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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.

Presents results of investigations carried out on high fly ash content concrete (HFCC) cores removed from several structures constructed in the U.K. since 1979. Structures investigated included a road pavement, a major road viaduct, water-retaining and industrial structures, and a slipway subjected to marine exposure. Concrete properties measured after 10 years of service include compressive strength, depth of carbonation, permeability, and chloride and sulfate penetration profiles. In addition, petrographic analysis of thin sections was also undertaken. The HFCCs studied were designed considering the fly ash to be just a further ingredient in the concrete rather than as a cement replacement. This led to higher fly ash contents and lower cement contents than is generally normal practice. The structures examined were in excellent condition after 10 years. Results show a durable concrete exhibiting increases in compressive strength beyond 28 days, little evidence of carbonation, low to average permeability, and resistance to chloride penetration. In this respect, it is significant that at the marine exposure sites, the chloride concentrations decreased significantly with depth. No evidence of alkali-silica reaction was detected in spite of reactive aggregates being present in some of the concretes.

The results of a great number of trial mixes for mix design of roller compacted concrete (RCC) are presented. This particular RCC used a local fly ash, rich in lime and sulfates, which does not meet any existing specification. This fly ash's performance in concrete has been studied for some time at the Laboratory of Reinforced Concrete of Aristotle University of Thessaloniki. Recently, this fly ash was used in the construction of a large RCC dam in northern Greece. Measurements of the strength development and the elasticity of RCC mixes showed that the 80 percent (by weight) of the cementitious material could be substituted for this fly ash. Therefore, it was proven that in RCC mixes this fly ash is more effective than in conventional concrete.

In normal strength concretes, the compressive strength is limited by the strength of the binder and the binder-aggregate bond. In high-strength concretes, however, the binder strength and the bond may be fully comparable to the strength of the aggregate. This fact may lead to the conclusion that the strength of high-strength concretes may be improved by replacing an ordinary aggregate type with a high-strength aggregate. A number of aggregate types have been combined with high-strength binders to evaluate the impact of the aggregate strength on concrete compressive strength. The significance of the aggregate strength has been compared with the effect of the cement type and the use of silica fume. According to the obtained results, the impact of the aggregate strength on the strength of high-strength concrete is limited, compared to the binder type, while the difference in E-moduli between the different aggregate types is fully reflected in the concrete E-moduli. This contradiction is explained by a hypothesis based on stress concentrations due to the difference in rigidity between the binder and aggregate.

The mechanical and structural properties of mortars containing silica fume were studied. Mortars containing 5 and 10 percent active silica additive were made. Mortars without silica fume (standard mortars) were also prepared. A first set of mortar specimens was cured entirely in water. A second set of mortars was cured in air. The third was immersed in water and then subjected to alternating cycles of storage in water and air. The results show a very close relation between the conditions of the mortars' curing and their mechanical properties. The flexural strengths of mortars containing silica fume, subjected to variable curing conditions, show periodic increases and reductions. SEM observations confirmed the relations found in the flexural strength tests.