International Concrete Abstracts Portal

The total chemical shrinkage of silica fume and Class F fly ash, both as pozzolanic materials reacting with lime and as mineral additives replacing portland cement, was studied. The external chemical shrinkage of cement paste with silica fume and fly ash replacement was studied as well. By increasing pH the rate related to the pozzolanic reaction decreased for sitica fume and increased for fly ash. Although the presence of alkalis are catalytically necessary for a rapid pozzolanic reaction of silica fume, the pH increase reduces the solubihty of calcium hydroxide (CH) due to the common ion effect. This may explain why the reaction rate decreases if dissolution of CH, followed by precipitation of CSH, is the rate-limiting step. The increased reactivity of fly ash, caused by a pH increase, indicates that the dissolution of the glassy aluminosilicate phase by alkalis was determining the overall rate of the process. The total chemical shrinkage was crudely estimated to be 8.8 ml/l00 g of reacted silica fume and 10.0 ml/l00 g of reacted fly ash, as compared with 6.3 ml/l00 g of portland cement. The measured shrinkage for silica fume could be higher than the above value since minor amounts of silicon metal in the silica fume could produce an expansion due to evolved hydrogen gas.

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.

The behavior of superplasticizers has been studied in highly alkaline suspensions of magnesium hydroxide and silica fume, which can be considered as good model system for cementitious systems containing silica fume. Rheology showed that as superplasticizer dosage is increased, suspensions pass from behaving as Bingham fluids to Newtonian fluids. Beyond a critical concentration large dispersed particles sediment due to the absence of yield stress. The critical concentrations required to obtain Newtonian fluids has allowed to elucidate the dependence between adsorption and dispersion. Indeed, dispersion appears to be only linked to adsorbed polymers and can therefore be attributed either to electrostatic or steric repulsion mechanisms. On the other hand, superplasticizer requirement increases with silica fume fraction in particular with the less ionic polymer. This indicates important electrostatic interactions with the surface in the process of adsorption.