Everyday Physics: How Seashells Have Built Our Cities

"I have spent the past few weeks watching humans make artificial rock. It didn't require a visit to a special laboratory or factory; all I had to do was stare out of my window at the building site across the road, where concrete is being poured every day. It's not a material we often think about, even though our cities are literally built on concrete foundations and supported by concrete pillars and it's the second most-used resource in the world after water. But when you stop and think, it's strange that the way we reshape our world is so dependent on one material. It got me thinking about how this human-made stone works.

Concrete itself is mostly gravel and sand -- known as aggregate. The broken lumps of natural rock are individually strong, but irregularly shaped. They're only useful if you can hold them together, and that's the job of cement. It's easy to imagine how aggregate can be mixed into wet cement, but the real trick is to get that cement to set hard, turning a sticky paste into a permanent solid. If you get the recipe right, it's really durable. The Romans built both the Pantheon and the Colosseum out of an early form of concrete, and they're still standing nearly 2,000 years later.

The ingredients are mundane. The five most common elements in the Earth's crust (oxygen, silicon, aluminum, iron and calcium) are also the main ingredients of cement. It makes sense that to make an artificial stone, you use the same materials that nature has been using to make natural stones for billions of years. Once a builder has poured the concrete, his or her job is done, but the real cement construction is only just getting started.

The critical building blocks of the stone-to-be are calcium oxide (known as quicklime), silicates and water, and they react together and rearrange themselves into a complex new structure. The water is an integral part of the cement -- it builds itself into the chemical matrix, bonding everything together, and it can take days and sometimes months before the full strengthening is complete. Once the cement ingredients have cured, there is no turning back: This process is irreversible and the aggregate is now locked into its new shape. A new stone has been created.

My favorite part of this is the back story. Cement only works because of calcium. Although calcium makes up around 4% of Earth's crust, it's very dilute, and there are no physical processes that concentrate it. Instead, calcium-containing rocks are eroded by rain, and calcium is washed into the ocean, where it becomes even more dilute: around 0.04% of ocean water.

Humans were not the first to discover calcium as a building material. Many tiny ocean creatures -- some just a single cell and some large enough to recognize (such as mussels and marine snails) -- construct themselves out of calcium carbonate, filching calcium ions from seawater one at a time, collecting and repositioning these valuable atoms to build their bodies or shells. If those creatures die in the right circumstances, their carefully harvested calcium will fall to the seabed, accumulating with that of billions of others over millions of years, and may eventually make its way back to the sunlight as calcium-rich limestone. Physics doesn't concentrate this atom, but biology will, if you have the patience to wait.

All of our concrete, all the foundations, pillars, sculptures and pavements, could only be built by us because the sea creatures went first, concentrating calcium into one kind of rock so that we could turn it into another. So concrete is really a two-stage invention, as humans modify what ocean life provided. Once you know this, cities never look quite the same again." [1]

 

 Now we are constantly driving carbon dioxide into the air with our diesels and other this type of equipment. That carbon dioxide dissolves in the seawater and acidifies it. That acid eats away, dissolves, the calcium houses of the sea inhabitants.

 

1. REVIEW --- Everyday Physics: How Seashells Have Built Our Cities
Czerski, Helen.
Wall Street Journal, Eastern edition; New York, N.Y. [New York, N.Y]. 30 Apr 2022: C.4.