Concrete is used more than any other material in the world. Its made by mixing cement, sand and/or gravel and water which triggers complex chemical reactions.
Concrete is a composite material made from a mixture of ingredients all with different properties. Around 10-15% of concrete is cement, the main ingredient of which is lime (calcium carbonate); between 60% and 65% is made up of aggregates such as gravel or sand and 15-20% is water.
Making concrete is like baking a cake: selected ingredients are mixed together, heated, and allowed to set. Just as different recipes create different cakes, the texture, strength, resilience, and colour of concrete can vary depending on its mixture and how it is made.
Adding water to the cement kicks off a series of complex chemical reactions forming new compounds (cement hydration), the most important of which is calcium silicate hydrate gel which gives cement its strength. As the mixture hardens it binds the aggregates (sand, gravel, stone) together to form a hard-solid mass, concrete.
2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2
Commercially, the hardening process or curing is helped by heating, steam and pressure. Best quality concrete is steamed at 160°C at 6 times atmospheric pressure for 24 hours. Concrete can take up to 20 years to gain maximum strength.
Concrete is easily shaped and poured, it is incredibly strong and hard wearing, highly resistant to fire and water and is one of the few materials used for underwater construction eg dams and bridges. The properties of concrete can be improved by reinforcing it with steel rods or mesh to increase its tensile (bending) strength.
Concrete has vast uses in construction including roads, bridges, buildings, flooring etc.The world record for the largest concrete pour in a single project is the Three Gorges Dam in Hubei Province, China. It is estimated that construction required 16 million cubic meters over 17 years. Did you know there is a world’s largest concrete garden gnome in the USA, its 15 feet tall!
The Romans famously triggered a concrete revolution and used volcanic ash “pozzolana” (a siliceous and aluminous material), quick-lime (calcium carbonate) and water to make cement. Most famously, the Pantheon in Rome, built almost 2000 years ago, is still the largest unsupported concrete structure in the world. The Egyptians also used concrete as a filling material when building the Pyramids of Giza, one of the ancient wonders of the world.
All the concrete we use is manmade but you can find concrete in nature. In Israel there’s a giant, 12 million year old natural deposit formed from oil shale in contact with limestone.
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