In practice, the amount of fly ash added to portland cement concrete varies depending upon the desired end properties of the concrete. Generally, when a given portland cement concrete is redesigned to include fly ash, between 10 and 50% of the cement is replaced by a volume of fly ash equal to that of the cement. Sometimes as much as twice the volume of the cement replaced, although 45.4 kg (100 lbs) of cement will only produce enough calcium hydroxide during its reaction with water to react with about 9 kg (20 lbs) of a typical fly ash. The combination of large amounts of certain fly ashes with small amounts of portland cement in concrete has been found to produce surprisingly high compressive strengths, which cannot be accounted for by the conventional “pozzolanic reaction”. Ratios of cement to fly ash as high as 1:15 by weight can produce compressive strengths of 20.7 MPa (3,000 psi) at I day and over 41.4 MPa (6,000 psi) at 28 days. Methods of identifying these “hyperactive” fly ashes along with some of the startling results, with and without chemical admixtures are described.

In an experimental program, the suitability of various methods for testing the pozzolanic activity of fly ash was investigated. The research was conducted on virtually all fly ashes approved as concrete additives in Germany. This paper discusses differences in the particle size distribution and the solubility on dissolution with hydrochloric acid and potassium hydroxide of the fly ashes. It also reports on interrelationships between the physical and chemical properties of the fly ashes and the workability and strength of mortars containing fly ash.

Based on the characterization of 12 clay and 5 fly ash samples, samples were selected for the following laboratory scale experiments: preparation of raw mixes, determination of the correlation between water content and plasticity, rate-controlled sintering in a laser-dilatometer, DTA-TGA-analyses, preparation, firing and testing of bars. Laboratory experiments have shown that it is necessary to bind the soluble sulphates as barium sulphate under acidic conditions and bind the free lime as phosphate when wet production methods are to be used. A full scale experiment has been carried out, resulting in the production of 16,000 red bricks containing 33% fly ash by volume in the raw mix. These fly ash bricks look satisfactory and have 10% higher strength and 2% less density than the corresponding bricks without fly ash. The water extractable matter in the two kinds of bricks were compared. The bricks were used for constructing a wall with the purpose of investigating their weather resistance.

The benefit of adding up to 35 wt% Class C high calcium fly ash to various types of fine and coarse aggregate pavement mixes is described and quantified. The mixes, which were compacted to maximum dry density at optimum moisture content, had variable compressive strengths during the first 28 days of curing; after that they assumed a relatively uniform pattern of strength gain reaching values as high as 11 MPa (1600 psi). Mixes containing 15% fly ash gave unacceptably low strengths. XRD measurements indicated massive formation of ettringite, transforming to monosulfoaluminate and the poorly crystallized hydrated phases of C-A-H, C-A-S-H and C-S-H. This transformation helps explain the gain in strength of the mixes with extended curing. SEM observations depicted progressive packing and densification of the skeletal matrix as the hexagonal phases and C-S-H gained higher crystallinity and formed aggregated masses. Furthermore, these observations suggest that fly ash acts predominantly as a chemical binder and partly as a filler in the aggregate mixes tested.

Compacted fly ash, lime, bentonite and water mixtures were cured at 23° and 50°C, for 1, 28, 90 and 180 days. Cementitious products and microstructure were observed by scanning electron microscopy, energy dispersive x-ray spectrometry and x-ray diffractometry. Unconfined compressive strength changes are correlated to the formation of new mineral phases. For bentonite-limefly ash mixtures, strength increased from 1050 kPa (I day) to 2,300 kPa (90 days) and then slightly increased to 2,400 kPa after 180 days at ∼ 230C. Ettringite is the most abundant mineral associated with the increased compressive strength.

New minerals identified in the 23°C mixtures include calcium silicate hydrate - Type 1, afwillite and ettringite. Acicular crystals of these and other minerals were formed by the hydration of lime and fly ash in the montmorillonitic clay. The cementitious phases create a rigid framework joining spheres and clay aggregates. Continued reaction dissolves some of the spheres and slightly reduces the rigidity of the cured samples.

