International Concrete Abstracts Portal

The purpose of this study was to evaluate selected transport properties (resistance to chloride-ion penetration and rate of water absorption) of ternary concrete mixtures containing fly ash and silica fume using different test methods. The resistance to chloride-ion penetration was evaluated using two non-steady, electric field migration tests: rapid chloride permeability (RCP) (AASHTO T 277) test and rapid migration procedure (AASHTO TP 64). The rate of water absorption was evaluated using the ASTM C 1585 sorptivity test and the non-standard absorptivity (ponding water absorption) test. Four mixture compositions (with varying contents of fly ash and silica fume) and five different curing conditions were used in the study. The test results were statistically analyzed (using regression method) to explore the relationships between composition, age of concrete and transport properties. In addition, the existence of possible interrelationships between the transport properties themselves was examined. For continuously moist cured specimens (Series 1 mixtures) the migration-type tests were found to be more reproducible than the absorption-based methods. However, the repeatability of absorptivity test was greatly improved if specimens were dried for certain period of time prior to testing (Series 2 mixtures). Although the curing method was found to have a very significant impact on the RCP and initial absorptivity values of all tested concrete mixtures, at later age much of the observed differences in those properties diminished.

The production of lightweight aggregates entails heating a silica-rich raw material, which is usually shale, clay or slate to about 1150°C. This heat treatment appears to activate the surface of the expanded aggregates so as to produce pozzolanic properties that reduce expansion due to alkali-aggregate reaction. When these heat-treated aggregates are incorporated into a concrete mixture, beneficial effects result that enhance the properties of the concrete. These effects are similar to, but of a significant lower magnitude, than when a pozzolan such as fly ash or silica fume is added to a concrete mixture. This is to be expected because expanded shale, clay and slate that have been ground to a fineness somewhat greater than that of portland cement have been supplied to the concrete industry as a pozzolan for some time. Also, it has been shown in micrographs that the vesicular nature of lightweight aggregates provides a space for any reactants that might form to precipitate without causing expansion.

The application of building rubble collected from damaged structures has become an important issue in Taiwan. The reuse of construction wastes not only conserves the finite raw materials and reduces energy consumption, but also can be a solution for environmental protection. Among the wastes, bricks and tiles were produced under high temperature sintering, which is similar to the producing process of pozzolanic materials like fly ash or slag. Such material might lead to the pozzolanic reaction in concrete. This research intends to investigate the probability to replace the natural sand or cement by using waste bricks and tiles. First, the gradient of fine aggregate was fixed. Four replacement percentages (25%, 50%, 75%, and 100%) were selected to replace part of natural sand in the mortar mixtures. Then the influence to the proporties of mortars was estimated according to the test results. The bricks and tiles were ground into powder with the same fineness of cement. Three replacement percentages (10%, 20%, and 30%) were selected to replace part of cement. The influence to the proporties of mortars was estimated as well. Test results show that the strength of mortars using waste bricks and tiles as fine aggregate is slightly lower than that of the control batch. However, the strength efficiency is good. On the other hand, the mortar containing brick or tile powder possesses higher long-term compressive strength because of the pozzolanic reaction.

The purpose of this study was to examine the effect of a Class F fly ash on mechanical and durability properties of concrete. The fly ash used in this study, from a Mexican source, was characterized by chemical and mineralogical analysis, and by its pozzolanic activity. Concrete mixtures were prepared with 20, 25, and 30% fly ash by mass of total cementitious material. Concrete specimens were cast and tested to determine the durability of fly ash concrete; the tests used included water, rapid chloride permeability tests (RCPT), migration test, acid resistance and abrasion resistance tests. Also the compressive strength was determined. Mortar specimens were used to evaluate sulfate resistance and alkali-silica reaction. The results of this study confirmed that the Mexican Class F fly ash was suitable in improving the durability characteristics of concrete when used at 25% or higher dosage of cement replacement.

Reject fly ash (rFA) represents a significant portion of the fly ashes produced from coal or lignite-fired power plants. Due to the high carbon content and large mean particle diameter, rFA is not used in the construction sector (for example as a supplementary cementing material) and is currently dumped into landfills, thus representing an additional environmental burden. In order to maximize the use of solid wastes, there is a need to investigate potential applications for rFAs. The benefits, however, of the inclusion of recycled materials will only be realized if they result in improved performance and economic savings for the final product. In this work, critical mechanical and hydration properties of blended cements made by blending the coarse fraction (after grinding) of a lignite high-calcium fly ash with ordinary cement were measured and compared with the respective properties of a good quality fly ash-cement blend. The results reveal that a relatively cheap, two-step classification-grinding method is able to enhance the pozzolanic behaviour of the rFAs, so that the overall performance of rFA containing cement is significantly improved.