A database of chemical, mineralogical and physical characteristics of North American fly ashes has been assembled by the Western Fly Ash Research, Development and Data Center. One-hundred and seventy-eight representative ashes were divided into three groups according to their analytical CaO content: low-calcium, <10%; intermediate-calcium, 10–20%; high-calcium, 20+%. Statistical analyses were performed within each of the three groups. Thirty-two plots relating chemical composition, mineralogy and physical test results are presented. Extensive discussions relating the chemistry and mineralogy of the ash to the source coal, the distribution of the chemical constituents among crystalline and glassy phases, and the reactions of these phases in concrete are given. The consistency of high-calcium fly ash generated at a Midwestern U.S. power station fired with Wyoming bituminous coal was studied using ninety-three ashes collected over a two year period. The availability of mineralogy for each ash leads to a more thorough understanding of the bulk chemical and physical test results used in evaluating fly ashes for utilization, and in modeling their behavior after disposal

The applicability of differential thermal analysis to determine the fusibility behavior of coal ashes was investigated. The technique proved useful to obtain four specific temperature points. These are: the minimum sintering temperature, the softening point, the point of complete fluidity and the reaction temperature which is tentatively identified as the eutectic temperature. In addition, differential scanning calorimetry was used to determine the heat capacity of the ash powders between 100K and 850K. Ash heat capacities were correlated with ash constitution

The application of different characterization techniques to the study of coal fly ash formation and of its behaviour in particulate control devices is discussed. Particle Induced X-ray Emission (PIXE) and Proton Induced Gamma-ray Emission (PIGE) are used for bulk elemental analyses, while X-Ray Photoelectron and Auger Spectroscopies are used for surface elemental analyses. Structural characterization is carried out by powder X-Ray Diffraction and Raman Microfocus Spectroscopy. The effect of different collection efficiencies of electrostatic precipitators on emitted fly ash characteristics is addressed. Furthermore size fractionated fly ash is analyzed to investigate the mechanisms of particle growth.

The kinetics and mechanism of the hydration reactions in slag cements were explored in blends containing either 40 or 50 wt% ground granulated blast furnace slag, made to a water:solid ratio of either 0.50 or 0.60 and aged 7d to 885d at 20°C. The microstructure of the slag rapidly differentiates into zonal structures. Analytical electron microscopy has been used to analyze the composition and indirectly determine the mineralogy of the zones. It is shown that considerable chemical matter is transported out of the slag grains, resulting in porosification of the former slag grains and densification of the cement hydrate matrix. This is supported by mass balance calculations. The driving force for these migrations is accounted for by the existence of a chemical potential gradient of water; hydrophyllic species such as Ca, Si, etc. migrate into regions of high chemical potential for water. The densification of the phase matrix is probably responsible for the low permeabilities of matured slag-cement blends.

Due to the particle to particle variation of coal mineral properties and random coalescence of mineral particles during coal burnout, fly ash particle properties change from particle to particle. The variations of particle properties (e.g. SiO2 content, viscosity) can be mathematically described by random variables. Since bulk analysis of fly ash gives only the mean values of chosen random variables, it is considered insufficient to describe the fly ash behavior either in boiler slagging/fouling or in different concrete structures. SEM-AIA-EDAX technique was used to supply raw data for estimating the distribution functions of particle size and chemical compounds in Texas lignite minerals and fly ash and Eagle Butte fly ash. To determine the volume based size distributions of these samples from their area-based size distributions, Abelian transformation was used. To estimate the distribution functions of CaO and SiO2 contents of the samples, particle area fractions were used. The confidence limits were also calculated for the estimated parameters. By determining the distribution functions of particle viscosity and chemical composition, it was shown that in the case of Texas lignite the coal burnout does not cause significant changes in the mineral matter properties. It was observed that the properties of fly ash depended solely on the mineral matter properties. However, in the Eagle Butte case the coal burnout has a major effect on the fly ash size distribution and its chemical composition.

