Construction aggregate, or simply aggregate, is a broad category of coarse- to medium-grained particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined materials in the world. Aggregates are a component of composite materials such as concrete and asphalt concrete; the aggregate serves as reinforcement to add strength to the overall composite material. Due to the relatively high hydraulic conductivity value as compared to most soils, aggregates are widely used in drainage applications such as foundation and French drains, septic drain fields, retaining wall drains, and roadside edge drains. Aggregates are also used as base material under foundations, roads, and railroads. In other words, aggregates are used as a stable foundation or road/rail base with predictable, uniform properties (e.g. to help prevent differential settling under the road or building), or as a low-cost extender that binds with more expensive cement or asphalt to form concrete.
Preferred bituminous aggregate sizes for road construction are given in EN 13043 as d/D (where the range shows the smallest and largest square mesh grating that the particles can pass). The same classification sizing is used for larger armour stone sizes in EN 13383, EN 12620 for concrete aggregate, EN 13242 for base layers of road construction and EN 13450 for railway ballast.
Sources for these basic materials can be grouped into three main areas: Mining of mineral aggregate deposits, including sand, gravel, and stone; use of waste slag from the manufacture of iron and steel; and recycling of concrete, which is itself chiefly manufactured from mineral aggregates. In addition, there are some (minor) materials that are used as specialty lightweight aggregates: clay, pumice, perlite, and vermiculite.
Biorock, also known as Seacrete or Seament, is a trademark name used by Biorock, Inc. to refer to the substance formed by electro-accumulation of minerals dissolved in seawater. Wolf Hilbertz developed the process and patented it in 1979. The building process, popularly called accretion, is not to be confused with Biorock sewage treatment. The biorock building process grows cement-like engineering structures and marine ecosystems, often for mariculture of corals, oysters, clams, lobsters and fish in salt water. It works by passing a small electric current through electrodes in the water. The structure grows more or less without limit as long as current flows.
Artificial reefs have been built since the 1950s using materials including sunken ships, concrete blocks and discarded tires. However, most of these plans failed to provide coral habitat. Most notoriously, tires were strapped down off the shore of Fort Lauderdale and became an environmental disaster. Some artificial reefs succeeded, but most remain relatively barren compared with natural reefs.
Biorock technology arose from experiments in the 1970s when Hilbertz was studying how seashells and reefs grow, by passing electric currents through salt water. In 1974, he found that as the salt water electrolyzes, calcium carbonate (aragonite) combines with magnesium, chloride and hydroxyl ions to slowly accrete around the cathode, eventually coating the electrode with a material similar in composition to magnesium oxychloride cements and as strong as concrete. Over time cathodic protection replaces the negative chloride ion (Cl-) with dissolved bicarbonate (HCO3-) to harden the coating to a hydromagnesite-aragonite mixture with gaseous oxygen evolving through the porous structure. Later experiments showed that the coatings can thicken at the rate of 5 cm per year. As long as current flows, the structure continues to grow and strengthen. It can heal itself if damaged, making it particularly useful as a replacement for concrete in hard-to-access locations. High levels of dissolved oxygen make it particularly attractive to marine organisms, particularly fin fish.
Hilbertz originally called his invention, on which he obtained several patents, underwater mineral accretion or accretion for short. Hilbertz’s original plan was to use this technology to grow low-cost structures in the ocean for developing countries. He also envisioned accreting large aquadynamic OTEC ocean thermal energy conversion plants, both for generating power and for producing hydrogen, ammonia, and magnesium hydroxide. This appeared to result in a building process largely independent of land-based resources.
To build a biorock reef, a welded, electrically conductive frame, often made from construction grade rebar or wire mesh, is submerged and attached to the sea bottom. A low voltage direct current is applied. This initiates an electrolytic reaction that precipitates mineral crystals naturally found in seawater, mainly calcium carbonate and magnesium hydroxide, on the structure.
Main components of biorock include magnesium hydroxide and calcium carbonate. This composition is chiefly the result of the ionic composition of seawater. One kilowatt hour of electricity accretes about 0.4 to 1.5 kg (0.9 to 3.3 lb) of biorock, depending on parameters such as depth, electric current, salinity and water temperature.