To make cement, you need calcium. To get calcium, you start with limestone—which contains calcium plus carbon and oxygen—and burn it in a furnace along with some other materials. Given the presence of carbon and oxygen, you can probably see where this is going. After burning the limestone, you end up with the thing you want—calcium for your cement—plus something you don’t want: carbon dioxide. Nobody knows of a way to make cement without going through this process. It’s a chemical reaction—limestone plus heat equals calcium oxide plus carbon dioxide—and there’s no way around it. It’s a
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