Long-term batch leaching studies of composites of lignite combustion and gasification ashes with a calcium-based scrubber waste have shown the prominent formation of an ettringite structure phase, accompanied by reductions in solution concentrations of potentially hazardous elements such as boron and selenium. The possibility that oxyanions such as arsenate, borate, chromate, molybdate, selenate and vanadate might substitute for sulfate in the ettringite structure has been explored. There are literature reports of fully substituted borate, chromate and selenate [ettringites]*, and of two minerals with partial borate for sulfate substitution. X-ray diffraction phase pure products with chromate and selenate, substituted completely for the sulfate in ettringite, and with arsenate, borate and vanadate partially substituted for sulfate, have been synthesized at room temperature by mixing soluble Ca, Al and sulfate sources and maintaining a pH> 12 with NaOH additions. Attempts to substitute substantial amount of molybdate for sulfate were unsuccessful. The resulting phases were characterized chemically by ICAP spectrometry, ion chromatography and thermal analysis, and for phase purity and unit cell size by XRD. The speciation of the oxyanions in the substituted [ettringite] were confirmed by FTIR spectrometry. In solid solutions, the sulfate/oxyanion ratio was greater in the precipitated solid than in the synthesis solution. Chemical analyses of the [ettringites] did not give simple stoichiometries analogous to Ca6Al2(SO4)(OH)12 ·26H2O. Because nonstoichiometry can be attributed to numerous possibilities for charge balance and defects, structural formulae of the oxyanion substituted [ettringites] could not be established.

The natural weathering process of fly ashes was studied in artificial fly ash beds. The ashes were previously classified according to their total alkali content into acid, neutral and alkaline. After 4 years the effects of weathering are minor. The formation of concretions due to local cementation by calcite is observed in alkaline fly ash. Small brown pellets, highly enriched in some heavy metals, are present in the acid fly ash beds. Under natural conditions leaching is mainly confined to the alkaline fly ash. Elements which are leached most are sulphur and some alkalis as well as minor amounts of some trace elements.

Shake- and column experiments of the weathered fly ashes with acids of pH 1 and 4, the latter also in combination with complexing agents, were carried out in order to check if the natural leaching process may be accelerated artificially. Strong acids (pH= 1) were most effective, but relatively large quantities of major elements were also leached. Experiments with the weak acids, combined with complexing agents, resulted in leaching percentages which did not significantly differ from experiments without these agents. This indicates that a major part of the elements are present in an immobile form; because of the use of sulphuric acid, it is likely that they are in some cases precipitated as sulphates (e.g. barium, calcium).

The chemistry of individual grains of several Class F fly ashes has been investigated by electron microprobe analysis. To interpret the analytical data-set, a combination of two multivariate statistical procedures was applied. The method involves the use of both fuzzy c-means cluster analysis (FCM) and non-linear mapping (NLM). It allows for a quick visual interpretation of the fly ash analysis and provides information which is not obtained through the use of the commonly used classical two-or three dimensional data representation plots alone. The method provides information on the occurrence of “outlying” samples, such as quartz grains, lime, calcium-sulfate or iron-containing particles. When the data-set is truncated for these outlying particles, cluster analysis of all ash data indicates that a model of 4–5 clusters can best explain chemical differences between the bulk of the particles of several class-F fly ashes. These clusters are related to each other by an inverse relation between the SiO2 and the A12O3 content, and some clusters display marked differences of either the oxides of Ti, Fe, Ca, Mg, K and/or K and Na. Inter-grain variation of one separate fly ash indicates that similar variations occur within individual fly ashes, only the absolute differences being smaller. The results are interpreted to represent either the coalescence or fusion of chemically different particles or small chemical differences in precursor clay minerals. The method of data-reduction presented here provides quick information on chemical relationships and is particularly suited to evaluate small data-sets. The method has potential to be combined with other input parameters, such as pozzolanic activity, glass content, particle size distribution and others.

