Siliceous fly ash

Foamed cement slurries have been used to provide a low density cement slurry to reduce permeability damage to highly sensitive formations through reduced fluid loss (29). Glass microspheres have also been used to substantially reduce cement slurry density (30, 31). Other additives which reduce cement slurry density to a lesser extent include bentonite, fly ash, silicates, perlite, gilsonite, diatomaceous earth, and oil emulsions (see citations in reference 29). 

These disadvantages make fusion a less than ideal technique for extreme trace element determination. However, for the determination of major, minor, and even some trace elements in such matrices as fly ash, silicates, slags, and dust good results can be obtained. 

The significant increase in the use of supplementary cementing materials (such as fly ash and slag) in the last decade has dictated the need for an admixture that can offset the slowed hydration that results when such materials are incorporated in concrete. Strong basic salts such as sodium aluminate, alkali hydroxides, silicates, sulfates and thiosulfates have shown some promise. A number of proprietary admixtures which claim to catalyze the pozzalanic and thereby increase the rate of hydration are now marketed. 

Pozzolanic S/S systems use portland cement and pozzolan materials (e.g., fly ash) to produce a strucmrally stronger waste/concrete composite. The waste is contained in the concrete matrix by microencapsulation (physical entrapment). It is a chemical treatment that uses commercially available soluble silicate solutions and various cementious materials such as cement, lime, poz-zolans, and fly ash. By addition of these reagents and rigorous mixing, the waste is fixed or stabilized. Contaminant mobility is reduced through the binding of contaminants within a solid matrix, which reduces permeability and the amount of surface area available for the release of toxic components. 

Terrafix units are trailer-mounted, fully transportable units that can be operational within hours after arrival at the site. The units are designed to screen out large material such as rocks and metal parts as well as remove ferrous material using magnetic separation. The remaining material is then mixed in a pug mill with cementitious materials (i.e., cement, fly ash, pozzilime) and/or silicates to produce a material in which the heavy metals are chemically fixed. 

Fig. 2. Pseudotemary diagram showing projected compositions in mol% of coal combustion fly ashes and of phases present in ash. Polymorphs of dicalcium silicate include lamite. Bulk composition of fly ash lies within the field bounded by the shaded thick line.

Sterling, R.O. and Helble, J J. (2003) Reaction of arsenic vapor species with fly ash compounds kinetics and speciation of the reaction with calcium silicates. Chemosphere, 51(10), 1111-19. 

Fly ash, as it is a large-volume industrial waste, is both cheap and abundant, so that there is an economic incentive to use fly-ash-modified cements. In addition, C02 is also produced as a waste by-product of industrial processes (power generation, cement manufacture, etc.), and its permanent sequestration into cement is an added environmental benefit. A fully carbonated Portland cement permanently sequesters about 130 L of C02 per kilogram of cement. Figure 15.8 shows the structural and chemical modifications produced in cemented fly ash microspheres as a result of the supercritical C02 treatment. As is the case with fly ash, kiln dusts are primarily siliceous, so that the same benefits can be derived from their use as modifiers in immobilization and S/S matrices. 

Fe203 and 55 weight % silica, as support, with 1% sodium silicate as a binder. In the preliminary work fly ash was considered as a support material but the maximum amount of iron oxide which could be incorporated with it was 25 weight % Based on this and the other criteria discussed above, the iron oxide-silica composition was found to be the most suitable sorbent for bulk H2S removal,... 

A correlation between the differences in the amount of Ni and Fe present in a majority of these ashes and their leachability was not observed. This lack of correlation suggests that these elements are in a matrix that is not readily solubilized by the water used to generate the leachate. Hansen and Fisher (4) have shown the iron to be primarily in the insoluble silicate matrix and nickel appears to be associated with an acid insoluble phase. However, for all of the other elements, the differences in amounts leached from the fly ashes were correlated with the bulk differences found in the fly ashes. 

Fly ash the airborne combustion residue from burning coal or other fuels consists mainly of various oxides and silicates. 

Lime/fly ash pozzolanic processes combine the properties of lime and fly ash to produce low-strength cementation. Kiln dust processes involve the addition of kiln dust to eliminate free liquids and usually form a low-strength solid. Lime-based processes for solidification use reactions of lime with water and pozzolanic (siliceous) materials, such as fly ash or dust from cement kilns, to form concrete, called a pozzolanic concrete. Wastes of desulfurization of gases and other inorganic wastes can be immobilized by this method. 

C-S-H = poorly crystalline or amorphous calcium silicate hydrate of unspecified composition. Ggbfs = ground granulated blast furnace slag. Hep = hardened cement paste. Pfa = pulverised fuel ash (fly ash). 

Due to the increasing cost of energy, the need to preserve the environment, and the non-existence or exhaustion of suitable natural raw materials in some areas, industrial and other waste materials are of interest as possible raw materials or supplementary fuels or both. Energy can be saved if even a part of the CaO can be provided by a material, such as blastfurnace slag, that does not require decarbonation. Supplementary fuels include such materials as used or reject tyres and pulverized household refuse, which can be introduced into the system in various ways. Some materials, such as pulverized fuel ash (pfa fly ash) can serve as raw materials that also possess some fuel content. Other wastes that have been used include calcium silicate residues from aluminium extraction, mining residues, and precipitated calcium carbonate from various industries. 

In the lime-based process, both lime and siliceous materials are used to solidify waste. Additives such as fly ash are also added to increase the... 

Common rock-forming mineral, although most Al-bearing silicates quite insoluble. Potentially soluble Al-hydroxides, hydroxysulfates form in lateric ore deposits, tropical soils, and precipitate in streams affected by acid-rock drainage. Al-rich soluble salts can occur in evaporative lake sediments, and in mine wastes. Potentially reactive forms in cement, concrete, smelter emissions, coal fly ash. 

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