Limestone is a calcium-based rock that has many uses in daily life. As a construction stone, it may decorate the facade of the office building or school that you are sitting in right now, and form the retaining wall around its parking garage and lawn. Powdered, it is a primary ingredient of the Portland cement used to build that garage and also of the soil-enriching materials that keep the lawn green.
This versatile stone was used to make the sidewalk that runs alongside your lawn and the chalk with which the street artist made a colorful drawing on that walk. Crushed limestone forms the sturdy foundation underneath the roads you drive, and you may also have encountered this rock in its purified and processed form in your toothpaste this morning, as a mineral supplement to strengthen your bones, or in the makeup you might be wearing.
♦ What is limestone?
Limestone is a sedimentary rock that forms over geologic time from the accumulation of organic debris with a high calcium content, such as shells and corals. This most often happens in warm shallow seas. The rock can also form when calcium precipitates out of water, for example, when a stalagmite forms and slowly builds up from the floor of a cave underneath dripping ground water.
Today there is a lot of limestone forming, especially in the seas 30 degrees of latitude north and south of the Equator. However, since the process has been going on for millions of years, vast deposits of the rock already exist on land, and humans have been finding uses for this sturdy but easily shaped material since at least 5000 years ago, when somebody in what are now called the Berkshire Downs in Oxfordshire, England, carved trenches, shaped in the form of a stylized figure of an animal, into a hillside and filled them in with bright white chalk. Not long after that, in Egypt, a pyramid of over 2 million huge limestone blocks was built at what is now called Giza to house a pharaoh.
♦ How is limestone used?
Since the days of hill figures and pyramid building, we have found many uses for limestone, some outdated, like stage lights made out of quicklime flares that led to the phrase “in the lime light.” Other old applications, like its use in mortar, continue with modifications. Limestone also fits well into the requirements of our modern world.
Most commercial limestone is sold as crushed stone or aggregate for use in foundations, road bases, as ballast for railroad track ties, for agricultural drainage, and in any industry that involves advanced chemical processing or metallurgy. Limestone is also used extensively in making Portland cement and concrete (cement with the addition of sand and gravel). The rock is so important that the US Geological Survey has noted that “[d]espite the low value of its basic products, the crushed stone industry is a major contributor to and an indicator of the economic well-being of the Nation.”
Ground limestone can be used as roofing granules or as rock dust in mines to prevent explosions. Very finely ground and purified limestone is also used as a mild abrasive in toothpaste. Processed limestone also serves as both a white pigment and inexpensive filler in makeup and in paints, and is an excellent source of calcium carbonate for vitamin supplement manufacturers.
♦ What are aglime, quicklime and lime?
Because of this high calcium content, limestone is also finely ground and used as agricultural lime, or aglime (link is to PDF file), to neutralize soil acidity.
When burned, limestone forms unstable and caustic quicklime (calcium oxide), but with the addition of water this dangerous material turns into the much safer and stable slaked lime (calcium hydroxide). Slaked lime has very low toxicity and is traditionally used by native peoples in making dried corn more tasty and easy to digest. Norwegians use it to make lutefisk, and it is also popular for home pickling. Slaked lime can filter small particles and so it is also used in pollution control and in the chemical and petroleum industries.
Limestone is good for stone carving. Its natural beauty can also be appreciated in many places throughout the world, from the White Cliffs of Dover to Mammoth Cave in the United States to beautiful Ha Long Bay in Vietnam. It also has possibly more practical uses than any other rock today. Laid down in vast deposits over many millenia, limestone is one of the most common and yet most valuable resources on our planet.