Cement paste and concrete specimens containing three different mixtures of portland cement (PC) and high-lime fly ash (FA) were subjected to various curing conditions, and the strengths, non-evaporable water, and porosities were compared with control mixes containing portland cement only. Strength and porosity data indicated that the cementing action of the mixtures was, in all cases, inferior to portland cement. For each mixture the strength loss resulting from imperfect curing, i.e. sealed or exposed to 50% relative humidity, was greater than for portland cement. In the case of concrete drying from one surface, strength differences were found, but these were not as clearly defined as those obtained in the paste specimens.

The predominant spherical shape of fly ash particles combined with mainly glassy composition and texture of its surfaces have a special effect on rheology of cement pastes containing fly ash. The early ages rheological behavior of cement pastes (ASTM Type I) incorporating 30% low-calcium fly ash was monitored by measuring viscosity of the fresh pastes prior to initial hardening and stiffening (up to −2 hours) as a function of time. The viscosities were determined using a co-axial rotoviscometer (HAAKE). The effects of fly ash content, water to cement ratio, and presence and concentration of superplasticizer, were evaluated. In addition, the dispersivity of fly ash spheres was evaluated by determining the zeta-potential of fly ash suspensions in water using a microelectrophoresis technique and the results were correlated to the chemical composition of fly ash as well as the viscosities of fresh pastes.

Fly ashes having three distinctly different levels of calcium, designated low-calcium (Class F), intermediate-calcium (Class F/C), and high-calcium (Class C) comprised the basic material for the present study. Pastes and mortars were made out of the three types of fly ashes and one type of cement (Type I) at various levels of replacement as well as different water/solid ratios (w/c). Chloride ion diffusion was measured by applying an electrical potential across cured cylindrical samples and measuring the amount of current passed in a certain period of time (proportional to amount of Cl− passed in this time). Other supportive measurements were made, e.g., porosity and pore size distribution, water permeability and surface area. The Cl− ion diffusivities were correlated with the chemical composition of fly ash (FA), mix proportioning, and water permeabilities of the tested hardened pastes or mortars.

Factor analyses and cluster analyses were the modeling tools used to relate the chemical and physical characteristics of fly ash and cement to the strength, sulfate resistance, and freeze-thaw durability of fly ash-modified concrete. A Type I Portland cement was mixed with base load and upset load condition fly ashes from three different power plants in each of five regions in the United States. Based on the interactions between the reactive constittuents of the cement and fly ash, common factor loadings were identified. Cement loaded onto the early strength factor; fly ash loaded onto the later strength factor. In some subgroups the quantity of mixing liquid loaded separately as representative of the high water/cement ratio, which masked the reactive interactions between the fly ash and cement. In other subgroups the inter-relationships between sulfate resistance and strength with fly ash/cemment fineness, CaO content, and alkali content were represented in the factor analysis as well as in the numerical analysis models.

Temperatures in large cementitious monoliths (works) can rise very high due to unfavorable thermal properties such as low conductivity and high diffusivity of the monolith and the surrounding media. Heat moderation becomes necessary in such situations to avoid excessive thermal stresses. Moderation due to the addition of inert additives such as sand in mortars is compared to that obtained by the addition of reactive but low heat evolution substituents such as Class C and Class F fly ashes. Substitution of cement by slag has also been considered. The hydration temperatures for the extreme conditions (adiabatic) have been experimentally measured and compared to those predicted under real conditions. Such a simulation has been made by measuring the thermal properties and analyzing the temperature distribution due to exothermic reactions as predicted by a finite differences computer model. In general, lower temperatures can be maintained by increasing the thermal conductivity and heat capacity of the hydrating material. This material can be tailored for both heat evolution and setting times by incorporating inert additives such a sand (quartz) and/or reactive additives such as slag and fly ash.

