International Concrete Abstracts Portal

Conditioning coal-burning power-plant flue gases with ammonia reduces the emission of nitrous oxide compounds. But the ammonia often combines with available sulfur and other compounds that attach to the fly ash. If the ammoniated fly ash is then used in concrete, the high-pH environment causes a release of ammonia and a strong, objectionable ammonia smell. This can make the fly ash unmarketable. What’s the solution? Fly ash beneficiation processes that can remove ammonia and also reduce the unburned carbon content. Some of the processes are described in one of the 54 papers included in ACI SP-199, Seventh CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete. Other papers deal with effects of fly ash and admixture combinations on setting time, use of slag concrete to reduce corrosion of reinforcement, and the role of chemical and mineral admixtures in concrete made with recycled concrete as aggregate. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP199

In developed countries use of mineral admixtures such as fly ash, silica fume has already been adopted in making concrete. This includes commercial application on a large scale either for addition or for replacement of cement. In India too such replacements have been readily accepted. With the introduction of ready mixed concrete the process has been accelerated in recent times. An investigation was undertaken to study the effects of fly ash and silica fume in concrete. Compressive strengths at different levels of replacements were found. Silica fume from a local source and fly ash from Ramagundam thermal power station of the State of Andhra Pradesh were used. Maximum size of coarse aggregate was 12.5 mm. Water to cementious materials ratio was 0.32 and aggregate-cementitious materials ratio was 3.2. Cement replacement levels by fly ash were 0, 10, 20, 30, and 40 percents and by silica fume were 0,4, 8, 12, and 16 percents. Thus a total of 25 mixtures were studied. Strengths at the ages of 28 days and 56 days were found. The results are presented in tables and figures. It was found that the highest replacement level of 40 % by fly ash and 16 % by silica fume, simultaneously, i.e. a total replacement of cement to the extent of 56 % gave a 10 % increase in the 28-day compressive strength compared to that of control concrete. Maximum increase of 43 % in the 28-day compressive strength was observed at a 32 % level of cement replacement ( 20 % by fly ash and 12 % by silica fume).

Good adhesion of a repair material to concrete is of vital importance in the application and performance of underwater concrete repair. This paper reviews techniques and results of bond strength test methods: compressive slant-shear test and tensile bond (load-point) test. The objective of this paper is to evaluate the effects of variation in water-to-binder ratio and silica fume and fly ash replacements on bond strength of underwater concrete repairs. The mixtures were proportioned with a Canadian Type 10 cement and two other binders, one with 10% silica fume, and secondly a ternary cement containing 6% silica fume and 20% Class F fly ash replacements. The water-to-binder ratios tested were 0.41 and 0.47. The mixtures were cast in water, on slabs placed in the bottom of forms, in blocks measuring 0.50 x 0.45 x 1 m with the free fall height in water 0.35 m. Cores were obtained from experimental blocks cast in water to evaluate bond strength test between old and new concretes by tensile load-point. The slant-shear adhesion was determined by casting cylinders above water with consolidation, and in water without consolidation. The samples were composed of old concrete cast in air (sawed and smooth surface) with new concrete above. The incorporation of 10% of silica fume, or 20% of fly ash and 6% of silica fume and the reductions of water-to-binder ratio from 0.47 to 0.41 resulted in significant increases in bond strength.

The paper presents the results of the chloride diffusivity of a wide spectrum of concretes with compressive strengths ranging from 30 - 120MPa having GGBS incorporation ranging from 0 - 85%. All these concretes were designed to have specific compressive strengths at the various percentages of replacement through an efficiency concept presented earlier. The chloride diffusivity of these concretes was assessed in accordance with the method suggested by ASTM C 1202 after 90 days of water curing. Tt was observed that a significant improvement in the chloride diffusivity could be achieved even in the lower strength concretes at the higher replacement levels. This clearly shows that it is not always necessary to have high strength to achieve the high chloride impermeability performance in concrete.

Both fly ash and granulated blast-furnace slag contain reactive alumina, the strength potential of which can be tapped through addition of gypsum. This has resulted in the development of cementitious mixture of fly ash, lime and gypsum, called FaL-G. In the presence of gypsum, some fly ash-lime mixtures render 3 to 6 times strength enhancement at all ages. The authors have found that high-volume fly ash blended portland cements also increase in strength with the addition of gypsum or anhydrite. Similar behaviour is observed in ground granulated blast-furnace slag blended portland cements also. This improved strength is attributed to the formation of additional calcium sulphoaluminate hydrates. The research findings are highly significant from standpoint of conservation of agricultural soils, minerals and energy, and better utilization of fly ashes and blast-furnace slag in tropical climates.