will be the world’s tallest building when it’s completed and we described some of the challenges involved in the construction process, including the need to pump the half a million cubic metres of concrete that will be necessary more than half a mile into the sky. This has inspired us to take a closer look at concrete, one of the most versatile and innovative materials on the planet.
Concrete is the most widespread artificial material on our planet and was first used thousands of years ago but the knowledge of how to make it was lost for a long time. The word ‘concrete’ actually refers to a range of substances that combine rocks or gravel with some sort of adhesive material – it’s basically just a pile of rubble mixed with water and cement. When combined together, these materials can be poured into moulds and shaped into whatever you want – the only limits are your imagination! This is why concrete is often referred to as ‘liquid stone’.
There is historical evidence that human beings have been playing around with concrete for thousands of years, but the first civilization to really harness the craft of concrete was the Romans. At the height of the Roman Empire (more than two thousand years ago) the port city of Pozzuoli was an important centre for commerce and military activity. Ships laden with grains, weapons, iron and pozzolana (an ashy volcanic sand from the Campi Flegrei supervolcano) set sail every day for trading ports in all corners of the known world.
The Romans had discovered that the volcanic ash that spewed out from the Campi Flegrei was special – when mixed with water, it turned into a mortar that was strong enough to bind together lumps of rock into an impenetrable, strong load-bearing material. The Roman philosopher Seneca had noted that the “dust at Puteoli (the city’s Latin name) became stone if it touches water”. Nobody knew why this happened, but it seems that the Romans had built a city right on top of what was, in effect, a natural cement factory!
Pozzolana, it seems, is a mixture of lime and silica oxides which are two of the main ingredients in cement (water is the third). It’s only recently (this year, in fact) that a Stanford geochemist figured out how this ash is formed. Deep inside the caldera of Campi Flegrei, the surface is covered in limestone which is a soft, powdery rock composed of calcium carbonate (CaCOȝ). Geothermally heated water washes over the limestone walls of the caldera triggering a decarbonation reaction. This causes CO₂) gas to be released and leaves behind calcium hydroxide (also known as hydrated lime).
As the geothermal fluids inside the volcano bring the lime closer to the surface, it is combined with the silica-rich ash and forms a cement-like cap that is impenetrable. However, when enough pressure builds inside the volcano, the cap starts to bend and crack and eventually breaks. This causes the cement-forming ingredients to rocket upwards as pozzalana ash.
Historians think that the ancient Romans probably first discovered the pozzolana hardening in the sea water surrounding the volcano and then experimented with it, mixing in pumice to make concrete. However, with the fall of the Roman Empire, this knowledge was lost.
Next week, we’ll have some more information on this fascinating and versatile material – there is so much to learn about concrete.