The available alkali test has been used for years as a method for preventing the use of a high-alkali fly ash in concrete containing reactive aggregate. This report presents data on fifteen good-quality Western ashes, both Class C and Class F, that are currently used in readymixed concrete. All fifteen ashes were effective in reducing the expansion due to the alkaliaggregate reaction. The reduction in mortar bar expansion (ASTM C 441) varied from 16% to 81%. Correlation of the percent reduction was made with available alkalies as Na2O, silica content, lime content, sulfur content, and the sum of silica, alumina and iron oxide. Surprisingly, the best correlation to percent reduction was with sulfur and the least was with available alkalies. This data indicates that most, if not all, fly ashes are effective in reducing the alkali aggregate reaction and that the available alkali test is not the best method for predicting the effectiveness of a fly ash in controlling this reaction.

Scanning electron microscope studies were carried out on concretes obtained from structures in Australia, England and the United States as part of an assessment of their durability performances. The ages of the structures ranged from nine to thirty years. All structures contained fly ash concretes in some sections, and, in a few cases, direct comparison between these concretes and ordinary portland cement (OPC) concretes was possible. Water: cement ratios of the concretes varied from 0.44 to 0.90. The sources of fly ash were identified in each case. The structures chosen had a range of use classifications encompassing building, hydraulic structures, bridge structures, towers and foundations. Deterioration effects of concretes in some structures were more common than in others. Based on observations, conclusions were drawn on the morphological features and hydration characteristics of the concretes.

The microstructures of steel reinforced ordinary Portland cement mortar samples, and those containing 40% and 70% slag as cement replacement, have been studied by electron optical techniques, after exposure to stagnant sea-water at 23°C for 320 days. Fracture surfaces along the interface with steel were examined using secondary electron imaging in the SEM. This revealed differences between the OPC and slag specimens as regards the morphology of the phases and the amounts of calcium hydroxide present. The microstructure at the interface with steel was studied by means of back scattered electron imaging combined with quantitative image analysis. Chloride concentrations at sections around the interface were determined by means of an EDXA facility linked to the SEM. Even after 320 days immersion in sea-water, there was some calcium hydroxide present in the interfacial zone.

The use of fly ash as a partial replacement in cements is widespread because of reduced costs and the reported improvement in performance. Fly ash/cement blends have been used offshore for underbase grouting of gravity platforms, although the cold marine exposure may slow the pozzolanic reaction which is very sensitive to temperature. Sea-water could be used as mixing water under these conditions. This paper looks at how curing at 8°C, the use of sea-water as mixing water, and direct exposure to sea-water, influence the hydration, strength and microstructural development of an OPC and an SRPC with 30% fly ash replacement. The early heat evolution was measured with a conduction calorimeter, while the presence and development of hydration products were identified by DTA, TG and x-ray analysis. Scanning electron microscopy was used in two modes; secondary electron images of fracture surfaces were examined, and a quantitative description of microstructure was made possible by analysis of backscattered electron images of polished sections. The morphology and distribution of hydration products in the wet state was studied using an environmental cell in a high voltage electron microscope.

Fly ash-cement pastes are known to develop fine pore structures that may retard the transport of ionic species. The rapid chloride permeability technique for studying the Cl- ion diffusion in hydrated fly ash/cement pastes, mortars and concrete was used. The technique applies an electrical potential across a cylindrical sample and measures the charge passed in a certain period of time. The results obtained on pastes and mortars cured for 28 days were reported previously and contrasted with those of neat cement pastes and mortars. The present paper reports more extensive studies made to examine the chloride permeabilities of pastes and mortars cured for up to 90 days. In addition, the effect of variable fly ash contents was examined. Concrete samples were included in the test scheme and the data were compared with pastes and mortars. Two important factors controlling the test results are discussed: first the mix design and curing conditions; second the experimental conditions during Cl- permeability measurements. In the second factor, the amount and rate of heat build up and the chloride ion concentrations are compared with the current passed. In addition, measuring current versus resistivity are critically discussed in terms of the voltage-current varistic characteristics of cement matrices.