The evolution and distribution of calcium hydroxide, CH, and the development of microstructure during the hydration of three low-calcium fly ash-Portland cement blends with water-solids ratio (w/s) of 0.40 have been investigated. During the first month of hydration, the CH content of the blends was found to be relatively higher than the plain mix, if a dilution effect due to replacement of cement by an inert material is taken into account. After 28 days of hydration the CH content in the blends began to decrease. SEM observations of specimens revealed the occurrence of large, well-crystallized CH plates in intimate contact with some of the fly ash particles at younger ages and even after six months of aging. The study also showed that the chemistry of the pore solution in contact with the hydrating cement system and the characteristics of the fly ashes, such as the glass content and the fineness of the ash particles seem to exert major influences on the rate of evolution of CH in the fly ash blends. Semi-quantitative X-ray diffraction analyses performed on specimens cast against polypropylene plastic plates used to “model coarse aggregates” showed reduction in the thickness of the interfacial zone for the fly ash-Portland cement pastes from about 60μm to less than 15μm within one month of hydration. In the case of the plain Portland cement paste no significant change was observed. The degree of orientation of CH crystals within the interfacial region also was significantly affected by the fly ashes, although by this age of hydration the CH data showed little or no evidence of pozzolanic reaction.

The reactivity of fly ash in cementituous systems has been an object of many studies, and is only understood in broad terms. It is generally agreed that the particle size distribution of fly ash correlates with strength development. It has also been demonstrated that temperature and pH development of the blended cement paste have an effect on fly ash reactivity. It is however not clear whether ash chemistry and/or ash glass structure have an influence on its reactivity, as is the case with some blast furnace slag cements. In order to discriminate between the many variables controlling fly ash reactivity, the dissolution kinetics of class-F fly ash have been studied. The fly ash was first separated into equal size and density fractions. These were reacted with NaOH, with a pH ranging from 13.0 to 13.7 and temperatures between 20 and 40°C. If the results of the dissolution experiments are corrected for differences in glass content and particle size, the effects of temperature and pH seem to be of more importance than differences in ash chemistry. The dissolution proceeded virtually congruent. TEM micrographs of some of the ash fractions indicate that a wide variety of glasses may be encountered in fly ashes; most glasses show evidence of phase separation. The results are discussed with respect to glass structure and glass dissolution-theory.

The Taiwan Power Company's (TPC) coal combustion power plants produced approximately 1 million tons of class F fly ash in 1988 but only 25% of it was effectively utilized. Accordingly, this study investigated the use of separation techniques to improve the quality of the fly ash in order to increase its application. In the experiment, the fly ash was dry-sieved through sieves No. 200, 300 and 400 mesh respectively. The chemical, physical and some of the engineering properties of those classified class F fine fly ashes were ascertained.

Partial substitution of Portland cement (PC) with fly ash (FA) offers the practitioner expanded scope for optimal utilisation of materials. This method requires careful evaluation of the resulting blended cement beyond the basic age-strength relationship. For the construction practitioner, workability, rate of strength gain, and inherent variability are major factors in meeting strength requirements and determining costs. A blend may be characterised by a mass efficiency factor which is the mass ratio of PC to FAwhich will yield the same target strength at fixed maturity and workability. Alternatively, the maturity efficiency factor may be determined as the ratio of log maturity values for the cements to reach a given target strength. Both efficiency factors incorporate important economic parameters of interest to the practitioner.

Because of its glassy character, FA attracts calcium hydroxide as well as sodium and potassium ions released into solution by the PC component. The risk of alkali-aggregate reaction may therefore be reduced. This characteristic may prove to be of such over-riding importance that a new approach to mix design may become necessary. The essence of such an approach would be the provision of sufficient FA to absorb reactive species released by the PC. This might involve increased PC+FA and water, with concomitant reduction in aggregate and in fineness modulus.

Dimensional changes such as shrinkage and creep affect the distribution of load between steel and concrete in columns, the deflection of beams and slabs, and cracking where displacements are restrained. In the case of PC concrete, both creep and shrinkage depend on the volume ratio of hydration products and the movement of pore water caused by drying and by mechanical load. In general the magnitude of creep and shrinkage varies with Vp, the volume proportion of cement paste, and reduces with wo*. Part substitution of PC with FA generally results in increase of Vp, but this is compensated by decrease in wo*. The nature of the hydration products varies with PCiFA ratio, and thus quantities such as the internal surface area, the hydraulic mean radius of pores and the effective modulus—the ratio of stress to total strain—must be evaluated.

