Thermal power stations use pulverized coal as fuel. They produce enormous quantities of coal ash as a by-product of combustion. This calls for the development of strategies to encourage and establish technological concepts which will ensure consumption of fly ash in bulk. Among the various uses of fly ash, its bulk utilization is possible only in geotechnical engineering applications. This necessitates characterization of the fly ash with reference to geotechnical applications. This paper presents a review of such studies carried out. The results show that fly ash is a freely draining material with angle of internal friction of more than 30 degrees. The specific gravity is lower leading to lower unit weights resulting in lower earth pressures. It can be summarized that fly ash (with some modifications/additives, if required) can be effectively utilized in geotechnical applications.

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Fly ash characterization with reference to geotechnical applications

At present, nearly 100 million tonnes of fly ash is being generated annually in India posing serious health and environmental problems. To control these problems, the most commonly used method is addition of fly ash as a stabilizing agent usually used in combination with soils. In the present study, high-calcium (ASTM Class C—Neyveli fly) and low-calcium (ASTM Class F—Badarpur fly ash) fly ashes in different proportions by weight (10, 20, 40, 60 and 80 %) were added to a highly expansive soil [known as black cotton (BC) soil] from India. Laboratory tests involved determination of physical properties, compaction characteristics and swell potential. The test results show that the consistency limits, compaction characteristics and swelling potential of expansive soil–fly ash mixtures are significantly modified and improved. It is seen that 40 % fly ash content is the optimum quantity to improve the plasticity characteristics of BC soil. The fly ashes exhibit low dry unit weight compared to BC soil. With the addition of fly ash to BC soil the maximum dry unit weight (γdmax) of the soil–fly ash mixtures decreases with increase in optimum moisture content (OMC), which can be mainly attributed to the improvement in gradation of the fly ash. It is also observed that 10 % of Neyveli fly ash is the optimum amount required to minimize the swell potential compared to 40 % of Badarpur fly ash. Therefore, the main objective of the study was to study the effect of fly ashes on the physical, compaction, and swelling potential of BC soils, and bulk utilization of industrial waste by-product without adversely affecting the environment.

Physical and Compaction Behaviour of Clay Soil–Fly Ash Mixtures

Preface The idea of developing a haul road manual or collection of guidelines was initiated by the late Professor Muirhead at the University of Alberta in 1999. Support and financial contributions for this work were obtained from NSERC, SMART (Surface Mining Association for Research and Technology) ATCO Power, and Finning. Both Syncrude Canada Ltd. and Suncor Ltd. provided access to road design data and methodologies. Syncrude Canada Ltd.'s internal reports were instrumental in verifying the strain-based design approach for haul roads that is advocated in this manual. Two closely related documents form a basis for this manual. The oldest is the report by Kaufman and Ault (1977) entitled Design of Surface Mining Haulage. Monenco (1989) took the Kaufman and Ault report and updated it to reflect conditions relevant to Canadian mines. The Monenco report is unpublished and difficult to obtain. The questionnaire survey that is found in the Monenco report was repeated in 1999 for this document. Content from the M.Sc. thesis by Kumar (2000) is also used in this document, especially in sections dealing with design of a haul road cross-section. Bruce Regensburg put together the section on haul road economics. In addition, haul road design issues were also gathered from various published sources that are referenced in these Guidelines for Mine Haul Road Design.

Guidelines for Mine Haul Road Design

The strength of fly ash mixture often needs to be enhanced for its better utilization in geotechnical and environmental applications. Many fly ashes often improve their strength with lime but may not meet the requirements. Gypsum, which reduces the lime leachability, further improves the strength. An attempt is made in this paper to study the effect of gypsum on the strength development of two Class F fly ashes with different lime contents after curing them for different periods. The sustainability of improved strength has been examined after soaking the cured specimens in water and with different leachates containing heavy-metal ions. The strength of both the fly ashes investigated improved markedly up to a particular amount of the lime content, which can be taken as optimum lime content, and thereafter the improvement is gradual. The improvement in strength at higher lime contents continues for a longer period (even up to 180 days). Gypsum accelerates the gain in strength for lime-stabilized fly ashes, particularly in the initial curing periods at about optimum lime content. At high lime contents gypsum attributes very high strength after curing for long periods mainly due to the alteration of fly ash lime reaction compounds. Gypsum not only improves the reduction in the loss of strength due to soaking even at low curing periods but also improves the durability of stabilized fly ashes due to repeated cycles of wetting and drying.

Role of Gypsum in the Strength Development of Fly Ashes with Lime

This report documents and presents the results of a comprehensive survey of the technical and economic optential for making use of waste materials as alternative aggregates in highway construction and maintenance. The survey included an assessment of the availability of conventional aggregates to determine where competitive opportunities for the introduction of new materials are most likely to exist. Information was obtained from over 300 relevant documents and supplemented by numerous contacts with agencies and individuals in both the highway and waste management fields. Of an initial group of 34 waste products that were examined for use in highway aggregate production, 30 were judged to have some promise. Technical, economic, and environmental evaluations that were made through a system developed on the project showed iron blast furnace slag; fly ash, bottom ash and boiler slag from electric power generating stations; reclaimed paving material; and anthracite coal mine refuse to offer the best opportunities for further development.

Waste Materials As Potential Replacements for Highway Aggregates

THE REPORT IS DIVIDED INTO FOUR PARTS CONSISTING OF A STUDY OF THE VARIATION IN THE PHYSICAL AND CHEMICAL PROPERTIES OF PULVERIZED FUEL ASH, AN INVESTIGATION OF THE USE OF MIXTURES OF LIME AND P.F.A. AS STABILIZING AGENTS FOR NATURAL SOILS, AN ASSESSMENT OF THE SUITABILITY OF P.F.A. FOR STABILIZATION WITH LIME OR CEMENT, AND AN EXAMINATION OF THE SELF-HARDENING PROPERTIES OF P.F.A. THE INVESTIGATIONS SHOW THAT PULVERIZED FUEL ASH IS SUITABLE FOR USE IN THE CONSTRUCTION OF ROAD BASES AND SUBBASES. IT IS A CHEAP AND READILY AVAILABLE MATERIAL AND ITS USE WILL HELP TO REDUCE THE DEMAND FOR NATURAL AGGREGATE IN GREAT BRITAIN AND AT THE SAME TIME ELIMINATE THE DISPOSAL PROBLEMS THAT ARE ENCOUNTERED AT POWER STATIONS. /RRL/A/

M D Ryley

Papers

The use of stabilized pulverized fuel ash in road construction. a laboratory investigation