Fly ashes are currently being produced that are much more widely different from each other in composition and other characteristics than had been previously experienced, owing to the widespread use of low rank subbituminous and lignitic coals. The current ASTM classifications into Class F and Class C pozzolan categories are not adequate to describe all their important properties. Current characterization methods are reviewed, including physical characterization by particle size distribution, shape, apparent specific gravity, content of hollow grains and of residual coal fragments, etc., chemical procedures of various kinds, and SEM, EDXA, XRD, and other methods for the determination of mineralogical content and glass character. Etching and chemical dissolution procedures are particularly important. The state of these various methods, current results of their use in rly ash characterizations, and the relations of these to reactivity and performance of fly ashes in cement and concrete are discussed.

Fly ashes of high magnetic content taken from two midwestern power plants were examined to determine the mineralogy of the magnetic and nonmagnetic fractions. Fly ash spheres from the magnetic fraction are predominantly composed of ferrite spinel, hematite and silicate glass. The hematite appears to be a replacement product of the original ferrite spinel. Nonmagnetic phases include mullite, lime, small amounts of hematite and silicate glass. Quartz morphology indicates that it did not fuse in the furnace. Mullite and lime have morphologies indicative of crystallization in the furnace. Hematite is bonded to the nonmagnetic particles or as a complete replacement of ferrite spinel spheres.

X-ray diffraction and scanning electron microscopic analyses have been performed on the various Western Canadial fly ash fractions and on mixtures of cement and fly ash fractions. The ash mineralogy of hydrated and unhydrated fly ash samples was examined by x-ray diffraction. Results from a study of the morphology of ash particles etched in dilute hydrofluoric acid are also included. Scanning electron microscopy indicates that most of the reactive portion of the ash is on the particle surface. Also, as the fly ash particle size increases, the amount of crystalline SiO2 increases but the quantity of calcium containing compounds decreases. Crys~alline SiO2 (the majority of which is present in the +75 μm fraction) does not take part in self-hardening reactions but does appear to take part in pozzolanic reactions.

The zeta-potentials of two fly ashes were studied (high-calcium and low-calcium). It was found that they possess a point of charge reversal at pH = 10.5 to 12. The point of zero charge (low-calcium fly ash) was found to be at pH = 5. Furthermore, it shifted to more acidic values after the fly ash is aged in several calcium-containing solutions. The surficial changes that could happen when mixing fly ashes with cement and concrete were further evaluated by aging fly ashes in different solutions: Ca(OH)2, CaSO4·2H2O, NaOH and water solutions. Information from analyses for different ionic species in the solutions and characterization of the solid residues (XRD and SEM) was used in tentative explanations for the different behavior of the two types of fly ash in cementitious mixtures and concrete.

Many fly ashes contain free lime (CaO) and periclase (MgO) [1,2]. These two compounds, when present in excessive amounts, are known to cause soundness problems in portland cement [3,4]. Recent work [5] has indicated that the autoclave expansion of portland cement-fly ash pastes is related to the concentration of CaO and MgO in a given paste, free lime typically being more detrimental than periclase. The purpose of this technical note is to briefly discuss two methods that are currently available for determining the free lime content of fly ash, and to suggest a supplement to the autoclave test (described in ASTM C 151). The major drawback of the autoclave test is that it requires approximately two days to complete and therefore it would be helpful to have a quick chemical test that could be used to indicate the soundness properties of a given fly ash.

The laser Raman microprobe was used to interrogate individual fly ash particles as small as 1 μm diameter and record “fingerprint” Raman spectra from both crystalline and non-crystalline components.of the fly ash. When compared to reference patterns of known crystalline phases, the Raman spectra can be used to identify crystalline phases and can give some structural information on other phases. Furthermore, because this method characterizes fly ash particles on an individual basis, correlations to both color and morphology of the particles can be made.

Coal combustion produces enormous quantities of residual ash often called bottom ash or fly ash. The fly ash component contains lightweight cenospheres giving the ash a fluffy character. Fly ash is captured in the coal combustion process by air pollution control devices as the gases exit the stack. Fly ash compositions are usually highly siliceous consisting mainly of the oxides of silica, alumina, calcia, and iron oxides; minor constituents such as MgO, alkali oxides, TiO2 etc. are also almost invariably present in quantities of ≈0.5 to 3 wt%. Two important aspects of crystallization of fly ash melts and glasses relate to the prevention of boiler slagging [1] and, from a waste utilization point of view, to the development of new products using fly ash as a raw material[2–4]. Ash devitrification on cooling of the melt results in friable material that does not stick to boiler walls as easily as glassy slag [5]. From another standpoint, crystallization of glassy ash slag to a fine grained equiaxed microstructure is considered a desirable glass-ceramic body for thermomechanical reasons [6].

