International Concrete Abstracts Portal

Blast furnace slags have been successfully used to reduce the leachability of technetium from cement-based waste forms because the slag produces a less permeable product or reduces the pertechnetate to a less mobile form. Waste contaminated with technetium is of particular concern to the U.S. Nuclear Regulatory Commission, Department of Energy, and Environmental Protection Agency because of its mobility as the pertechnetate ion. Results on the technetium leachability of cement-based waste forms with and without a slag component and for different slags are presented. The mass transfer parameter (e.g., diffusivity) for leaching technetium from these waste forms decreased by five orders of magnitude when slag was used (i.e., using slag can increase the ANS 16.1 leachability index by five). Results of bulk and surface examinations of the slags are presented.

Chemical characteristics of the cement paste-aggregate interfacial zone have been considered to influence the durability and mechanical properties of concrete. Particularly, effects of mineral admixtures such as fly ash and slag on the microstructure of the interfacial zone deserve attention. An x-ray diffraction technique was used to evaluate the amounts of Ca(OH)2, ettringite, and the orientation of Ca(OH)2 crystals in the interfacial zone. Composite specimens with several types of rocks were broken to produce a fracture surface on the cement paste prism to which the x-ray diffraction analysis was applied. The analyses showed that the addition of fly ash or slag considerably affected the peak height and orientation of Ca(OH)2 crystals in the interfacial zone, which normally extends up to 50 to 100 æm from the interface. The formation of ettringite in the vicinity of the aggregate surface was restricted by the addition of the admixtures. These results also suggest that the addition of the mineral admixtures favorably affects the resistance of the interfacial zone against aggressive agents from the surroundings. The x-ray fluorescence analysis was conducted to quantify calcium and silicon in the zone. The results obtained complemented the conclusions described previously. 137-389

Many marine and hydraulic structures must be constructed and repaired while submerged under water. Frequently, this requires placement of relatively thin (0.5 m or less) layers of concrete to fill voids in exposed surfaces or submerged formwork. Concretes placed underwater should flow readily and with little segregation and resist erosion from underwater currents. The hardened concrete should achieve excellent adhesion to underlying surfaces and develop high strengths. To achieve the desired performance, the concrete should contain a moderate amount of anti-washout admixture, a cement content of approximately 350 kg/m3, 25 kg/m3 of silica fume to enhance durability, and 18 kg/m3 fly ash to improve workability of the fresh concrete. A hard, natural gravel, representing approximately 54 percent of the aggregate content, should be used for wear resistance, and with the lowest possible w/c (0.41 ñ 0.03, typically) consistent with placement requirements, to maintain strengths. Prior to the actual field placement, several rheological and mechanical properties should be determined to insure proper placability, homogeneity, and therefore increase the success probability and cost effectiveness of site trials and subsequent actual field placements.

Durability properties of concrete and mortar based on a special type of alkali-activated slag called F-cement have been studied. The microstructure was found to possess a high occurrence of microcracks that had an obvious influence on the flexural strength and rate of carbonation. The rate of chloride-ion diffusion was about 30 times lower than in the portland cement concrete. Salt scaling was found to depend solely on the water-to-binder ratio and is independent of the air content. Early freezing takes place when the strength exceeds 5 MPa, and F-mortar shows high chemical resistance against solutions of sodium, calcium, and magnesium chloride.

Updates a 1983 critical review on pozzolanic and cementitious by-products for use in concrete. The by-products included in this report are fly ash, granulated blast-furnace slag, and condensed silica fume. Recently available worldwide statistics on production and utilization rates of these mineral admixtures are given. New information is presented on their physical and chemical characteristics, structure, and reactivity of the glassy phase, mechanisms by which concrete properties are enhanced, and engineering properties of concrete containing siliceous by-products. A special emphasis is given to durability aspects of concretes incorporating fly ash, blast-furnace slag, or condensed silica fume. Finally, the status of standard specifications and test methods is reviewed, and the contribution of siliceous by-products to make concrete an environment-friendly material of construction is emphasized.