First things first: cement is not the same thing as concrete. Here’s the difference: cement is a substance that reacts quickly with water to form a bonding agent that holds other ingredients in place. Various forms of cement have been in use for thousands of years. The most common type of cement in use today is known as Portland cement, a mixture of limestone, clay minerals, and gypsum, heated and pulverized. Portland cement is used in grout, mortar, plaster, stucco, and concrete.
Concrete is a mixture of cement and water with fine and coarse materials, such as gravel and sand. This kind of concrete is used in exterior and interior walls, footings and floors - just about everywhere people build. In fact, more concrete is used around the world each year than any other human-made building material. Concrete depends on natural minerals for most of its substance, plus one manufactured product - cement.
As a manufactured product, cement was first made in 1824. It is not expensive or difficult to make - but another material is proving to be still more economical. Fly ash, a by-product of combustion in a coal-burning power plant, has the same properties as cement when used in concrete. It’s often less expensive, and its use helps keep waste products out of landfills. One other benefit is that fly ash is ready to use, which saves energy and effort compared to making cement. This reduces manufacturing waste, too.
To help reduce airborne pollution, coal-burning power plants use equipment that removes ash from the plant’s exhaust gases. This ash was most often added to water and diverted into “ash ponds” that cost money to create and maintain. The process of creating and disposing of fly ash was only an expense for energy producers, with no apparent benefit.
Construction industry specialists looked at the properties of fly ash and found that it could be used in place of cement in many kinds of concrete. In order to fulfill the requirements of professional builders, any substitute would have to meet standards set by ASTM International, originally known as the American Society for Testing and Materials. Fly ash does.
The money earned through the sale of fly ash for use in concrete contributes to the income of a power plant that collects and sells it. What it saves in disposal costs is even greater. What had been only a waste material has become a valuable resource in two ways. Power plants that direct fly ash toward construction use can reduce their landfill needs by 80 percent or more.
Technically speaking, fly ash acts as a pozzolan - a siliceous, or siliceous and aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. That’s how ASTM International defines it. The word “pozzolan” comes from Pozzuoli, Italy, where volcanic ash was first used in Roman times as cement in concrete.
Most environment-friendly building materials are fairly new, and their performance over time is still unknown. If you compare fly ash to volcanic ash used in concrete, though, some examples built more than 2000 years ago are still standing: structures in Cosa, Italy, that have withstood millennia of direct exposure to seawater; and the Pantheon in Rome, Italy, a pozzolan-and-lime concrete structure with a cast concrete dome 124 feet in diameter.
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