Class C fly ash

As with pfa low in CaO, fine, spherical particles consisting largely of glass predominate, but the glasses are higher in CaO and the assemblages of crystalline phases are different. In a study of 26 ashes from the western USA. McCarthy el al. (M84) found that quartz, lime, periclase, anhydrite and a ferrite spinel were ubiquitous or nearly so. Other crystalline phases present in some cases or in minor amounts included C3A, merwinite, alkali sulphates, melilites, mullite, sodalite and possibly others. Due to the presence of 

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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... 

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. 

The density of the class C fly ash product is slightly higher than that of class F. This difference may be because class F ash contains more carbon and, hence, is slightly lighter. Since the ash and the binder powders have nearly the same density, the ash loading has hardly any effect on the density. Overall, the ash products are approximately 25% lighter than the corresponding cement products. 

Compressive strength of Ceramicrete with 60 wt% Class C fly ash, as a function of curing time. 

Ceramicrete formulations with Class C fly ash are suitable for shallow wells. Though Class C fly ash reacts more rapidly due to its calcium content, it also responds favorably... 

Fly ash starts out as impurities in coal, mostly clay, shales, limestone, and dolomite, which ends up as ash, and fuse at high temperature becoming glass. Two U.S. classifications of fly ash are produced. Class C and Class F, according to the type of coal used. Class C fly ash, typically obtained from subbituminous and lignite coals, must have more than 50% total of silica, alumina, and iron oxide. Class F fly ash, typically obtained from bituminous and anthracite coals, has more than 70% of these oxides. 

In mixes made with class C fly ash, the reactive constituents of this ash, other than the glass phase, such as C3 A, C2S, C, and CS, also participate in the hydration process. 

Figure 9.2 Compressive strength of Portland-fly ash cements made with different additions of class C fly ash. Composition of ash Si02>...

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