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Q & A: Burning rocks

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Most recent answer: 10/22/2007
Q:
Why does concrete and stone not combust
- Austin (age 13)
Green Cove Springs
A:
Concrete is often made up of sand and cement. The sand is often silicon dioxide, which is already oxidized. Some sands are made up at least partly of calcium carbonate, which also has oxygen in it. Calcium carbonate is not a good material to include in concrete because it is water soluble and would wash away.

Most stones are oxides of some material or other (most often silicon, calcium, boron, aluminum, magnesium, phosphorus, potassium, manganese, and lots of other materials). In this sense, they are already "burnt" as well. Many rocks are formed in volcanic eruptions and begin their lives as hot lava. If they weren't already combusted at that point, then they certainly would catch fire.

In fact, most combustible stuff is living or has been living at some time in the past. Wood, oils, fats, gasoline, and natural gas all receive their energy from the sun as transformed by living organisms. Some metals burn (like iron -- very fine steel wool can be ignited, as can pure magnesium, and sodium and calcium and cesium will all explode in contact with water), but they are not found in their pure state in nature (otherwise they would have burned long ago).

Tom

(published on 10/22/2007)

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