International Concrete Abstracts Portal

Paper is concerned with the research of the effective utilization of fly ash produced from power plants. Three classes of "classified fly ash" produced by classifying conventional fly ash by air separation with the maximum particle diameters of about 20, 10, and 5 æm have been investigated. Special attention has been given to concrete strength enhancement effect due to classified fly ash. Experimental studies have reported on the basic properties of fresh concrete and hardened concrete having low water-binder ratio and high strength, produced by mixing the classified fly ash having the maximum particle diameter of about 10 æm, alone or in combination with ground granulated blast furnace slag. It is shown that the classified fly ash is an effective material that contributes to the reduction of superplasticizer requirements that are generally used in high-strength concrete, improvement of workability by reduced viscosity, and improvement of strength development, whether the classified fly ash is used alone or in combination with ground granulated blast furnace slag.

The purpose of this research was to study the ability of Class C fly ash for high-volume concretes when its sulfate content reaches the limit in its specifications. The laboratory test items were compressive strength, length change in concrete cylinders 70 x 780 mm, and quantitative analysis of all materials. Calcium hydroxide content in concretes was also measured with a new method by selective extraction. This is suggested to determine the decreasing content of the calcium oxide in concretes, as responsible for he pozzolanic behavior or its carbonation degree. One type of Class C fly ash with different ages and three classes of cements with different C3A content were tested. The influence of curing conditions in tap water immersion, the different potential contents of ettringite, and calcium hydroxide were taken into account. The following results were obtained. The content of SO3 in fly ash very near the 5 percent specification limit used in high-volume concretes with substitutions of 60 percent of cement gave no undue expansions for given conditions. No significant length changes in concrete were observed in any of the tests. The strength development shows good values, especially when the concrete was cured in tap water.

Adequate curing is essential for all concrete, whether it contains fly ash or not, if the potential properties of concrete are to be fully realized. However, since the long-term benefits associated with the pozzolanic reaction have become more evident in well-cured concrete, it has been generally considered that concrete containing fly ash has a greater susceptibility to poor curing than plain concrete. Tests were carried out at the Department of Civil Engineering of the K. U. Leuven on a series of mortar mixes with a range of fly ash-cement ratios to study the effect of curing on the strength development of mortar. Mortar specimens were subjected to a range of moist-curing periods prior to air-storage. Compressive strength was determined at various ages. The results confirm the importance of curing, with reductions in curing period resulting in lower strength. The strength of the mortar containing fly ash appears to be more sensitive to poor curing than the plain mortar.

There is an increasing tendency worldwide toward using cements blended with fly ash, silica fume, blast furnace slag, and natural pozzolans. Incorporation of these materials in concrete makes it dense and impermeable. While the effect of chloride and sulfate ions on the durability of blended cements is well documented, meager data are available on the synergistic effect of high concentrations of these salts on the durability performance of these cements. Since the structural components, especially foundations in the coastal areas in some parts of the world, are subjected to high concentrations of these salts, it is imperative to investigate the performance of blended cements in such environments. In this investigation, mortar and concrete specimens made with Type I cement blended with fly ash, silica fume, and blast furnace slag were exposed to a highly concentrated chloride-sulfate (2.1 percent SO4-- and 15 percent Cl- solution for a period of 540 days. The performance of these cements in resisting reinforcement corrosion was evaluated by monitoring half-cell potentials and measuring corrosion rates at periodic intervals. Deterioration due to sulfate ions was evaluated by visual survey, and measuring loss in compressive strength. Results indicate that surface deterioration and loss in strength was greater in blast furnace slag and silica-fume cement specimens compared to fly ash and plain cement specimens. Severe surface scaling and considerable reduction in strength (55 to 75 percent) was observed in the former cements. Moderate surface deterioration and loss in strength of about 25 percent was observed in fly ash and Type I cements. Corrosion of steel in silica fume and blast furnace slag was, however, much lower than in fly ash blended and Type I cements.

Describes the basic properties of concrete containing ultrafine particles, which are produced from fly ash. The ultrafine particles are produced from fly ash with ultra-high temperature treatment. This treatment enables control of the specific surface area, from 20 to 130 mý/g, by controlling the quenching speed. The main chemical component is SiO2, over 60 percent of which is amorphous. Ignition loss, which is 1 to 5 percent with fly ash, is below 0.2 percent. The properties of concrete with these ultrafine particles differ greatly in the specific surface area of the particles. Experiments showed that ultrafine particles with a specific surface area of 71 mý/g develop a compressive strength of approximately 118 MPa (w/c = 25 percent), while plain concrete develops approximately 105 MPa. Ultrafine particles with a specific surface area of 35 mý/g improve the consistency of fresh concrete, especially in a low water/cement (w/c = 20 to 25 percent), enabling concrete to be easily mixed without increasing the dosage of high-range air-entraining (AE) water reducer. Results show ultrafine particles to be highly active and useful as an admix material for high strength concrete.