The hydration of high-calcium and low-calcium fly ash-cementmixtures was investigated to determine the effect of fly ash upon the hydration of a Type I portland cement, and to determine the associated mechanisms of hydration. When blended with portland cement, both fly ashes retarded the early hydration process, the high-Ca more so than the low-Ca. Analyses of solution compositions and calorimetric (heat of hydration) measurements were made. The retardation and hydration effects are discussed in terms of solution composition data and solid phase characterization. The hydration effects were interpreted and compared with the results of previous work.

The setting and strength gaining process of PC concrete containing Class C high lime fly ash were related to the reaction products identified using XRD and SEM. Four fly ash concrete mixes (20, 30, 40, and 50 percent replacement of cement by fly ash) and similar paste mixes were compared to control mixes for curing periods up to one year. Setting time and early compressive strength were adversely affected by the addition of fly ash. Beyond one week all of the fly ash concrete mixes gained strength at a faster rate than the corresponding control mixes. XRD studies suggest that the retardation mechanism may be associated with the high levels of ettringite formed early in the hydration process and its conversion to monosulfoaluminate. A decrease in the level of calcium hydroxide, typical of pozzolanic activity, was not in evidence. SEM micrographs of fly ash spheres in concrete at the various stages of hydration reveal an intricate crystal framework. A simple heat of hydration test is presented which helps explain the strength gains observed.

The influence of the chemical composition of fly ash on the autoclave expansion of portland cement-fly ash pastes has been studied. The autoclave expansion tests were performed as described in ASTM C 151–81. Several of the cement-fly ash pastes tested exhibited severe expansive behavior. Free lime (CaO) was the only single variable that correlated well with the expansion observed in the autoclave tests. A multivariable model was generated in which lime, magnesium oxide, and tricalcium aluminate were the significant variables. The autoclave expansion of portland cement-fly ash pastes appears to be quite sensitive to the amount of free lime present in a given mixture.

Cementitious slurries composed of an oil well cement, a high calcium fly ash, a low calcium fly ash, and three commercially available superplasticizers (two different sulfonated naphthalene formaldehyde condensates and a sulfonated melamine formaldehyde condensate)were mixed to contrast the two fly ashes and to determine the effectiveness of each of the superplasticizing agents. Most commercial superplasticizers and cements are relatively expansive; therefore a partial substitution by fly ash and other by-products represents a substantial savings in both quantity of chemical admixture required and energy consumption for the manufacture of cement. In the cement/fly ash mixtures of 100/0, 90/10, 60/40, 40/60, and 10/90, with both high calcium fly ash and low calcium fly ash, the mixture containing the low calcium fly ash was consistently less workable. The rheological properties of the high vs. low calcium fly ash mixtures were controlled by the differences in fly ash particle size and the presence of irregular large particles, rather than by the differences in chemistry between the two. The low calcium fly ash was the coarser material.

The flexural strength, fracture toughness and Young's Modulus of an ordinary Portland cement plus 25 wt% fly ash paste (FACP) were compared with those of a control paste (HCP) at times ranging from one day to one year. At comparatively early times these properties develop more slowly in FACP. However, at comparatively late ages, e.g. ten months, FACP showed improvements over HCP for all of these properties. A complicating factor is that for both FACP and HCP, the flexural strength and toughness show maxima and the “crossover” times for the various properties differ. This paper analyses these results in terms of the observed fracture mechanism for HCP by taking into account the changes brought about in the microstructure of FACP, due to the incorporation of fly ash.

A fluidised bed combustion (FBC) process has been developed to a stage that both coarse rejects and tailings from coal washery plants may be successfully combusted. During the combustion process excess heat is produced, and the products from the process, spent rejects and primary and secondary cyclone fines, are reduced in volume compared to the input material. Results of preliminary investigations into the properties of the spent rejects and cyclone fines are reported. A program has been initiated to investigate and evaluate the potential uses of the FBC process, including the use of the products in stabilised and unstabilised forms in road construction, as a material suitable for the construction of embankments, and in lean concrete mixes either as an aggregate or as a pozzolan.The characteristics of the FBC products are compared, where appropriate, with those of the unburnt coal washery wastes.

