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Chemistry

What is Cement?

This weekend I was tiling a bathroom. The stuff you stick the tiles to the floor is a mixture of cement, sand and a few other things. That kind of mixture is similar to what is in concrete concrete, which is a remarkable material used in all kinds of construction. The mystery to me was, “Why does cement harden when the water dries?” And also, “Why do they warn not to let touch the wet mixture?” The answers, it turns out, involves the chemistry of Calcium and Silicon — the same element that is used to make computer chips.
Cement is made from powdered di- and tri- calcium silicates. These chemicals react with water to form Calcium Silicate Hydrate. Calcium Silcate Hydrate is insoluable, so it will form solid crystals that glue the cement together. The reaction is remarkable in a number of ways. First, it is slow — as the reaction proceeds, the growing crystals get in the way of the water molecules that are trying to get to the calcium silicates. That slows down how quickly reaction goes. This slowing process allows concrete to be mixed and kept liquid for about three hours as it is shipped to where it is needed.
Another fascinating feature of cement is that it is made from extremely common materials: limestone and sand. At a cement plant, limestone (Calcium Carbonate) and sand (Silicon Dioxide) are ground into a very fine powder. Then they are heated. As the mixture reaches 800°C water boils away from the raw materials. At about 1200°C the limestone breaks down into Calcium Hydroxide, releasing carbon dioxide into the air. At 1400°C the sand reacts with the Calcium Hydroxide to produce molton cement. The cement hardens to form small pebbles called clinker that are then ground up to produce regular cement.
The calcium hydroxide that is produced as an intermediate ingredient is the reason that it’s not good to leave the cement mixture on your skin. More calcium hydroxide is released as the cement is reacting with the water. Calcium hydroxide is a very caustic base, and can cause burns on your skin.
The carbon dioxide that is released is significant: The manufacture of cement in the United States releases about 40 million metric tons of carbon dioxide into the atmosphere each year. Carbon dioxide is suspected to cause Global Warming.
There are a lot of other subtleties in the use of cement. Compounds of aluminum and polymer additives can change the properties of the final product. The diverse properties, inexpensive starting materials and ease of use make cement one of the best building materials in the world.

3 replies on “What is Cement?”

Very interesting. But did you or Becka feel any burning when you did get it on your hands? How long is the “prolonged exposure” period?

No, I didn’t notice anything, but then again we were reasonably careful not to immerse our hands in it. I have a feeling that it’s the sort of thing that can cause rather serious damage over time without you really noticing. You might notice a slight burning sensation and then suddenly your epidermis is missing…

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