Phosphate concrete

In most applications, a small amount of binder powders is mixed with a large volume of inexpensive hllers and then the entire mixture is stirred in water to form the reaction slurry. For example, if the phosphate binders are used for manufacturing construction products, invariably the hllers are sand, gravel, ash, soil, or some mineral waste. The phosphate binders provide adhesion between the particles of these hllers and bind them into a solid object. Thus, these mixtures mimic conventional concrete mixmres in which Portland cement binder is mixed with large volume of sand and gravel to produce cement concrete. When phosphate binders are used, the products may be termed as phosphate concrete . In waste stabilization, the waste itself becomes the hller and the hnal product is termed as a waste form . 

Generally, a small amount of the binder is used in cement or phosphate concrete the major volume of the concrete is aggregates. The small amount of binder keeps the overall cost of the product low. The components of the aggregates, however, do not participate in the setting reaction. On the other hand, in the case of a CBPC the reaction of the aggregate will entirely depend on its solubility in the acidic phosphate solution. Since silica (Si02) is insoluble in an acid solution, it is safe to assume that it will not participate in any setting... 

Fig. 3.1.9. Phosphatic concretion from the valley of the Dniester River, Uk.R.S.S.R. showing euhedral crystals of a variety of apatite — presumably francolite — within a matrix of quartz. The concretions also contain glauconite, pyrite, etc. Magnification 67x. Reproduced with permis.sion (McConnell, 1950).

Fig. 3.1.10. Phosphatic concretion from Brazos County, Texas. The large dark object is an appendage of a fossil crab, the tip of which is truncated by a calcite vein. The matrix material is essentially collophane (isotropic francolite), but contains glauconite, quartz, limonite, pyrite, gypsum, etc. in addition to fossil fragments. Magnification 27x. Reproduced with permission (McConnell, 1950).

Fig. 3.1.11. Phosphatic concretion from the middle Eocene (Claiborne fauna), Brazos County, Texas (see also Fig. 3.1.10). Dense, almost opaque, collophane shows contraction fractures some of which now contain pyrite. The light-colored outer rim is the result of weathering. Reduced to about 1/2. Specimen courtesy of H.B. Stenzel.

These materials are now widely used for coating both steel and concrete surfaces that are subject to a particularly aggressive environment (e.g. North Sea oil platforms). There is less validity for their use under normal atmospheric conditions since they are relatively expensive and tend to chalk on exposure to sunlight. However, their use as zinc phosphate, pretreatment or blast primers for blast-cleaned steel which is subsequently overcoated by any other paint system is an extremely valuable contribution to the painting of new steel work. 

Abdelrazig, B. E. I. Sharp, J. H. (1988). Phase changes on heating ammonium magnesium phosphate hydrates. Thermochimica Acta, 129, 197-215. Abdelrazig, B. E. I., Sharp, J. H. El-Jazairi, B. (1988). The chemical composition of mortars made from magnesia-phosphate cement. Cement Concrete Research, 18, 415-25. 

Sugama, T. Kukacka, L. E. (1983a). Magnesium monophosphate cements derived from diammonium phosphate solutions. Cement Concrete Research, 13, 407-16. 

Miller, W.L., N.J. Blake, and R.H. Byrne. 1985. Uptake of Zn65 and Mn54 into body tissues and renal concretions by the southern quahog, Mercenaria campechiensis (Gmelin) effects of elevated phosphate and metal concentrations. Mar. Environ. Res. 17 167-171. 

Prayon One of the Wet processes for making phosphoric acid by reacting phosphate rock with sulfuric acid. The byproduct is gypsum, calcium sulfate dihydrate. It uses a compartmentalized, multi-section, lined, concrete reactor, with finishing tanks in which the gypsum crystals mature. In 1990 one third of the wet process phosphoric acid made in the Western World was made in this way. The process was developed in 1977 by the Societe de Prayon, Belgium. Variations are known as PH2, PHI 1, and PH12. One variation uses solvent extraction with isopropyl ether and tri-n-butyl phosphate. 

There are two useful side products. The H2Sip6 is shipped as a 20-25 % aqueous solution for fluoridation of drinking water. Fluorosilicate salts find use in ceramics, pesticides, wood preservatives, and concrete hardeners. Uranium, which occurs in many phosphate rocks in the range of 0.005-0.03% of UsOg, can be extracted from the dilute phosphoric acid after the filtration step, but this is not a primary source of the radioactive substance. The extraction plants are expensive and can only be justified when uranium prices are high. 

The mechanism of action by which silane and siloxanes reduce expansion has been attributed to water repellence and air entrainment. Phosphate addition or coatings may interfere with the dissolution of silica gel and the formation of gel. It is also possible that phosphate reduces the osmotic potential and the swelling pressure in the gel. The manner in which air entrainment reduced expansion was attributed to the accommodation of alkali-silica gel in the air void system. For example, it was found that air-entrained concrete with 4% air voids could reduce AAR expansion by 40% [23]. 

Ceramicrete is an ex situ stabilization technology that uses chemically bonded phosphate ceramics to stabilize low-level radioactive waste and hazardous waste containing radionuclides and heavy metals. The technology mixes phosphates with acidic solution, causing an exothermic reaction similar to that used in forming concrete. But while concrete is based on relatively weak hydrogen and van der Waals bonding, Ceramicrete uses a combination of ionic, covalenf and van der Waals bonds to stabilize contaminants. 

ISOTRON Corporation s electrokinetic decontamination process is a patented, in situ process for the removal of contaminants from soil, groundwater, and porous concrete. The technology applies a low-intensity direct current (DC) across electrode pairs to facilitate electromigration and electro-osmosis of contaminants. The process works primarily on highly soluble ionized inorganics including alkah metals, chlorides, nitrates, and phosphates. Heavy metals such as lead, mercury, cadmium, and chromium have also responded favorably. 

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