Sand is a loose, fragmented, naturally-occurring material consisting of very small particles of decomposed rocks, corals, or shells. Sand is used to provide bulk, strength, and other properties to construction materials like asphalt and concrete. It is also used as a decorative material in landscaping. Specific types of sand are used in the manufacture of glass and as a molding material for metal casting. Other sand is used as an abrasive in sandblasting and to make sandpaper.
Sand was used as early as 6000 B.C. to grind and polish stones to make sharpened tools and other objects. The stones were rubbed on a piece of wetted sandstone to hone the cutting edge. In some cases, loose sand was scattered on a flat rock, and objects were rubbed against the sandy surface to smooth them. The first beads with a glass glaze appeared in Egypt in about 3,500-3,000 B.C. The glass was made by melting sand, although naturally-occurring glass formed by volcanic activity was probably known long before that time.
In the United States, sand was used to produce glass as early as 1607 with the founding of the short-lived Jamestown colony in Virginia. The first sustained glass-making venture was formed in 1739 in Wistarburgh, New Jersey, by Caspar Wistar. The production of sand for construction purposes grew significantly with the push for paved roads during World War I and through the 1920s. The housing boom of the late 1940s and early 1950s, coupled with the increased use of concrete for building construction, provided another boost in production.
Today, the processing of sand is a multi-billion dollar business with operations ranging from very small plants supplying sand and gravel to a few local building contractors to very large, highly automated plants supplying hundreds of truckloads of sand per day to a wide variety of customers over a large area.
The most common sand is composed of particles of quartz and feldspar. Quartz sand particles are colorless or slightly pink, while feldspar sand has a pink or amber color. Black sands, such as those found in Hawaii, are composed of particles of obsidian formed by volcanic activity. Other black sands include materials such as magnetite and homblende. Coral sands are white or gray, and sands composed of broken shell fragments are usually light brown. The white sands on the Gulf of Mexico are made of smooth particles of limestone known as oolite, derived from the Greek word meaning egg stone. The white sands of White Sands, New Mexico, are made of gypsum crystals. Ordinarily, gypsum is dissolved by rain water, but the area around White Sands is so arid that the crystals survive to form undulating dunes.
Quartz sands, which are high in silica content, are used to make glass. When quartz sands are crushed they produce particles with sharp, angular edges that are sometimes used to make sandpaper for smoothing wood. Some quartz sand is found in the form of sandstone. Sandstone is a sedimentary, rock-like material formed under pressure and composed of sand particles held together by a cementing material such as calcium carbonate. A few sandstones are composed of almost pure quartz particles and are the source of the silicon used to make semiconductor silicon chips for microprocessors.
Molding sands, or foundry sands, are used for metal casting. They are composed of about 80%-92% silica, up to 15% alumina, and 2% iron oxide. The alumina content gives the molding sand the proper binding properties required to hold the shape of the mold cavity.
Sand that is scooped up from the bank of a river and is not washed or sorted in any way is known as bank-run sand. It is used in general construction and landscaping.
The definition of the size of sand particles varies, but in general sand contains particles measuring about 0.0025-0.08 in (0.063-2.0 mm) in diameter. Particles smaller than this are classified as silt. Larger particles are either granules or gravel, depending on their size. In the construction business, all aggregate materials with particles smaller than 0.25 in (6.4 mm) are classified as fine aggregates. This includes sand. Materials with particles from 0.25 in (6.4 mm) up to about 6.0 in (15.2 cm) are classified as coarse aggregates.
Sand has a density of 2,600-3,100 lb per cubic yard (1,538-1,842 kg per cubic meter). The trapped water content between the sand particles can cause the density to vary substantially.
The preparation of sand consists of five basic processes: natural decomposition, extraction, sorting, washing, and in some cases crushing. The first process, natural decomposition, usually takes millions of years. The other processes take considerably less time.
The processing plant is located in the immediate vicinity of the natural deposit of material to minimize the costs of transportation. If the plant is located next to a sand dune or beach, the plant may process only sand. If it is located next to a riverbed, it will usually process both sand and gravel because the two materials are often intermixed. Most plants are stationary and may operate in the same location for decades. Some plants are mobile and can be broken into separate components to be towed to the quarry site. Mobile plants are used for remote construction projects, where there are not any stationary plants nearby.
The capacity of the processing plant is measured in tons per hour output of finished product. Stationary plants can produce several thousand tons per hour. Mobile plants are smaller and their output is usually in the range of 50-500 tons (50.8-508 metric tons) per hour.
In many locations, an asphalt production plant or a ready mixed concrete plant operates on the same site as the sand and gravel plant. In those cases, much of the sand and gravel output is conveyed directly into stockpiles for the asphalt and concrete plants.
The following steps are commonly used to process sand and gravel for construction purposes.
Most large aggregate processing plants use a computer to control the flow of materials. The feed rate of incoming material, the vibration rate of the sorting screens, and the flow rate of the water through the sand classifying tank all determine the proportions of the finished products and must be monitored and controlled. Many specifications for asphalt and concrete mixes require a certain distribution of aggregate sizes and shapes, and the aggregate producer must
The production of sand and gravel in many areas has come under increasingly stringent restrictions. The United States Army Corps of Engineers, operating under the Federal Clean Water Act, has required permits for sand extraction from rivers, streams, and other waterways. The cost of the special studies required to obtain these permits is often too expensive to allow smaller companies to continue operation. In other cases, residential development in the vicinity of existing aggregate processing plants has led to restrictions regarding noise, dust, and truck traffic. The overall result of these restrictions in certain areas is that sand and gravel used for construction will have to be transported from outside the area at a significantly increased cost in the future.
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Hornbostel, Caleb. Construction Materials, 2nd Edition. John Wiley and Sons, Inc., 1991.
Siever, Raymond. Sand. W.H. Freeman and Company, 1988.
Grover, Jennifer E., Bob Drake, and Steven Prokopy. "100 Years of Rock Products, History of an Industry: 1896-1996." Rock Products, July 1996, pp. 29+.
Mack, Walter N. and Elizabeth A. Leistikow. "Sands of the World." Scientific American, August 1996, pp. 62-67.
Miller, Russell V. "Changes in Construction Aggregate Availability in Major Urban Areas of California Between the Early 1980s and the Early 1990s." California Geology, January/February 1997, pp. 3-17.
— Chris Cavette