Concrete was a composite material made of aggregate, cement, and water. Its use became more common during the 20th and 21st centuries.
Concrete was a common material for construction during the time of the Roman Empire. This concrete was made of quickline, pozzolana, and pumice. No need for stone or brick. Concrete was used to build the Roman Colosseum. After the Fall of Rome, concrete became rare in construction until the 1600s. This new concrete used Portland cement based on limestone. However, it was not created or patented until 1824. Before that, conventional Roman concrete was still used. Both Roman concrete and Portland cement concrete had one major drawback. Despite their high compressive strength, the tensile strength was very low. That was one reason why the Colosseum looked the way it did in the 20th century. A solution was found in steel reinforcing bars or rebar.
Tech Level: 8-10
Reinforced concrete was first used in 1853 when Francois Coignet built a four-story house just outside of Paris. Reinforced concrete was stronger than wood and even drywall. This became clear after 9/11. When the North Tower was hit, the drywall in the middle which protected the stairs was torn off. It became clear that drywall was embarrasingly weak. Thus, future skyscrapers were built with reinforced concrete. Examples included Burj Khalifa in Dubai and One World Trade Center in New York City. One World Trade Center was important because it meant that nothing could destroy the American Dream. Not only that, but the concrete protected the elevators and the stairs. A plane crashing into it would never destroy the tower. Concrete was used for other purposes, too. It was even to build submarines.