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SP-178 This Symposium Publication contains the proceedings of the Fourth CANMET/ACI/JCI International Conference held in Tokushima, Japan, in June 1998. Sixty-two refereed papers were accepted for presentation at this conference and for this publication.

The evolution of the microstructure of a hydrated fly ash-belite cement (HFABC) has been studied during a period of 90 days after mixing. The cement was synthesized from a mixture of fly ash (ASTM Class F), lime and water by a hydrothermal procedure. The microstructure characterization at different times was followed by mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and EDS, X-ray diffraction (XRD), thermogravimetry (TG) and analytical techniques. Besides, the compressive strength was determined and correlated to the microstructure characteristics. The pore solution was expressed and analyzed at different periods of the hydration reaction.

In India approximately 45 million tonnes per annum of fly ash is generated from the thermal power plants. Disposal of these fly ashes is a major issue endangering the environment. It is a challenge to the Research Scientists to innovate technologies for bulk usage. As a part of a comprehensive programme on utilisation of fly ashes, several approaches are being pursued at The Research and Development Division of The Associated Cement Companies Ltd. Thane, India, both In traditional and non traditional technological areas. Among the newer technologies being developed, one approach which merits attention is the synthesis of reactive belite cement and cementitious composites through hydrothermal processing, as developed by the Material Research Laboratory, Pennstate University USA. The Indian fly ashes were found to be not responsive to such hydrothermal activation. Keeping in view the chemistry of Indian fly ashes a novel process route has been developed in the author’s laboratory. This process involves activation of fly ash with alkali under non-hydrothermal conditions. The activated fly ash is reacted with hydrated lime in pre-determined proportions to produce the Calcium silicate /aluminate hydrates gels (hydrogel), which is de-watered, dried and sintered at 11 50-1350°C to produce the cement clinker. The sintering temperature depends on the type of cement intended to be produced. The above hydrogel process makes it possible to use about 30 % fly ash in the manufacture of cement clinkers. The present paper highlights the activation technique and the process of hydrogel clinkering route. The chemico-mineralogical, microstructural characteristics of the clinkers produced through this process is also illustrated.

This paper describes the approach for customization of fly ash, focused on the proportioning of concrete mixtures for applications where specific performance is needed such as strength, durability and workabili-ty. In order to tailor these specific concrete mixtures, bulk fly ashes can be selected and upgraded to fulfil the requirements with respect to for instance, grain size distribution and carbon content (LOI). By customizati-on of fly ash its value will increase, which is beneficial inspite of the cost for upgrading. For customization, basic knowledge and relationships are needed about the decisive influential factors of the total system of concre-te components and design. To this end, descriptive models have been evaluated and applied with the emphasis on models for fly ash fineness and packing. These models have been used as a background for classifica-tion and micronization of fly ash. In experimental work, a relation was found between the dry pac-king of the solid constituents of mortar and the workability after the addition of water. Then fine and ultra-fine fly ashes were processed throu-gh air-classification and grinding (micronization). These processed fly ashes were added to concrete and the strength development was compared to concrete with silica fume and with reference concrete. In full scale tests the suitability of micronized fly ash for the production of very workable concrete, was confirmed.

In many standard specifications there is a limit for the maximum amount of unburnt carbon of fly ashes often referred to as LOI. In particular, according to the European norm EN 450, this limit is 5% on the continental basis of the European Unity, or 7% on the domestic national basis. Therefore, fly ashes with LO1 over 7% should be rejected as a supplementary cementitious material in concrete mixtures. Four fly ashes from coal-fired electric generating plants, with LO1 content of about 4, 7, 9, and 1 l%, were used to manufacture concrete mixtures. They had the water-cement (W/C) ratio of 0.68, corresponding to a water-binder ratio of 0.48 and a fly ash/binder ratio of 0.30. A small amount of superplasticizer (0.3- 0.4% by cement mass) was required to compensate the slump decrease caused by fly ash with higher LOI (> 7%). Two reference concrete mixtures, without fly ash, were also produced with a w/c of 0.68 and 0.48. The performance of all these concrete mixtures was assessed in terms of compressive strength at early and later ages (l-l 80 days), water-permeability, chloride diffusion, and carbonation rate. There was no evidence available which indicated that the LO1 content of the fly ash affected negatively any of the properties studied. In particular, due perhaps to its peculiar pozzolanic activity, the fly ash with the highest LO1 content (11.30%) performed better than that with the smallest amount of LO1 material, (4.19%). This occurred in terms of higher compressive strength, lower water-permeability, slower chloride diffusion, and decreased carbonation rate in the corresponding concretes. Therefore, the conformity criteria adopted by some standard specifications in rejecting fly ashes only on the basis of the relatively high LO1 content, without determining the corresponding concrete performance in terms of strength and durability, appear to be technologically inadequate and