How can you not love the narrator/professor from the Periodic Table of Videos? His upper crust British accent, his halting delivery, his shock of white hair (sadly under a hard hat here), and his periodic table ties all make him an endearing character.
His explanation of the making of concrete is nicely simple and well laid out...
calcium carbonate ---heat--> calcium oxide + carbon dioxide
calcium oxide + sand (silicon dioxide) + aggregate (stone) + water ---> concrete
From there, the silicon dioxide and the calcium oxide make (from 2:21) "a sort of jelly that gets harder and harder." This 'jelly' then reacts with the carbon dioxide from the air to get even harder still, reacting over hundreds of years to react a final hardness.
Then the professor goes through the differences in concrete strength under tension and compression, explaining how adding in the steel reinforcement to overcome the material's weakness in tension.
There's even a great, if quick, explanation of the environmental issue with creating carbon dioxide in the process of producing the calcium oxide.
Very well done, professor.
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