Aggregate Crushed Stone
Aggregate Crushed Stone in Quarry Industry
Bedrock, the source material for crushed stone, is classified on the basis of origin as sedimentary, igneous, or metamorphic. Sedimentary rocks form by consolidation of loose sediment by chemical, biochemical, or mechanical processes. Chemically or biochemically deposited carbonate sedimentary rocks, such as hard, dense limestone (calcium carbonate) and dolomite (calciummagnesium carbonate), commonly are referred to as limestone in the aggregate industry. Generally these rocks make good sources of crushed stone, however, some are too soft and absorptive, or may contain too much poor quality material to yield high-quality aggregate.
Chert, also known as flint, is a tough finegrained sedimentary rock made of quartz. Chert is used as aggregate but it may react with adverse consequences when used in concrete. Hard, dense sandstone, a mechanically-deposited sedimentary rock, is occasionally used as crushed stone.
Many igneous rocks are hard, tough, and dense, and they make excellent crushed stone for construction uses. However, some igneous rocks are chemically reactive when used as aggregate in concrete. Igneous rocks solidify from naturally occurring molten rock (magma) generated within the Earth, and they are classified further by their origin, composition, and grain size. Hard, coarse-grained rocks form from molten magma that cools slowly deep within the Earth. There rocks commonly are referred to as granite in the aggregate industry. Fine-grained volcanic rocks form as molten lava flows onto the Earths surface and cools and solidifies relatively quickly. These igneous rocks commonly are referred to as traprock in the aggregate industry.
Metamorphic rocks form when existing rocks are subjected to heat and pressure within the Earth. Some metamorphic rocks are hard, tough, and dense and can be used as aggregate. These include gneiss (a banded crystalline rock); marble (metamorphosed limestone), and quartzite (metamorphosed sandstone).
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Aggregate Producers
In the United States, more than 1,200 companies produce crushed stone from some 3,300 quarries, and 4,000 companies produce sand and gravel from about 6,400 operations. Five companies account for nearly 25 percent of the aggregate production. Even so, more than 5,000 companies are active in the aggregate business, and no single producer dominates the industry. Even the largest producers must compete at the local level. Five of the top 10 crushed stone companies and three of the top 10 sand and gravel companies are foreign owned. Consequently, acquisitions of companies have become commonplace.
One of the major reasons for acquisitions is to obtain new high-quality reserves. Acquisitions are also being used by larger companies to spread the cost of technology over more production, thus achieving higher operating efficiencies.
Opening a new aggregate operation is a complicated process that can cost millions of dollars and take many years. Natural aggregate producers expend tremendous amounts of time and money locating potential aggregate resources and determining their quantity and quality. They also spend large amounts of money and effort determining the feasibility of production; identifying potential environmental impacts from production; making certain their operation will conform to the relevant laws; and obtaining the necessary permits to extract, process, and transport the aggregate.
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