International Concrete Abstracts Portal

SP-132 Published in two volumes...The first volume contains papers dealing with fly ash and natural pozzolans. The second volume consists of papers dealing with condensed silica fume and ferrous and non-ferrous slags.

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.

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.

Class F fly ash was used as the basic granulating material. Catalysts and binders were added to evaluate the behavior of granulated material. The accelerated curing method was also considered. Results indicate that granulation rate depends closely on the slant angles of the disc, the revolving rate, the methods of adding admixtures, and granulation time. Though aluminum powder reduced unit weight and raised the strength of fresh particles, it had a detrimental effect on other properties. Addition of hydrophilic seed reduced the granulation time and increased productivity. The results showed that at a constant relative humidity, the higher the temperature, the more rapid and higher the strength development. It is important to maintain constant temperatures or low strengths may result. Normal steam curing, autoclave curing, and microwave steam curing have beneficial effects on the strength of fly ash lightweight aggregates. The differences in curing results are due to differences in mix proportioning of aggregates and duration of curing.

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.