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Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.[1]

The composition of sand varies, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide, or SiO2), usually in the form of quartz. The second most common type of sand is calcium carbonate, for example, aragonite, which has mostly been created, over the past half billion years, by various forms of life, like coral and shellfish. For example, it is the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years like the Caribbean.

Sand is a non-renewable resource over human timescales, and sand suitable for making concrete is in high demand.[2] Desert sand, although plentiful, is not suitable for concrete. 50 billion tons of beach sand and fossil sand is used each year for construction.[3]

Composition


The exact definition of sand varies.

ISO 14688 grades sands as fine, medium, and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm. In the United States, sand is commonly divided into five sub-categories based on size: very fine sand (1⁄16 – 1⁄8 mm diameter), fine sand (1⁄8 mm – 1⁄4 mm), medium sand (1⁄4 mm – 1⁄2 mm), coarse sand (1⁄2 mm – 1 mm), and very coarse sand (1 mm – 2 mm). These sizes are based on the Krumbein phi scale, where size in Φ = -log2D; D being the particle size in mm. On this scale, for sand the value of Φ varies from −1 to +4, with the divisions between sub-categories at whole numbers.

The most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica (silicon dioxide, or SiO2), usually in the form of quartz, which, because of its chemical inertness and considerable hardness, is the most common mineral resistant to weathering.

The composition of mineral sand is highly variable, depending on the local rock sources and conditions.

Sources


Rocks erode or weather over a long period of time, mainly by water and wind, and their sediments are transported downstream. These sediments continue to break apart into smaller pieces until they become fine grains of sand. The type of rock the sediment originated from and the intensity of the environment gives different compositions of sand. The most common rock to form sand is granite, where the feldspar minerals dissolve faster than the quartz, causing the rock to break apart into small pieces. In high energy environments rocks break apart much faster than in more calm settings. For example, Granite rocks this means more Feldspar minerals in the sand because it wouldn't have had time to dissolve. The term for sand formed by weathering is epiclastic.[7]

Sand from rivers are collected either from the river itself or its flood plain and accounts for the majority of the sand used in the construction industry. Because of this, many small rivers have been depleted, causing environmental concern and economic losses to adjacent land. The rate of sand mining in such areas greatly outweighs the rate the sand can replenish, making it a non-renewable resource.[8]

Sand dunes are a consequence of dry conditions or wind deposition.

Beach sand is also formed by erosion.

Marine sand (or ocean sand) comes from sediments transported into the ocean and the erosion of ocean rocks.

Study


The study of individual grains can reveal much historical information as to the origin and kind of transport of the grain.[12] Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. It is called grus in geology or sharp sand in the building trade where it is preferred for concrete, and in gardening where it is used as a soil amendment to loosen clay soils. Sand that is transported long distances by water or wind will be rounded, with characteristic abrasion patterns on the grain surface. Desert sand is typically rounded.

People who collect sand as a hobby are known as arenophiles. Organisms that thrive in sandy environments are psammophiles.[13]

