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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.

A study was made of the effects of Saskatchewan lignite fly ash on the resistance of concrete to freezing and thawing. Concrete was made with either ASTM Types I or V cement and different percentages of fly ash with an air content of 4 to 6 percent. Performance of the concrete was evaluated by measuring the changes in its dynamic modulus and its mass. A scanning electron microscope was also used to examine the changes in the microstructure of the cement paste due to exposure to freezing and thawing. Results show that the use of high percentages of fly ash in concrete (35 and 50 percent) reduced its resistance to freezing and thawing even though it contained about 6 percent air and was cured in water for 80 days. However, concrete containing 20 percent fly ash gave satisfactory performance, provided its air content and strength were comparable to control concrete that contained no fly ash. Results from the SEM examination show that the decrease in resistance of fly ash concrete to freezing and thawing may be due to the slow migration of portlandite and ettringite crystals from the dense C-S-H zones to the air voids. Concrete with fly ash was less susceptible to the migration of portlandite, but its air voids contained more fibrous hydrates, which may have led to an increase in the past porosity.

Red mud is a by-product from the aluminum industry. To investigate the possibility of using this waste material as a pozzolan in the cement and concrete industries, tests were carried out to examine the pozzolanic properties of calcined red mud. Red mud was calcined for 5 hr at five different temperatures: 600, 650, 700, 750, and 800 C. Blended portland cements containing 30 or 50 percent of the calcined red mud were studied for hydration products, strength, and durability. The results indicated that the red mud had the maximum reactivity when calcined at 600 C, because on hydration the lime content of the blended cement was considerably reduced. The calcined red mud when used in combination with portland cement contributed to the formation of hydrated alumina-silicates and hydrogarnets. Very good compressive strengths were obtained with the blended cement containing 30 percent calcined red mud. Mortars cast with these blended cements were placed in solutions of seawater and acetic acid. The results indicated good stability of mortars to these environments.

Three of the major uses of silica fume (microsilica) additions to concrete have been to improve mechanical properties, improve corrosion resistance by reducing permeability to aggressive anions such as chlorides, and improve concrete resistance to chemical degradation. In the last two uses, the mechanical properties are also enhanced beyond those of ordinary portland cement concretes of the same mix proportions without silica fume. Thus, the production of durable concrete often leads to an improvement in mechanical properties. Long-term resistance in accelerated laboratory corrosion testing in sodium chloride solutions is documented. It is shown that silica fume significantly lowers chloride ingress with increasing efficiency as the water-cementitious ratio decreases. A clear improvement in corrosion performance with the addition of calcium nitrite corrosion inhibitor became evident in this long-term program. It is also documented that high concrete resistivities do not necessarily prevent severe corrosion from occurring. Chemical resistance of silica fume (microsilica) concretes to numerous acids, bases, and salts is also examined. The results show significant improvements with the addition of silica fume in the time to 25 percent mass loss in cyclic and continuous ponding experiments for most chemicals. For some highly alkaline solutions, there is no improvement with microsilica. Improvements in compressive strength are documented for the mixtures used in the corrosion and chemical resistance studies. Additional mixtures were examined to determine flexural strength and modulus of elasticity. These mixtures were similar in composition to those typically used for corrosion protection. The results showed that silica fume significantly increased strengths and the modulus of elasticity. The improvement in flexural strength was greater than that expected from formulas typically used for moderate strength concretes and the increase in modulus of elasticity was less. It is hoped that the design engineer will be able to utilize the data to take full advantage of the property improvements and not merely durability or strength improvements with silica fume.