Sand

Sand: 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.

Raw Materials: 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. The white sands of White Sands, New Mexico, are made of gypsum crystals. Gypsum is dissolved by rain water, but the area around White Sands is so arid that the crystals survive to form undulating dunes.

The Manufacturing Process: 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 following steps are commonly used to process sand and gravel for construction purposes.

Natural decomposition: Solid rock is broken down into chunks by natural mechanical forces such as the movement of glaciers, the expansion of water in cracks during freezing, and the impacts of rocks falling on each other.

The chunks of rock are further broken down into grains: By the chemical action of vegetation and rain combined with mechanical impacts as the progressively smaller particles are carried and worn by wind and water.

As the grains of rock are carried into waterways: Some are deposited along the bank, while others eventually reach the sea, where they may join with fragments of coral or shells to form beaches. Wind-borne sand may form dunes.

Extraction: Extraction of sand can be as simple as scooping it up from the riverbank with a rubber-tired vehicle called a front loader. Some sand is excavated from under water using floating dredges. These dredges have a long boom with a rotating cutter head to loosen the sand deposits and a suction pipe to suck up the sand.

Sorting: In the processing plant, the incoming material is first mixed with water, if it is not already mixed as part of a slurry, and is discharged through a large perforated screen in the feeder to separate out rocks, lumps of clay, sticks, and other foreign material. If the material is heavily bound together with clay or soil, it may then pass through a blade mill which breaks it up into smaller chunks.

Washing: The material that comes off the coarsest screen is washed in a log washer before it is further screened. The name for this piece of equipment comes from the early practice of putting short lengths of wood logs inside a rotating drum filled with sand and gravel to add to the scrubbing action.

A modern log washer consists of a slightly inclined horizontal trough with slowly rotating blades attached to a shaft that runs down the axis of the trough. The blades churn through the material as it passes through the trough to strip away any remaining clay or soft soil. The larger gravel particles are separated out and screened into different sizes, while any smaller sand particles that had been attached to the gravel may be carried back and added to the flow of incoming material.

Crushing: Some sand is crushed to produce a specific size or shape that is not available naturally. The crusher may be a rotating cone type in which the sand falls between an upper rotating cone and a lower fixed cone that are separated by a very small distance. Any particles larger than this separation distance are crushed between the heavy metal cones, and the resulting particles fall out the bottom.

Quality Control: 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.

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