When concrete is poured in large volumes, it is necessary to be able to predict the temperature rise which may occur inside the mass because of the effect this may have on the ultimate properties of the hardened concrete. It is known that the elevated temperatures generated may have a detrimental effect on final strengths and that if the difference in temperature between the centre and the surroundings exceeds 20–25°C, cracking may occur. In order to study these effects, a calorimetric controlled apparatus has been designed that is able to simulate the temperature rise profile occurring within any size of concrete pour. The apparatus is also used to control a curing bath thus enabling the compressive strength of match cured concrete to be determined. Results have been obtained for both temperature rise and strength growth at the centre of simulated 0.8, 1.5 and 3 m deep pours, using plain Portland and Portland PFA cement concrete. These show that in the larger sized pours the strength of concrete from a Portland PFA cement blend grows, after 2–3 days, at a greater rate than that of pure Portland cement, while giving lower temperature rises. Comparative strengths at 28 days are 48 Nmm−2 for the PFA cement concrete and 38 Nmm−2 for the pure Portland cement concrete. The influence of temperature on the reactivity of the PFA under these conditions has been studied using a dilute acid dissolution method. The hydrates formed and the progress of the pozzolanic reaction within the actual concretes has been monitored using scanning electron microscopy.

The Subcommittee of ASTM that is responsible for pozzolan specifications, (C09.03.10) is currently revising C 618, the standard specification, and C 311, the standard test methods. It is no small task. It is generally considered that dividing fly ashes into two classifications (Class F and Class C) is no longer acceptable. The Subcommittee has reached the tentative conclusion that one class of fly ash pozzolan is more appropriate and less ambiguous, provided it is accompanied by an optional table outlining the appropriate limits for sulfate resistance, heat of hydration, hydraulic properties, etc. That is, the engineer need specify only a fly ash pozzolan. If the engineer needs more than a “general purpose” fly ash he calls for special characteristics from the optional requirement table. Any major revision in the C 618 specification challenges the Subcommittee to look for better and more useful test methods in C 311. The committee is looking at C 1012 for a sulfate resistance procedure, conduction calorimetry as a heat of hydration method, and better methods to evaluate pozzolanic activity. The committee is aware of the limited usefulness of hydrated lime or sodium hydroxide in evaluating pozzolanic activity. Perhaps a more suitable term, i.e. the activity index with cement, is more appropriate. Fly ash properties that need to be evaluated include water reduction, activation by lime and alkalies, hydraulic or self-cementing properties and pozzolanic activity. Any test that tends to compensate for a fly ash's lower reactivity (i.e., accelerated thermal curing at 35°C or 65°C) probably leads to misleading results. A critical look at the national standards of other countries might allow the committee to take a visionary yet practical approach toward a new fly ash specification.

Fly ash and other pozzolans are increasingly used in concretes that must consistently achieve high compressive strength and other measures of performance. In contemporary design and construction practice, however, there is some concern as to whether or not use of commercial fly ash materials contribute to or reduce variations observed in field compressive strengths and properties of the fresh concrete. Regardless of its potential for improving the performance of the concrete, if the daily ongoing use of commercial fly ash will decrease the consistency of the concrete produced, the use of fly ash will be curtailed. In this paper, the experiences of geographically diverse, specific concrete suppliers using their respective sources of fly ash are summarized in detail. The nominal compressive strength of concretes, made with and without fly ash, are analyzed statistically. Special attention is given to moderate- and high-strength concretes. In conclusion, it is shown that concretes made with fly ash may not be more consistent than concretes with only portland cement.

High-calcium fly ashes with CaO contents >20% do not necessarily contain the glass type found to be characteristic of such ashes in previous results. Instead of this glass (asymmetrical x-ray maximum at 32 deg. CuK-alpha radiation), the fly ash may contain a lower calcium content glass (symmetrical x-ray maximum near 27 deg.) and a substantial content of crystalline CaO. The properties of a representative fly ash of each type are briefly illustrated, and the characteristics of fly ash pastes and of cement-fly ash pastes produced from the two fly ashes are examined.

An investigation has been made of the variability of physical and chemical properties of high-calcium (Class C) fly ashes from four Iowa power plants. The investigation summarizes results obtained from three years (1983 through 1985) of monitoring of the various power plants. All four of the power plants burn low-sulfur, sub-bituminous coal from Wyoming. Fly ash samples were obtained from the power plants in accordance to the procedures described in ASTM C 311. Laboratory testing methods were similar to those specified by ASTM C 311. During the three year period, 102 samples were subjected to chemical and physical analysis while an additional 349 samples were subjected to physical analysis only. In general, the four power plants produce fly ashes of similar mineralogy and chemical composition. The observed time variation of the chemical composition of fly ash from a single power plant was quite small. The sulfur content consistently showed the largest coefficient of variation of the 10 elements studied. Physical characteristics of the fly ashes (as measured by ASTM tests) were also fairly uniform over long periods of time, when considered on an individual power plant basis. Fineness, when measured by wet washing using a 325 mesh sieve, consistently exhibited the largest coefficient of variation of any of the physical properties studied.