In contrast with pozzolanic bituminous fly ashes, the western American lignite and subbituminous fly ashes are both pozzolanic and cementitious. During the last three years, there has been much research and actual use of western ash alone and with lime or cement for roadbed stabilization. In contrast with lime stabilization which only applies to clay, the western ashes can be used to stabilize clay as well as silty and granular soils. Fly ash is moving from the experimental to standard alternative for pavement in many states. Some examples of western U.S. experience will be presented.

During cementing operations, the hydrostatic pressure of the cement slurry column is utilized to control the reservoir pressure. However, when the well extends through weak formations, it is often necessary to lighten the hydrostatic pressure of the slurry by lowering the density to avoid hydraulically fracturing the reservoir rock. Densities of cement slurries can be lowered by adding pozzolanic lightweight additives such as fly ash. Fly ashes have found utility in numerous oil and gas well cementing applications, namely: 1) fly ash can be dry-blended with regular Portland cements or hydrated lime for use in primary cementing; 2) lightweight cement compositions can be formulated by adding lime and finely ground quartz to fly ash/cement mixtures; and 3) an ultralight-weight pozzolanic microsphere cement, with densities as low as 0.96–1.64 g/cm3, can be formulated by blending Portland cements with hollow microspheres isolated from fly ash. This paper discusses the physical and chemical properties of each of these cement slurries, along with their relative advantages and disadvantages. Areas of future research are also discussed.

Recent studies at Oak Ridge National Laboratory (ORNL) and elsewhere have identified various chemical processes for recovering useful materials, such as alumina and iron oxides, from coal combustion fly ash. Based on certain assumptions, each of these processes can yield useful products at economical prices. Most processes leave a residual solid waste with a volume only slightly less than that of the original waste. This residue may not present a serious hazard, but its volume alone makes disposal difficult for utilities with limited land available for disposal sites. Characteristics of some of the residues are being studied to determine possible beneficial uses or applications of these by-products.

Utilization or disposal of gasification ash requires detailed characterization of its chemistry and phase formation (mineralogy). A North Dakota lignite ash produced in the Morgantown Energy Technology Center (METC) gasifier has been studied in detail by x-ray diffraction and electron microprobe analysis. The ash was coarse (84% of grains larger than 1.0 mm) but a typical grain was composed of a dozen or more crystalline phases with dimensions on the micrometer scale as well as less abundant glass phases. Hard centimeter-size clinkers suggested partial melting followed by crystallization. Silicates (dicalcium silicates (C2S), merwinite, Ca-Na-silicate (CNS), quartz), aluminosilicates (melilite, nepheline, carnegieite), oxides (ferrite spinels, periclase, hematite), calcite and minor zeolites comprised the dominant mineralogy. Microprobe analyses were obtained for large numbers of grains of the C2S phases, CNS, merwinite, melilite, ferrite spinels and calcite. The remaining phases had crystal sizes too small for analysis. A model is proposed for the genesis of this ash based on the inorganic constituents of lignite and the gasifier operating conditions.

Lignite coal ash produced in a stirred, fixed-bed gasifier at the Morgantown Energy Technology Center (METC) has been studied by scanning electron microscopy using both morphologic and chemical analyses. Detailed chemical analyses of phases, grains, and traverses across grains illustrate the heterogeneous nature of the ash. These data support the concept that most of the ash was formed by partial melting and subsequent crystallization and agglomeration.

Lignite gasification ash from the Morgantown Energy Technology Center (METC) gasifier was subiected to two short-term leaching treatments. The cnncentrations of regulated elements in the EPA EP procedure leachate from the bulk METC ash did not exceed the “EP Trigger Limits.” A modification of this procedure that uses a basic synthetic groundwater instead of the acid EP extractant was also performed on the bulk ash and its eleven size fractions. Water equilibria modeling was used to explain the concentrations of major elements in solution. Corcentrations of minor and trace elements in solution after leaching with synthetic groundwater were also below “EP Trigger Limits.”

Fly ash and gasification ash have been evaluated economically for the following uses: partial replacement of Portland cement; mineral wool; blended cement; Sulfurcrete®, high flexural strength ceramic products; ash to upgrade roads; glazed ceramic wall tile; and unglazed floor tile. The ash evaluated is a high-calcium, high-sodium material derived from the Beulah-Zap lignite mined in the Beulah region of North Dakota. Of the uses examined, concrete replacement provided an 8.0% cost saving, blended cement 37.3%, high flexural strength ceramics 52.8%, ash in road construction 44.4%, and wall tile 5.2%. Mineral wool had no replacement savings calculated because blast furnace slag is not available locally to provide a consistent basis. Sulfurcrete® did not provide a cost saving over concrete but its use life and properties are sufficiently different from those of concrete to justify use in some applications, provided that the raw materials are readily available.