Montana Earth Science Picture of the Week

Where Cement Comes From

Limestone Quarry . . .
The photos on this page were both taken near Montana City, a community located 5 miles southeast of Helena. Here a light-colored rock, called Madison Limestone, can be found just beneath the soil, making this a convenient place for a quarry. Ash Grove Cement Company removes limestone from the quarry and hauls it to a plant just a few miles away where it is used to make cement. A quarry is sort of like a mine. However, at a mine the rock (called ore) is removed because it contains a valuable metal, which must then be removed by processes such as milling, leaching, and smelting. On the other hand, at a quarry it is the rock itself that the company is after.

Why limestone . . .
Limestone is primarily made up of the mineral calcite whose chemical formula is CaCO3. At 1000 C the CaCO3 loses CO2 to become CaO. This step releases so much CO2 into the atmosphere that cement production is one of the top five sources of the CO2 emissions in the USA. In the next step, the CaO is heated to 1500 C and it reacts with silica to form calcium silicate.

The most common combination of materials used to make cement is limestone, clay and sand. These materials are crushed and then processed in a furnace called a kiln where temperatures reach 1500 C (2730 F). The intense heat causes chemical reactions that convert the partially molten raw materials into pellets of calcium silicate called clinker. After adding some gypsum and other key materials, the mixture is ground into the extremely fine gray powder that we call "cement".

What's the difference between cement and concrete? . . .
Although the terms cement and concrete often are used interchangeably, cement is actually one of the ingredients used to make concrete. The other ingredients are sand and/or gravel (aggregate), and water. Typically, concrete is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water.

If it can't be grown it has to be mined . . .
No one likes to see the land torn up, but we all benefit from buildings, roads, and sidewalks made of concrete. Although 93 % of U.S. highways are paved with asphalt, 40 % of our interstate highways are made of concrete.

Below: Here is a closer view of Ash Grove's limestone quarry at Montana City.

Term: silica, aggregate


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By Rod Benson
Earth Science Teacher at Helena High School

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