International Concrete Abstracts Portal

A new variety of applications are being found for fly ash. It is increasingly important to determine the structural properties of fly ash particles, especially for increasing use in composites, such as polymer matrix and cast metal matrix composites. In this paper, selected properties of precipitator fly ash and cenosphere fly ash were directly measured; other properties were indirectly estimated from the experimental measurements on the properties of composites containing fly ash. The properties studied included sphericity of fly ash particles, wall thickness, the ratio of wall thickness to diameter, and modulus. The data generated on properties of fly ash have been compared with other filler or reinforcement materials, such as glass microspheres, Al 2O 3 , SiO 2, and SiC particles. The results show that the deviation of cenospheres from sphericity is virtually independent of diameter. The ratio of wall thickness to diameter of cenospheres is about 0.1 and tends to decrease with increasing particle diameter. The calculated values of effective modulus of fly ash from the property measurements on metal matrix-fly ash composites are in the range of 140 to 310 GPa, which are of the same order of magnitude as of general ceramics and glasses. The morphology of precipitator and cenosphere fly ash particles studied using scanning electron microscope is discussed. The implications of the morphology and properties of fly ash on its utilization, especially in micro and macro composites, including cements, are also discussed.

A test program was conducted to establish criteria for a performance- based specification of concrete quality, as a opposed to a prescriptive specification, for a major project in South Africa. Concretes containing different combinations of portland cement, fly ash, ground granulated blast furnace slag, and silica fume were prepared over a range of water/binder (W/CM) ratios. The samples were stored in water for three days to simulate the probable effects of site curing practice. Each concrete was then subjected to three different tests: air permeability and water sorptivity, both conducted in an "Autoclam," and a rapid chloride conductivity test. Time constraints prevented the preparation of a performance specification, but the results were used to prescribe a W/CM ratio and binder type. The results of the investigation also provide the basis for future evaluation of the site concrete by conducting similar tests on cores extracted from the structure. It was established that specifying only on the basis of concrete strength is insufficient to insure a high potential durability.

Cementless fine-grained concrete based on high-calcium fly ash and slag from thermal power plants was developed by the Siberian Metallurgical Institute in 1990. This paper presents the results of a study of schedules of heat treatment of the cementless concrete aimed at improvement of quality and durability of concrete. Prior to heat treatment, concrete was cured for three, six, and 12 hours at 60, 80, and 100 C. The temperature rise and cooling took three hours each. This cycle was provided by an automatic steam-curing chamber. After moist curing at high temperature using the above cycle, the specimens were tested for compressive strength immediately after cooling to room temperature and at the age of 28 days. It was found that the temperature of the isothermal heating should be in the range of 80 to 100 C. The best results were obtained with 100 C, although it is difficult to achieve this temperature, especially in cast-in-place construction. It also demands a great amount of electrical energy. Therefore, 80 to 90 C should be acceptable as the optimum temperature range. The optimum time of the isothermal heating is 9 to 12 hours. However, the computer processing of the results of the investigation showed that the optimum time of curing was six to seven hours. The technology and recommendations for heating of cementless slag ash concrete, by means of heating wires used in the construction of low-rise houses both in summer and winter periods, have been developed.

Presents a study on high-early-strength cements based on calcium sulfoaluminate, C 4A 3S. These cements can be produced at temperatures about 300 C lower than normal portland cement and can also be synthesized using industrial process wastes and byproducts, such as fly ash, blast furnace slag, chemical gypsums, and other waste materials containing reactive sulfate and alumina. Cements designed to contain C 4A 3S, Beta-C 2S, and CS or C 4A 3S, calcium sulfosilicate, C 5S 2S, and CS have been synthesized using (a) pure analytical reagent (AR) calcium carbonate or commercial limestone as the source of CaO; (b) fly ash, blast furnace slag, bauxite, clay, or alumina as the source of Al 2O 3 and SiO 2; and (c) natural gypsum, phosphogypsum, or desulfogypsum as the source of sulfate. Ettringite, C 6AS 3H 32, generated by the hydration of C 4A 3S and CS is responsible for the high early strength of these cements. The hydration of the silicate phase and the presence of C 5S 2S contribute to ultimate strength. These ettringite-containing cements do not expand and, in fact, have dimensional stabilities similar to portland cement. In these types of cements, durability problems may arise from the poor resistance of ettringite to carbonation. Due to the higher resistance to carbonation of another calcium sulfoaluminate hydrate, monosulfate (C 4ASH 12), the investigation has been extended to a composition which included brownmillerite, C 4AF, whose presence promotes the conversion of ettringite to monosulfate during hydration.

The geological characteristics of Mexico permit an important number of materials to be considered for use as pozzolans in the construction industry. These materials have great importance, due to the relevance to problems of concrete durability. This situation has caused an increase in the use of pozzolanic cement. This increase of use of pozzolanic cement creates a need for characterization of these products and evaluation of their performance based on the specifications related to their use in concrete. Mexican natural pozzolans meet the requirements of the specification, with some deficiencies in some pozzolans. The objective of this work was a detailed characterization of all pozzolans actually used in Mexico and evaluation of their use as an admixture in concrete, using for this purpose ASTM C 311 and C 618. Those particular points of the specifications that are not satisfied completely and the elements that contribute to the fact are discussed in this paper.