The ordinary fine aggregate in concrete has been replaced by ground and sieved steel slag fine aggregate, treated and exposed to air for three months. Compared with concrete made from natural sand, properties such as compressive strength, flexural strength, elastic modulus, permeability and abrasion resistance are considerably improved. The improvement increases with a decrease in w/c ratio, an increase in curing time and an increase in the replacement weight of sand. These results are due to the fact that the steel slag contains some active minerals such as C3S, C2S, C4AF, etc., and shows favorable surface physical characteristics that improve the bond between steel slag particles and cement paste. The results of XRD, SEM and EPM microhardness showed that there are heavier concentration of ions, with finer crystals and a lower degree of CH orientation at the interfacial zone between steel slag particles and cement paste. The study also found small cementitious and fibrous C-S-H crystals growing from the fine aggregate, which are linked with hydrated products from cement paste making the bond and structural characteristic more favorable with cement. The steel slag fine aggregate is an active mineral similar to cement. The bond between the aggregate and cement paste is strengthened both physically and chemically.

The surface and near surface characteristics of alkaline fly ash particles were studied to evaluate the heterogeneous chemistry and structure of ash in order to provide data and understanding of the dissolution behavior of elemental constituents. Fly ash was subjected to a range of chemical weathering intensities in laboratory column experiments. Weathered ash from columns were then analyzed for surface morphology and chemical compositional change. Determinations included chemical characterization by instrumental neutron activation analyses, surface analysis by SEM-EDXA of whole and fractured particles, surface glass analysis by selective dissolution, and mineralogy by powder x-ray diffraction. These data were used to develop a physical model of fly ash particles that describes the heterogeneous distribution of elements on or in a reactive glass hull and in matrix glass and mullite. The model is discussed in terms of elemental partitioning and its application to the mechanistic approach advocated for prediction of leachate composition.

An application of a fundamental thermochemical approach that relates the aqueous concentrations of elements to specific chemical reactions in waste/water systems is described. The approach involves the concept of waste as consisting of mixtures of discrete solids (pure solid phases and solid solution phases) that control aqueous concentrations of individual solutes through their solubilization or adsorption/desorption characteristics. Thermochemical data can then be used in conjunction with knowledge of the specific solid phases present in the wastes to calculate the aqueous concentrations of elements. This approach does not explicitly consider specific rates of kinetically controlled reactions but does incorporate empirical knowledge of the relative rates of several reaction types. Overall, the thermochemical approach is applicable to all waste types and environments, and is appropriate because (1) most waste solids that either are present or are expected to form during weathering have relatively rapid precipitation/dissolution kinetics, and (2) experimental results, enrichment of many constituents on particle surfaces, and the large specific surface areas of wastes suggest that many of the constituents present in the wastes are accessible to solution attack and are mobilized fairly rapidly. Supporting evidence for the successful application of this approach to predicting pore-water concentrations of several major and trace elements in utility wastes is discussed.

The Pennsylvania Power & Light Company, in cooperation with the Electric Power Research Institute, is studying leachate generation at a large outdoor test cell of compacted coal combustion fly ash constructed at the Montour Steam Electric Station in north-central Pennsylvania. The test cell design is identical to that of the plant's active dry fly ash disposal facility. Leachate initially generated at the test cell contained concentrations of chromium and selenium approximately ten times above EPA's assigned Maximum Contaminant Levels (MCL's) for drinking water. Concentrations of these two parameters have since decreased approximately five-fold. Similar reductions have occurred in the concentrations of several of the other trace metals measured in the leachate. The dominant reaction mechanisms controlling leachate composition were examined in an attempt to explain these and other observations. Deterministic chemical modeling was performed to aid in the interpretation of the data. The dynamics of leachate generation were also examined through an evaluation of the infiltration and redistribution of moisture.

Composite waste forms prepared using solid and liquid waste streams from an integrated coal gasification facility at Beulah, North Dakota, were evaluated for the potential leachability of select trace elements using batch tests. Cylinders were fabricated from seven blends of the solid components using liquid waste incinerator blowdown, multi-effect evaporator concentrate, or tap water. Individual waste forms and composites were leached using the same protocols. Both long term and short term leaching tests were used to evaluate these composite waste forms.Comparisons of measured concentrations of elements with values calculated from data on individual components were used to determine any change in leachability that may have occurred. These changes were expressed as a fixation factor, which is a term derived by dividing the calculated leachability by the measured concentrations from the actual leaching tests. Results from these experiments indicated that leachability of some elements could be lowered by the mixing of various solid and liquid waste streams. Ettringite formation is one possible mechanism responsible for fixation of several of the elements.