🏗️ Materials science

Plastic is usually made from oil or natural gas, which chemists break down and rearrange into long, repeating chains of molecules called polymers, giving plastic its flexible yet sturdy structure. Those long chains are built from bonds that are extremely strong and stable, which is exactly what makes plastic so useful, but it is also why plastic resists breaking down naturally for an incredibly long time. Instead of fully decomposing like a banana peel or a leaf, plastic mostly just breaks into smaller and smaller pieces over many years, which is part of why reducing and recycling plastic matters so much.

Steel starts as iron ore, a rock containing iron mixed with other minerals, which gets heated to extremely high temperatures inside a furnace alongside coal to melt out the pure iron. That melted iron actually contains too much carbon to be strong and useful on its own, so workers blast oxygen through it to burn away the extra carbon until the mixture becomes steel. Different small amounts of other elements can then be added to make specialised kinds of steel, some rust resistant, some extra hard, depending on exactly what the steel will be used for.

At a recycling plant, mixed recycling first travels along conveyor belts past workers and machines that pull out anything that clearly does not belong, like plastic bags or food waste. Powerful magnets then pull out any metal that reacts to them, while other machines use blasts of air, spinning discs, or infrared light to sense and separate paper, cardboard, glass, and different types of plastic from one another. Each sorted material is then bundled together and sent to a different specialised facility, which melts, shreds, or reshapes it into the raw material for something brand new.

Concrete is made by mixing cement, water, sand, and small stones called gravel together into a thick paste that can be poured or shaped while still soft. Cement itself is made by heating limestone and clay to extremely high temperatures, then grinding the result into a fine powder that reacts with water in a chemical process rather than simply drying out. Once mixed and poured, concrete hardens through that chemical reaction over several days, eventually becoming strong enough to hold up buildings, bridges, and roads for decades.