High-lime fly ashes obtained from the combustion of lignites or subbituminous coals are common by-products of thermal power plants in many countries, including Turkey. Chemical analyses, mineralogical analyses, and the formation of hydration and pozzolanic reaction products at different ages of three Turkish high-lime fly ashes were carried out using X-ray diffraction (XRD) techniques. The relationships between the mineral phases in the fly ashes and the hydration and pozzolanic reaction products were investigated. Fly ash is formed at combustion temperatures of approximately 1000 C, at which the clay impurities decompose. These fly ashes contained highly reactive silica and alumina. The reaction of these oxides with the free lime and anhydrite present in two of the fly ashes led to the formation of C-S-H gel and ettringite starting with the beginning of hydration. The third fly ash, having anhydrite as the only major calcium-bearing compound, produced gypsum upon hydration. However, introduction of Ca(OH)2 into the system resulted in similar reaction products. At later ages, beside the previously mentioned products, C4ACH11 and C4AH13 were also observed in all three cases. Interpretation of the results indicated that although all three fly ashes were of high-lime type, two of them were hydraulic and autopozzolanic, whereas the third was pozzolanic only.

Summarizes the work conducted by the Virginia Department of Transportation to evaluate the characteristics of concrete containing silica fume in the overlays as a protective system to prevent the penetration of chlorides into concrete. The first three field installations of silica fume concrete overlays in Virginia are described. The practices of other states in the USA for low-permeability silica fume concretes are also compared. The results indicate that silica fume concretes can be placed successfully in thin overlays on bridge decks. These concretes can provide the low permeabilities required to prevent the penetration of chlorides and other detrimental solutions into the concrete. Adherence to good construction practices is necessary, especially for the prevention of plastic shrinkage cracking.

Mortars with and without fly ash are cured initially in distilled water or NaCl solution for 7, 28, 56, and 91 days and then exposed to the accelerated carbonation. The influence of chloride ion on the depth of carbonation is evaluated. Furthermore, mortars initially cured in distilled water are exposed to the accelerated carbonation condition and then immersed in NaCl solution to study the influence of carbonation on penetration of chloride ion. In both cases, electrochemical properties of steel reinforcement embedded in the specimen are measured. The penetration depth of chloride ion in fly ash mortar immersed in NaCl solution is larger at an early age, but becomes almost the same as that of the control mortar later. The depth of carbonation of mortar cured initially in NaCl solution is smaller than that in distilled water, and the same trend is observed, independent of initial curing period and the addition of fly ash. Fly ash mortar shows higher carbonation depth than the control mortar. Corrosion current of steel reinforcement in mortar is affected by both carbonation depth and chloride ion penetration.

Recently in Japan, super low-heat cement (SLHC) with a much lower heat of hydration than the moderate heat portland cement generally used in massive concrete structures has been demanded for reducing the thermal cracking of mass concrete. To satisfy the requirement, the SLHC (heat of hydration: 167 kJ/kg at 28 days; compressive strength: 30 MPa at 28 days, complying with Japanese industrial standards), incorporating Blaine fineness > 600 m¦/kg and a large amount of ground granulated blast furnace slag (GGBS), has been developed. The effects of fineness and content of GGBS on heat of hydration and compressive strength of cement are described. Included also are test results of the bleeding ratio, compressive strength, adiabatic temperature rise, drying shrinkage, and carbonation depth of concrete using the SLHC.

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.