In general the data required by the practitioner is not available in the literature and he must have recourse to ad hoc tests in order to establish basic workability-time-temperature-strength data. This is, potentially, a fruitful area of research.

A European view of the use of fly ash in concrete, if there is one, is very confused. Mehta [1] produced figures for the production and utilisation of fly ash in the cement and concrete industries for 1984, ranging from 10% utilisation in the UK where the cement makers are not enthusiastic, to 80% in the Federal Republic of Germany and even 94% in the Netherlands according to Fraay, Bijen, de Haan and Larbi [2]. Looking a little deeper into these figures the CEGB Ash Marketing Board says that it sells over 40% of its production of fly ash of some 14 million tons/year, much of it for landfill, while the figures for Germany relate only to the 2.6–2.9 million tons of ash from black coal; 70% of the German ash comes from brown coal, and all of this is returned to the open-pit mine for dumping. This paper will review some of the European papers in the Third CANMET/ACI International Conference on Fly ash, Silica Fume, Slag and Natural Pozzolans in Concrete held in Trondheim in June 1989 and some more recent work funded by the Ash Marketing Division of CEGB at Imperial College and the Building Research Establishment, Garston. Four specific aspects of the effects of fly ash on concrete will be considered: rheology and workability; the pozzolanic reaction and the effects of curing; the development of microstructure; and the development of strength and some aspects of durability.

Seven fluidized bed combustion ashes of different composition were studied as to their hydraulicity and pozzolanicity. Four of them exhibited hydraulic reactivity and hardened when mixed with water. All of them exhibited a positive effect on strength development if used as constituents of blended cements. However, the presence of significant amounts of Fe2+ and free lime in the ash, as well as excessive amounts of SO3 in the produced cement caused unsoundness associated with the decline of flexural strength after longer hydration times.

Inability of the Iowa fly ash industry to meet their demands for fly ash during the peak construction months led the Iowa Fly Ash Affiliates to initiate research into storage alternatives for high-calcium fly ashes. Conventional, closed storage facilities are extremely expensive and currently not cost effective. In addition, the industry is faced with the rising costs of landfill disposal. This paper presents the results of utilizing the rapid self-cementitious properties of high-calcium ashes to agglomerate them into discrete, aggregate size particles for stockpiling. The two fly ashes used in this study contained 25 to 30 percent calcium. Water was used as an agglomerating medium. Agglomeration was accomplished using three types of commercial equipment as follows: continuous rotary pan agglomerator, continuous auger agglomerator and a batch turbine agglomerator. All units produced relatively well graded aggregate material differing primarily in particle shape and texture. Research work discussed includes gradation, strength, and durability of the agglomerates. Agglomerates were also reground using a newlydeveloped, energy efficient, micronizing technique. Research results using the reground ash in concrete and soil stabilization are presented.

Fly ashes from the Lansing and Ottumwa power plants in Iowa were agglomerated by means of a continuous pan agglomerator, a continuous auger and a batch turbine agglomerator. In order to compare agglomeration mechanisms the following parameters were determined: (a) particle size distributions of the untreated fly ashes; (b) particle size distributions of the agglomerated fly ashes; (c) pore size distribution of agglomerates; (d) crystalline hydration products by X-ray diffraction; and (e) morphological characterization by scanning electron microscopy.

In the batch system coalescence mechanisms were favoured. The agglomerates were fairly irregular in shape and had a rough surface texture. As residence time in the system increased breakage of agglomerates occurred, reducing the average agglomerate size. In the continuous systems layering of the fine feed particles onto established agglomerates was the predominant growth mechanism. The agglomerates were smooth and spherical. The layer structure was observed by scanning electron microscopy. Agglomerates of widely varying size, strength, and pore matrix can be produced in both systems. It is envisaged that while agglomerates could be produced with characteristics essential for their proposed end use by either method, continuous pan agglomeration would be the most versatile system to utilize.

Sulphate-expansion tests were performed on mortar bars manufactured with two types of subbituminous fly ash and two types of cement. One fly ash was from Alberta, Canada while the second, a high-calcium ash, was from the United States. Ordinary and sulphate-resistant cements were used in the examination.

