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Condensed silica fume (CSF) greatly influences not only the mechanical but also the physical properties of concrete. The most striking effect is the reduced permeability, caused by a change in the pore structure. Another sign of this alteration, though not as evident, is the change in the form of the water vapor isotherm. Preliminary results from an investigation concerning the first desorption isotherms of mortar with CSF-cement ratio varying between 0 and 25 percent and a water-cement ratio varying from 0.3 to 0.6 are presented. The results show that CSF influences the pore size distribution not only in the mesopore range, as shown in earlier studies, but also in the micropore range. The drying courses were also recorded in the project and it is clear that CSF significantly prolongs the time in reaching equilibrium, especially in relative humidities below 80 percent. This indicates that the continuous pore system is much narrower when CSF is incorporated. The question of when the "true" equilibrium is attained is discussed.

Results of an investigation of cement paste structure, and strength, permeability, and frost resistance of concrete with admixtures of silica fume type are given. The admixtures are waste materials from metallic silicon, low-grade ferrosilicon, ferrosilicon chrome production, containing SiO2 in the amount of 92, 70, and 66 percent, and surface area of 25.0, 44.9, and 18.5 mý/g, respectively. The influence of the admixtures on the cement paste microstructure results in an increase of gel porosity volume, decrease of capillarity porosity, and in an increase of strength. Thus, concrete strength increases and its permeability decreases. Physical and chemical properties of the silica fume-type admixtures insignificantly affect gel pore volume, whereas they have significant influence on capillary porosity. An increased dosage of high-range water-reducing admixture (HRWR) is a beneficent factor in increasing hydration degree and gel porosity, decreasing capillary porosity volume, and, consequently, increasing strength. Concrete frost resistance with silica fume dosages up to 10 percent of cement mass is not lower than the reference concrete with the same amount of binder.

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.