Opportunities for development of the use of fly ash in blends with Portland cement by improvement of the relevant research are discussed in view of the currently modest growth of the fly ash utilization, despite copious research activity during the past decade. Refined characterization is suggested of the decisive parameters of the chemistry, mineralogy and granulometry of both the components of the reacting, blended system, and also of the effects induced by chemical admixtures. Effective dispersion of the finest particle fractions in the fresh paste, and monitored integral hydration are indispensable successive precursors for the creation of a blended, hardened cement paste which acts as a “microconcrete”, having dense matrix consisting of fully hydrated, finest fractions of both source materials and coarser unhydrated particles representing “microaggregates”. Surface chemistry and energetics are emphasized as essential scientific regimes to apply for the explanatory research having as their aim substantial increases of the replacement ratios of fly ash to cement in concrete.

The Materials Research Society (MRS) is a possible instrument for activity to encourage development of research planning and management methods, which are used in other fields of materials research, to strengthen research as an instrument for effective recovery of the chemical energy inherent in fly ash and for improvement of concrete durability.

Over the years there have been hundreds of papers written on benefication of fly ash to extract alumina, magnetite, activated carbon, cenospheres, etc., and to enhance the properties of the fly ash itself for concrete making purposes. Several of the processes have been patented. Few of them have been exploited commercially. We examine some of the more popular processes, the history of their success or failure, and current achievements with fly ash beneficated for use in Portland cement concrete.

A standard system for grinding of Portland clinker to cement and the different principles for producing fly ash cement is described. The quality is influenced by the grinding system selected. The preferred system for intergrinding of fly ash and clinker involves introduction of fly ash into the finishing compartment of the mill. Efforts made to relate cement quality to specifications are described. Much attention is put on the physical characteristics of the fly ash: bulk weight, specific weight, particle distribution, particle shape, degree of agglomeration and colour. Surface areas and pozzolanic activity matter, but the air void content, which is much affected by grinding, is more important. This observation is based on testing of fly ashes and fly ash cements from production and laboratory grindings. The testing includes compressive strength, specific gravity, laser granulometry and SEM. The results have led to the introduction of new analytical and control procedures. A review of fly ash quality factors of most importance for cement producers is given. The energy balance in the production of blended fly ash cement involves savings from reduced consumption of clinker, and expenditure on transport, handling of fly ash and extra grinding of clinker and fly ash; net savings may be considerable.

Since 1976 the rate of utilization of fly ash from Danish power stations has increased from about zero to more than 75% of production. The main reason for this growth is the co-operation between the utilities and the fly ash consumers, i.e. predominantly the Danish and the Norwegian cement producers. This paper deals mainly with the precautions against separation, with storage and shipment of quality ash, and with the main principles of the co-operation between the producer and the consumer.

The expansive shales used for roadbeds in Oklahoma are traditionally stabilized with lime. Stabilization with a Class C (high-calcium) fly ash was explored and compared to an optimum design utilizing the conjunctive use of fly ash, lime and cement, in the laboratory and in a field experimental project. Periodic visual observations indicated that the performance of test sections was excellent. Analyses of field samples showed that fly ash, either alone or mixed with lime and cement, was effective in ameliorating the texture and plasticity of the shale by reducing the amount of clay size particles and by imparting higher strength levels to the shale. Laboratory samples showed better stabilization than field samples, but the field samples performed at an acceptable level in measurements of compressive and beam strength, cohesion, angle of internal friction and resistance to deflection. The microstructure of stabilized shale was studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Nonbasal (hkl) reflections in stabilized oriented specimens suggest that the clay particles acquire high resistance to dispersive forces. The reduction of the areas under the peaks help explain the strength gain observed. The SEM observations indicated newly formed hydration products, possibly calcium aluminum silicate hydrate crystals, and a rather dense degree of packing as manifested by the substantial reduction of void areas as a result of stabilization.

An experimental programme was undertaken to determine whether Canadian fly ash, and in particular Western-Canadian fly ash, can be used to replace clay as the main ingredient in brick-masonry units.

These preliminary examinations show that Western-Canadian fly ash as the solid material, combined with an adequate binder, can be used to produce a superior ceramic material. Strengths are very much higher than that revuired for load-bearing masonry elements, densities are approximately 800 kg/m3 less than a normal clay brick and the material is durable to freezing and thawing. X-ray analysis and scanning electron microscopy clearly show chemical and physical changes which occur to the material due to firing. These chemical changes appear to be closely related to the subsequent strength which is developed.

Bricks manufactured from Alberta fly ash appear to be a viable alternative to standard clay brick. A substantially stronger and more economical brick may be possible; fly-ash bricks can be manufactured with lower firing temperatures of shorter duration. This, in itself, could lead to the realization of significant energy savings. Also, if large quantities of fly ash can be used in the brick industry, then significant gains will be made toward the elimination of this waste product from the environment and conservation of natural resources.