Uses


  • Abrasion: Before sandpaper, wet sand was used as an abrasive element between rotating devices with elastic surface and hard materials such as very hard stone (making of stone vases), or metal (removal of old stain before re-staining cupper cooking pots).
  • Agriculture: Sandy soils are ideal for crops such as watermelons, peaches, and peanuts, and their excellent drainage characteristics make them suitable for intensive dairy farming.
  • Aquaria: Sand makes a low-cost aquarium base material which some believe is better than gravel for home use. It is also a necessity for saltwater reef tanks, which emulate environments composed largely of aragonite sand broken down from coral and shellfish.
  • Artificial reefs: Geotextile bagged sand can serve as the foundation for new reefs.
  • Artificial islands in the Persian Gulf.
  • Beach nourishment: Governments move sand to beaches where tides, storms, or deliberate changes to the shoreline erode the original sand.[14]
  • Brick: Manufacturing plants add sand to a mixture of clay and other materials for manufacturing bricks.[15]
  • Cob: Coarse sand makes up as much as 75% of cob.
  • Concrete: Sand is often a principal component of this critical construction material.
  • Glass: Sand rich in silica is the principal component in common glasses.
  • Hydraulic fracturing: A drilling technique for natural gas, which uses rounded silica sand as a "proppant", a material to hold open cracks that are caused by the hydraulic fracturing process.
  • Landscaping: Sand makes small hills and slopes (golf courses would be an example).
  • Mortar: Sand is mixed with masonry cement or Portland cement and lime to be used in masonry construction.
  • Paint: Mixing sand with paint produces a textured finish for walls and ceilings or non-slip floor surfaces.
  • Railroads: Engine drivers and rail transit operators use sand to improve the traction of wheels on the rails.
  • Recreation: Playing with sand is a favorite beach activity.
  • Roads: Sand improves traction (and thus traffic safety) in icy or snowy conditions.
  • Sand animation: Performance artists draw images in sand. Makers of animated films use the same term to describe their use of sand on frontlit or backlit glass.
  • Sand casting: Casters moisten or oil molding sand, also known as foundry sand and then shape it into molds into which they pour molten material. This type of sand must be able to withstand high temperatures and pressure, allow gases to escape, have a uniform, small grain size, and be non-reactive with metals.
  • Sandbags: These protect against floods and gunfire. The inexpensive bags are easy to transport when empty, and unskilled volunteers can quickly fill them with local sand in emergencies.
  • Sandblasting: Graded sand serves as an abrasive in cleaning, preparing, and polishing.
  • Thermal weapon: While not in widespread use anymore, sand used to be heated and poured on invading troops in the classical and medieval time periods.
  • Water filtration: Media filters use sand for filtering water
  • Wuḍūʾ: an Islamic ritual wiping of parts of the body
  • Zoanthid "skeletons": Animals in this order of marine benthic cnidarians related to corals and sea anemones, incorporate sand into their mesoglea for structural strength, which they need because they lack a true skeleton.

Resources and environmental concerns


Only some sands are suitable for the construction industry, for example for making concrete. Because of the growth of population and of cities and the consequent construction activity there is a huge demand for these special kinds of sand, and natural sources are running low. In 2012 French director Denis Delestrac made a documentary called "Sand Wars" about the impact of the lack of construction sand. It shows the ecological and economic effects of both legal and illegal trade in construction sand.[16][17][18]

To retrieve the sand, the method of hydraulic dredging is used.

Sand's many uses require a significant dredging industry, raising environmental concerns over fish depletion, landslides, and flooding.[20] Countries such as China, Indonesia, Malaysia, and Cambodia ban sand exports, citing these issues as a major factor.[21] It is estimated that the annual consumption of sand and gravel is 40 billion tons and sand is a US$70 billion global industry.[22]

The global demand for sand in 2017 was 9.55 billion tons as part of a $99.5 billion industry.[23]

Hazards


While sand is generally non-toxic, sand-using activities such as sandblasting require precautions. Bags of silica sand used for sandblasting now carry labels warning the user to wear respiratory protection to avoid breathing the resulting fine silica dust. Safety data sheets for silica sand state that "excessive inhalation of crystalline silica is a serious health concern".[24]

In areas of high pore water pressure, sand and salt water can form quicksand, which is a colloid hydrogel that behaves like a liquid. Quicksand produces a considerable barrier to escape for creatures caught within, who often die from exposure (not from submersion) as a result.

Manufacture


Manufactured sand (M sand) is sand made from rock by artificial processes, usually for construction purposes in cement or concrete.

Case studies


In Dubai, United Arab Emirates, the use of sand has been very demanding in the construction of infrastructure and creating new islands. They used up their own reserves and now import most of their sand from Australia. There have been three projects to create artificial islands needing more than 835 million tonnes of sand, which cost more than US$26 billion.[26]

See also


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