The “quality” of fly ash produced during 1985 at Ottumwa Generation Station, was evaluated by two different experimental programs. The first consisted of the physical tests specified in ASTM C 311; these results are applicable to the use of fly ash as an admixture to portland cement concrete. The second consisted of monitoring the changes in the physical properties of fly ash pastes; these results would be applicable to the use of fly ash as a grout or a soil base stabilization agent. The physical properties monitored during the testing program were compressive strength, volume stability and setting time. In general, the results obtained from the two testing programs were quite different. When using testing procedures defined by ASTM C 311 the fly ash appeared quite uniform, but results obtained from the fly ash pastes were quite erratic. It was found that compressive strengths of the pastes can vary by a factor of five in rather short periods of time.

The utilization of ashes from combustion or gasification of western U.S. coals offers many possibilities for useful products. Of the possible uses, the following have been identified in an earlier study [1] as having sufficient potential for laboratory development and testing: mineral wool, sulfur concrete, high flexural-strength ceramics, replacement of cement in concrete, and road stabilization. Three lignite-derived ash products from the Beulah, ND, site were used in the present study: fixed-bed gasification ash; a dry scrubber ash; a combination bottom ash/economizer ash from an electrical power plant, Where possible, ASTM fabrication and testing procedures were used. Mineral wool of similar physical character to commercial wool and at lower potential cost was produced using 100 percent of various western ashes. Sulfur concrete utilizing 80% ash and 20% modified sulfur developed flexural and compressive strengths in excess of 2,250 and 5,000 psi, respectively. An economically competitive vitrified ceramic product with flexural strength above 7,800 psi was produced from a mixture of 50% ash, 45% sand, and 5% clay. By using a total ash mixture of 26% gasifier ash and 74% combustion ash, a very satisfactory, economical and durable roadbed material was developed. The replacement of up to 50% of the cement in concrete with western fly ash produces economical, high strength concrete.

This paper presents a brief review of the literature dealing with utilization of atmospheric-pressure fluidized bed combustion (AFBC) solid wastes. The uses that have been proposed for AFBC residues include the following: agricultural lime, waste neutralization and stabilization, low-strength backfill, soil cementing and asphaltic concrete aggregate. An evaluation of a high-Ca waste from a Canadian AFBC installation is discussed. The waste was found to be unsuitable for applications in Port-land cement concrete because of poor strength development and expansion in mortars. The waste was compatible with Portland cement for soil cementing purposes but the resulting mixes were not resistant to freezing and thawing. Applications in asphaltic concrete were found to be successful in the laboratory and a small field trial is in progress.

An investigation was conducted into the use of beneficiated ash fractions as feed materials for direct acid leaching to recover Al and other metals. A low-Ca (1.4% CaO) ash was selected from which four fractions (classifier fines, classifier rejects, magnetic concentrate, and non-magnetics) were produced by mineral beneficiation methods. The raw ash and the four ash fractions were leached with 6 M HCl under reflux. Mass balance data were obtained to determine extraction efficiency for Si, Al, Fe, Ca, Mg, Na, K and Ti. It was found that size separation of the ash had a marked influence in enhancing extraction efficiency from the fine fraction for all of the elements studied except Fe. Extraction of Fe from the magnetic fraction was less efficient than from the non-magnetic fraction. Examination of the solid residue after leaching showed that, although most of the acid-soluble components were removed from the glassy phases of the ash, a significant part of the ash comprised non-reactive glass of high-Si content. A relationship between extractable Al in ash and leachable alkali and alkaline earth metals was identified and discussed in terms of glass modification theory.

Carbochlorination of the metal oxides in fly ash by suspending the solid reactants in a NaCl-AlCl3 melt at 530–850°C and then sparging chlorine into the melt has been investigated. A mechanically agitated, semi-batch reactor was used to test the effects of temperature, oxide and carbon loading, salt composition and gas flow on the reaction rate. The process was modeled using the carbochlorination of pure alumina, the rate of which was found to be chemical reaction controlled at temperatures below about 650°C and gas-liquid mass transfer controlled at higher temperature. The carbochlorination rate of the mixed oxides in coal fly ash was also mass transfer controlled at higher temperatures when aluminum recoveries were less than about 50%. At higher aluminum recoveries, the overall rate was limited by the rate of ash dissolution into the melt.