Twenty-two ash samples from utility companies across the country were studied to determine their potential use for mineral recovery. Most tests were limited to chemical analyses and determination of leachable metal values in the ashes, but samples from two utilities were examined in more detail. The more detailed analyses included physical as well as chemical properties. This work was part of a broad-based effort supported by the Electric Power Research Institute (EPRI), and the results from this study were used in economic assessments of mineral recovery from fly ash made by Kaiser Engineers in another part of the EPRI program. Although the compositions and properties of the ash samples varied considerably, only two of the 22 ash samples studied appeared to be unsuitable for mineral recovery. These results indicate that large portions of the ash produced in this country are suitable for potential resource recovery, but further work to evaluate potential upgrade of the leached ash residue is recommended.

Anticipating the environmental effects of utilization or disposal of coal combustion and conversion by-products requires proper characterization of the inorganic constituents in coal. Inorganic constituents include minerals as well as the organically associated inorganic elements. Characterization of these constituents should not be limited to the types and abundances of the minerals and elements, but should also include their modes of occurrence (Table I and Figures 1–4). Information on modes of occurrence should include the textural relationships of the minerals and the chemical form of the elements (i.e. organic/inorganic associations). This will enable us to predict better how the inorganic constituents will behave upon cleaning, combustion, conversion, or leaching of the coal. For example, chalocophile elements (As, Bi, Cd, Cu, Hg, Pb, Se, Sb, Tl, Zn) associated with secondary cleat (vertical breaks in the coal) or with vein filling sulfides will likely be removed during coal cleaning. In contrast, these elements, when associated with dispersed accessory sulfides and selenides [sphalerite (ZnS), clausthalite (PbSe), chalcopyrite (CuFeS2), galena (PbS)] are commonly concentrated in the cleaned coal. Calcium present in coal as a carbonate would respond to technological processes in a different way than calcium present in organic association or as calcium sulfate, phosphate or silicate.

The mineralogy of nine ash samples derived from North Dakota lignite and gasified in three different fixed-bed producers was determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The gasifiers had maximum temperatures in the 1100–1250°C range, but included several differences in ash residence time, oxidizing gas and pressure, and ash handling conditions. The lignite gasified came from the same geologic formation, but the mines were separated by 20 km. Despite these differences, all of the ash specimens had the same basic mineralogy: glass at the 15–30 wt.% level; a crystalline phase assemblage of ortho- and pyro-silicates (merwinite, dicalcium silicate (C2S), C2S-like phases, and melilite), network aluminosilicates (nepheline, carnegieite, and a sodalite-structure phase), oxides (ferrite spinel, hematite, and periclase), and calcite along with residual lignite minerals (quartz and felspar). SEM morphologies and XRD comparisons of the amounts of crystalline phases in size fractions both indicated that some of the phases (particularly merwinite and carnegieite) crystallized during cooling of portions of the ash that had melted during gasification.

Morphological, chemical and mineralogical speciation of fly ash from a power plant burning sub-bituminous coal has been investigated by examination of size and density fractions. It was found that whereas, fractionation by size revealed little information as to speciation among particle types, separation of the ash into six density fractions showed major differences in properties associated with true particle density. In particular it was found that at least two types of glass co-exist in the ash: “Glass I” – a predominantly silico-aluminous glass associated with particles of low density (cenospheres); “Glass II” – a calcium alumino-silicate glass associated with high-density particles. These glasses were found to differ greatly in composition and to be characterized by shifts in the position of the 2-theta of the XRD-halo. In addition, it was shown that cryptocrystalline mullite is associated only with the low-density particles. It is proposed that particles comprising low-density fractions can be considered as glassceramics with low degrees of crystallization. Particles of high-density are better described as the products of internal lime-sinter reactions.

A procedure has been developed for organizing and displaying the results of particle size distribution determinations on fly ash so as to promote simple and useful comparisons. The procedure involves comparative plots of histograms displayed on a logarithmic size basis, and is illustrated by selected data drawn from a larger suite of analyses of Indiana fly ashes.

Factors affecting fly ash characteristics are examined by consideration of the various processes and constituents involved in its production. Coal conversion, boiler-furnace operations and collection procedures are discussed. Current classification schemes applied to fly ash, in particular to those for use as a pozzolanic mineral admixture in concrete are reviewed. The variability of the material is highlighted using physical and chemical data. Shape and size characteristics of selected fly ashes from sources within New South Wales, Australia are illustrated by electron micrographs. The influence of these properties on resultant concrete properties are briefly considered. The formation of nodules of fly ash material on the surface of fabric filters is chosen to illustrate the importance of physical and chemical conditions in the collection chamber, whereas pH changes with time of the water phase in contact with selected fly ashes demonstrate the importance of chemical characteristics of the collected materials. Such chemical characteristics may influence the path of pozzolanic reactions significantly.

