Fly ash Class

The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

Few comprehensive classification schemes for CCP exist. The American Society for Testing and Materials (ASTM 1994) classifies two catgories of fly ash (Class F and Class C) based upon chemical and physical properties of the fly ash (the total amount of Si + A1 + Fe, sulphate, loss on ignition). This classification system was developed for the use of fly ash as an admixture in concrete. More recently, new classification schemes have been developed that place emphasis on textural descriptions, the form of carbon (or char ), and the surface properties of fly ash (Hower Mastalerz 2001). These new classification schemes for fly ash may be the result of growing concern over mercury emissions from coal-fired boilers. Studies have shown that mercury adsorption onto the surface of fly ash particles is a function of both the total carbon content and the gas temperature at the point of fly ash collection (Hower et al. 2000). 

Activated fly ash (class F type (Si02 + AI2O3) > 70%) has been used as low-cost support of Ce(OTf)3 complexes (7 wt%) [102]. The acidic activation was realized to favor the formation of silanol groups at the surface and the subsequent interaction with the triflate complexes. 

Significant dispersion of hexachlorobutadiene has been confirmed by the detection of hexachlorobutadiene at areas which are far removed from release sources (Class and Ballschmiter 1987). A high partition coefficient (log Ko=) value of 3.67 (Montgomery and Welkom 1990) for hexachlorobutadiene indicates that adsorption to soils with high organic carbon content can occur. Wind erosion of contaminated surface soils can then lead to airborne hexachlorobutadiene-containing particulate matter. Levels of hexachlorobutadiene have been detected in fly ash from the incineration of hexachlorobutadiene-containing hazardous waste (Junk... 

Fig. 3. Phase diagram of a portion of ihe Sitb-AbO% system, showing regions of phase separation of undercooled melts. The projected composition of Class-F fly ash glasses lies in the range of 10-4051 mol /t AI,Ot. Adapted from Roth et al. (1987).

Qian, J. C., Lachowski, E. E. Glasser, F. P. 1988. Microstructure and chemical variation in Class F fly ash glass. Materials Research Society Symposium Proceedings, 113, 45-53. 

Fig. 3. X-ray diffractogram of Class-F bituminous coal fly ash. Analytical conditions diffraction data were collected using a Philips X-ray powder diffractometer (45 kV/30-40 mA CuKa theta-compensating variable divergence slit diffracted-beam graphite monochromator scintillation detector) automated with an MDI/Radix Databox. The scan parameters were typically 0.02° step size for 1 s count times over a range of 5-60° 2-theta. All data were analysed and displayed using a data reduction and display code (JADE) from Materials Data Inc., livermore, CA.

Fort Union lignite is low in S (<0.5 wt% S03) and it forms Class-C fly ash that contains Ca-and/or Mg-bearing phases such as lime, anhydrite, C3A, periclase, melilite and merwinite. As Ca and Mg concentrations in the coal increase, so too does the amount of Ca- and Mg-bearing minerals in the fly ash. At the lower range of CaO concentrations (15 wt%), only a small percentage of the total CaO is... 

The largest volume of coal mined in the USA comes from the Powder River Basin (PRB) located in Wyoming and Montana (EIA 1995). Combustion of the PRB coal produces a high-Ca Class-C fly ash (22-32 wt% CaO) that is widely used as a replacement for Portland cement in concrete. An XRD pattern of this... 

Fig. 4. X-ray diffractogram of Class-C lignite fly ash (from Fort Union coal). For analytical conditions, see Fig. 3.

Fig. 7. Scanning electron micrograph of high-Ca Class-C subbituminous fly ash (from Powder River Basin coal).

A relationship exists between fineness and SAI. While 34 wt% fineness meets the ASTM C-618 specifications, finer ash samples have been shown to achieve higher compressive strengths. While sizing on a 325 mesh screen (>45 pm) can be indicative of the amount of fine materials present, it does not directly measure the reactive component of the ash, which is in the very finest fractions (e.g., <5 pm). The presence of this ultra-fine material has a profound impact on strength development of the concrete, as the surface area available for pozzo-lanic cementitious reactions is contained in the finest fractions. For example, the surface area of a typical Class F fly ash, with a mean particle size (D50) of 24 pm will have over 90% of the total surface area accounted for in the <5 pm... 

Blended hydraulic cements are used to conserve energy. They are intimate and uniform blends of fine materials such as Pordand cement, ground blast furnace slag, fly ash, and other pozzolans, ie, fine, reactive silica sources. ASTM C595 lists five classes or types. 

Class E. Inorganic materials, which supply additional fine particles to the mortar pastes and thereby increase the thixotropy, such as fly ash, hydrated lime, kaolin, diatomaceous earth and other raw or calcined pozzolanic materials and various rock dusts. [40, 42],... 

Cold-weather concrete mixtures incorporating non-chloride antifreeze accelerating admixtures have been used in a number of projects. Two of these projects are profiled. All the concrete mixtures described below were treated with the sodium-thiocyanate-based CWA mentioned earlier. These projects illustrate the impact of this admixture on normal concrete mixtures containing Class C fly ash, since fly ash typically delays time of setting and, hence, would not be a logical choice for cold-weather concrete. 

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