International Concrete Abstracts Portal

The increasing construction of high-rise buildings in recent years had led to a demand for lightweight, high-strength concrete. In this study, the compositions of the matrix and the air void structure of aerated mortar containing silica fume were investigated as the basis for manufacturing lightweight, high-strength concrete. Mortars made with cement containing silica fume and fine or ultra-fine silica stone powder, having a particle size between that of cement and silica fume, were tested; the properties of cement paste in fresh and hardened conditions were improved. The compressive strength and the air void structure of prefoamed aerated mortars were determined and their relationship studied. Based on the results, it was confirmed that lightweight, high-strength concrete could be made with an effective combination of aerated mortar containing silica fume and lightweight coarse aggregate.

To assess the risk of corrosion due to high silica fume or fly ash content, hardened cement paste and concrete tests were performed at the Institute for Building Materials Research at the Aachen University of Technology to determine the influence of these concrete admixtures on the alkalinity of the pore solution and on chloride-induced corrosion of the reinforcing steel in the concrete. The fly ash content in the tests was up to 60 percent by mass and the silica fume content up to 25 percent by mass of total binder content. The mixtures were made up with a portland cement and a portland blast furnace slag cement (50 percent by mass blast furnace slag) at varying water-binder ratio. A combination of 45 percent by mass portland cement, 15 percent by mass silica fume, and 40 percent by mass fly ash was also included in the program. Reducing the portland cement clinker content in mixtures with high silica fume contents by the use of blast furnace slag or by the substitution of high amounts of fly ash leads to a rapid exhaustion of calcium hydroxide. Substantial quantities of alkalies are bound to reaction products, resulting in a dramatic drop of pH value in pore solution (below pH = 12.0) and, hence, increasing the risk of depassivation of the steel surface. The reduced alkalinity must be weighed against a significant refinement of pore structure through the rapid pozzolanic reaction of silica fume, clearly increasing the electrolytic resistance of concrete and reducing the corrosion rates to possibly negligible values.

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.