The effect of high volumes of ungraded fly ash on the properties of foamed concrete

Foamed concrete is produced by combining foam and slurry thus entrapping numerous small bubbles of air in the cement paste or mortar. The density of foamed concrete is a function of the volume of foam added to the slurry and the strength decreases with decreasing density. In this study the effect on the properties of foamed concrete, of replacing large volumes of cement with both classified and unclassified fly ash (pfa) was investigated. The casting densities of the materials used in this investigation varied between 1000 and 1500 kg/m3 and 50%, 66.7% and 75% of the cement (by weight) was replaced with pfa. The properties measured included compressive strength, dry density, porosity, permeability, water absorption, drying shrinkage, elastic deformation, creep, and void size distribution. The use of high volumes of unclassified ash did not appear to have any significant detrimental effects on the measured properties of the foamed concrete. Although the rate of gain in strength was reduced by the use of large volumes of ash, up to 67% of the cement can be replaced by ash without any significant reduction in, the long-term strength. The permeability increased with increased air content but the water absorption did not appear to be influenced by the volume of air entrained. The elastic modulus of foamed concrete reduced while the creep increased with reducing density. The drying shrinkage of foamed concrete does not seem to be a function of density and the shrinkage values were similar to those of cement paste with similar water/binder ratios; these values were between two to three times greater than those of conventional concrete. An increase in ash content lead to a slight reduction in the drying shrinkage. A mathematical model was developed relating the compressive strength of the foamed concrete to age, porosity and ash/cement ratio.

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