International Concrete Abstracts Portal

The use of autoclaving processes in producing cement-based products is well established. The main reason for using autoclaving processes is often to increase production rate and/or to decrease sensitivity towards variations in humidity, such as reducing moisture movements. The latter effect is mainly believed to be caused by an improved crystallinity of the finished material. The use of fly ash as the siliceous component in the base mixture is also well known. However, the crystallinity is ignored by the use of a noncrystalline base material, such as fly ash. From productions of autoclaved cement-based products utilizing silica sand it is known, that additions of small amounts of gypsum to the base mixture may improve strength and reduce moisture move-ments. This paper deals with the production of autoclaved materials utilizing fly ash and gypsum or its derivatives. Materials produced are characterized according to their density and strength-characteristics as well as to their crystallinity. The use of gypsum or its derivatives may cause significant improvements in strength as well as in crystallinity, but the op-timum design is closely related to the actual production process as well as to the chemical properties of the base materials and to the physical properties of the fly ash.

Experiments are reported where the strength development of concretes is investigated containing 75% by weight of portland cement and 25% fly ash with various water-cementitious materials ratios. The strength results of these concretes are compared not only to concretes containing no mineral admixtures but also to concretes containing 75% by weight of portland cement and 25% quartz powder. New as well as old experimental data seem to in-dicate that the contribution of fly ash to the quality of concrete is not a constant value determined solely by the physical and chemical characteristics of the fly ash but rather it can vary in different concretes. For instance, the relative contribution of a fly ash to concrete strength is increasing with decreasing water-cementitious materials ratio. The paper closes with a discussion of research needed for the clarification of the factors that maximize the contribution of fly ash to the strength of concrete.

SP91 This contains 78 symposium papers, bringing together the expertise of representatives from industry, government and universities. These volumes present the latest advances in the use of fly ash, silica fume, slag and natural pozzolans in concrete. New technologies are explored to provide ways in which these valuable mineral by-products can best be used to conserve both resources and energy. Case studies include: the effect of fly ash on physical properties of concrete; evaluation of kiln dust in concrete; effect of condensed silica fume on the strength development of concrete, and the influence of slag cement on the water sorptivity of concrete.

Recently, a high alkali high calcium fly ash, (6 t o 7.5 % Na20 equivalent) has started to be produced from Western Canadian lignite coal at Ontario Hydro’s Thunder Bay Generating Station. Because of its high alkali content, ASTM C 441 mortar bars containing Pyrex fine aggregate and 25 volume percent fly ash were tested initially. Even with its 2.9% available alkali content , the fly ash replacement reduced expansions at one year by approximately 50 percent relative to the high-alkali portland cement alone.Based on the C441 results and good pozzolanic properties, a program was undertaken to evaluate the performance of this lignite ash in air-entrained concrete. In this program, strength deve lopment, air-void parameters, resistance to freezing and thawing (ASTM C 666 Procedure A), permeability, and pore size parameters were evaluated. For up to 35 weight percent replacement with fly ash and with mixtures adjusted to constant slump, significantly lower water to cementing materials ratios were achieved, equal strengths were attained in only 6 days along with higher later-age strengths, freezing-and-thawing resistance was excel lent and permeabilities and porosities were reduced. In addition, while the sulfate resistance of many lignite ashes is poor , the performance of ASTM C 1012 mortar bars with 35 weight percent replacement for a 12.8% C 3 A portland cement was similar to that for moderate sulfate resisting portland cements (C3A L, 8 .0 %).

Tests of portland cement concretes contain-ing Class F and Class C fly ashes from ten different sources were conducted to evaluate mixing water requirement, time of setting, bleeding, compressive strength, drying shrinkage, abrasion resistance, and absorption. The effects of moisture availability and temperature during curing were also examined. Mixing water requirement was reduced for concretes with Class C fly ash. There was no consistent water reduction when Class F fly ashes were used. Slight to signifi-cant retardation of setting time was noted for concretes with fly ash. Setting time generally increased as concrete mixing water requirement increased. Concretes with fly ash showed less bleeding than control concretes. Concretes with Class C fly ash showed less bleeding than concretes with Class F fly ash. Concretes containing Class C fly ash developed higher early age compressive strength than concretes with Class F fly ash. Compressive strengths of concretes with Class F fly ash were more susceptible to low curing temperatures than those for concretes with Class C fly ash. At early ages, compressive strength of concretes with fly ash, regardless of class, was essentially unaffected by moisture availability. Abrasion resistance of control concretes and concretes containing fly ash was dependent on compressive strength. Drying shrinkage and absorption of the concretes were generally unaffected by the use o f f l y ash.