The available alkalis of six Iowa fly ashes, four Class C and two Class F, have been studied as outlined by the procedures listed in ASTM C 311. The purposes of the study were to: (1) assess the significance of the test when it is used to analyze different fly ashes; (2) to investigate the possibility of decreasing the time required to complete the test (it currently requires 28 days for curing). When cured for 28 days at 38 C, the available alkalis were found to be about 60% and 30% of the total equivalent alkalis (equivalent alkalis = %Na20 + 0.658 × %K20) for Class C and Class F fly ashes, respectively. However, more than 85% and more than 40% of the total equivalent alkalis for the Class C and Class F fly ashes, respectively, were mobilized after 5 to 6 months of curing at 38 C. It was concluded that the available alkali test described in ASTM C 311 tends to underestimate the amount of equivalent alkalis present in Class C fly ash-calcium hydroxide mixtures after long periods of time.

A method of mounting fly ash specimens for scanning electron microscopy is described that avoids almost all charging difficulties and permits imaging of close packed fields of representative particles. The procedure facilitates a true assessment of the kinds of particles present in that specific fly ash and their relative frequencies of occurrence.

The surface layers on coal fly ash particles are of special environmental interest in that concentration enrichments of trace elements may occur [1], thereby enhancing the potential bioavailability of toxic species. Little research, however, has been devoted to the analytical characterization of intraparticle and interparticle distributions of trace elements. The high detection sensitivity, spatial resolution, and depth profiling capabilities of secondary ion mass spectrometry (SIMS), coupled to digital image acquisition and processing [2], permit three-dimensional (3D) compositional maps for collections of individual micron-sized particles. The 3D analysis of trace element distributions in coal fly ash particles is the subject of this SIMS investigation

Proton-induced X-ray emmision and proton-induced gamma-ray emission analyses have been used to determine the elemental content of four oil fly ash samples collected from the flue line of an oil-fired power plant. X-ray absorption spectroscopy was used to characterize the vanadium and nickel compounds present in the fly ash. Vanadium was present primarily as VOSO4·3H2O and nickel was present as a mixture of NiSO4 and NiO.

Slagging difficulties encountered in large, pulverized coal-fired utility boilers can be alleviated by using recently developed chemical conditioners designed to inhibit the agglomeration of molten fly ash particles. Slagging results from the impaction and accretion on the boiler interior surfaces of molten ash particles in the flue gas resulting from the combustion of coal. Electron Spectroscopy for chemical Analysis (ESCA) and ion sputtering depth profiling were utilized to examine the mechanism whereby a fuel conditioner containing copper oxychloride effectively reduced slag deposition rates during a pilot-scale, pulverized coal combustion experiment.

Three new fly ash standard reference materials (SRM's) have been produced by CTL under contract with the U.S. National Bureau of Standards (NBS). Each unit of the SRM's consists of approximately ten grams of well-blended fly ash hermetically sealed in a glass vial. Twenty-thousand vials of each SRM were produced. The SRM's will be certified for major and minor chemical elements, loss on ignition, and for the percentage residue on the No. 325 standard sieve. The SRM's are suitable for use in developing new methods of chemical and physical analysis, for checking the performance of instruments and analysts, and for research into the composition and properties of fly ash.

An on-going fly ash research program, funded by six Iowa power companies and their marketing representative has been established at Iowa State University. The broad objective of the affiliate program is to conduct basic research on Iowa fly ashes that can be directly applied to increasing their utilization in the construction industry. The first year goal is to evaluate the influence of various Iowa fly ashes on the air void system of concrete. Previous research at Iowa State University had indicated that fly ash may be influencing air void distribution in fly ash concrete, and hence durability. A variety of combinations of aggregates, fly ashes, cements and admixtures are being evaluated for compatibility in concrete. Rapid diagnostic tests are being developed, including (1) a modified Hallimond tube flotation test to evaluate physico-chemical compatibility of combinations, and (2) LeMont image analysis techniques to analyze the air void system. Procedures and preliminary results are presented and discussed.

With financial support from utilities and ash brokers*, the Western Fly Ash Research, Development and Data Center was established under the aegis of the North Dakota Mining and Mineral Resources Research Institute in August of 1985. Research will be performed by the two North Dakota universities in Grand Forks and Fargo. The fundamental objective of the Center is to enhance the knowledge base of the properties (chemical, mineralogical and physical) and reactions of the coal by-products (principally fly ash, but including bottom ash and FGD waste) produced in the Midwestern and Great Plains regions of the US. Most of the study specimens will be high-calcium (ASTM Class C) ash derived from low-rank lignite and subbituminous coals mined in North Dakota, Montana and Wyoming, although ash from other regions and coals is also being studied. The enhanced knowledge base should lead to more widespread utilization of these by-products [1,2] or, where this is necessary, to their safe and cost-effective disposal [3].