Bars were exposed to attack when they reached a given strength level. Some bars were soaked in a mixed solution of sodium and magnesium sulphate under controlled conditions; the pH of each solution was maintained at either 7 or 9.5 by regular additions of sulphuric acid. For other bars the pH of the solution was not controlled. Behaviour of bars in water baths where pH was maintained approximately constant was also examined. Bars were soaked in individual three-bar groups so that possible correlation between amount of acid added and linear bar expansion with time could be observed. Post-expansion examinations included strength degradation, qualitative X-ray analysis and thermal analysis.

Results show some interesting correlations, or lack therof, between acid addition and sulphate attack. Mortars made with the Alberta ash performed better than the corresponding control mortars; however, in a mixed magnesium/sodium solution expansions were much greater than in previous tests where only sodium sulphate was used. Bars made with the high-calcium subbituminous ash performed very poorly under all conditions.

An expansive shale roadbase, stabilized with a Class C (high-calcium) fly ash received an 11–inch full–depth asphaltic concrete surface layer and the highway was opened to traffic six years ago. Periodic sampling and visual observations indicate that the performance of the pavement test sections are above average.

Analyses of field samples showed that fly ash was effective in ameliorating the texture and plasticity of the shale and imparting strength to it on a long term basis. Pavement deflections and the extent of cracking have not increased beyond acceptable levels during the six year period.

X-ray diffraction studies show a reduction of the areas under the peaks and the SEM observations reveal a dense degree of packing and reduction of the void areas. These modifications occur during the first two years of service and any changes beyond that period appear to be minor.

This paper summarizes findings of laboratory and field trial evaluations of ponded fly ash used as a component in a stabilized aggregate base course. Ponded fly ash is the fine portion of pond ash which is a by-product of a coal burning process and is disposed by sluicing to a disposal pond.

Three stabilized aggregate base mixtures containing various proportions of dense graded aggregate, ponded fly ash, and hydrated lime were evaluated in the laboratory relative to maximum dry density, optimum moisture content, unconfined compressive strength, and static chord modulus of elasticity. The mixture that was selected for field trial evaluation had the highest unconfined compressive strength and consisted of 84% dense graded aggregate, 11% ponded fly ash, and 5% hydrated lime.

A 750 foot section of a 22 foot wide roadway was constructed in May 1988. Approximately 88 tons of ponded fly ash were utilized in constructing the experimental base. Prior to construction, in-place California Bearing Ratio tests, moisture content determinations and Road Rater deflection tests were performed on the prepared subgrade. The stabilized aggregate base was placed in one 8 inch lift. During construction, relative compaction and moisture content of the base material were monitored by means of nuclear devices. Post construction evaluations included Road Rater deflection tests and coring to obtain samples for laboratory evaluation.

To date, the section containing the stabilized aggregate base is performing very well in comparison to the conventionally paved section.

Fly ash and bottom ash are being used extensively for stabilization of roads. Unpaved county roads in rural areas are often being resurfaced with bottom ash to improve their stability. A preliminary, uncontrolled examination was conducted to assess the environmental problems that may result from the use of fly ash and bottom ash on highways. To do this, soil, plant and run–off water samples were collected from county roads and highways in Oklahoma that were constructed using either of the two ash forms. These samples were analyzed for fourteen elements of which eight are under the USEPA regulation list of priority pollutants for solid waste and drinking water. The results indicated that the allowable limits for six out of the eight elements were exceeded in the run-off water samples. Compared with the control, fly ash, bottom ash, coal and soil-core samples all contained significantly higher levels of all elements; however, except for barium all were below the regulatory levels. The concentrations of As, Sb, Pb, Ni, Se, and TI in the run-off water samples are high enough to be of concern, although they are below the allowable limits for drinking water. These six elements are found at much higher levels in the fly ash and bottom ash than in the input coal. More Ba is released into the run–off water when the roads were under heavy traffic. Although there was no visual damage observed on the collected plant samples, much higher than normal levels of most metals, were obtained in the tissues.