The effect of microstructure on the properties of foamed concrete

Abstract

Foamed concrete is produced when a foaming agent IS added to cement-based slurry consisting of cement, water, cement extender and filler. This lightweight building material can contribute significantly to uplift disadvantaged communities when used during the development of infrastructure. To achieve this goal, in depth research into the structural properties of the material is essential. Since 1992 tests to determine structural properties of foamed concrete have been conducted at the University of Pretoria. The results show that the compressive strength of foamed concrete is a function of age, ash/cement ratio and porosity and for a given porosity and age there is an optimum ash content, resulting in the maximum compressive strength. The focus of this research is therefore on foamed concrete mixtures with target densities varying between 700 kg/m3 and 1500 kg/m3 where the question that needed to be answered is: what is the influence of the microstructure on the physical and structural properties of these mixtures? The study is restricted to the effect of the microstructure on the relation between the physical properties (such as density, ash/cement ratio and porosity), and the structural property (compressive strength) of foamed concrete. In order to evaluate the influence of the microstructure on these relationships it was necessary to develop parameters to explain and quantify the microstructure of foamed concrete. An image processing and analysis system was applied to develop the air-void size distribution parameters and the air-void spacing parameters. These parameters were used to represent the microstructure (entrained air-void structure) of the foamed concrete mixtures. It was therefore now possible to plot graphs showing the effect of the microstructure on the physical and structural properties of foamed concrete. It was established that the 28-day dry densities have an influence on the air-void size distribution. In turn the air-void size distribution has an influence on the average % porosities and 28-day compressive strength of foamed concrete. The 28-day dry densities have no influence on the spacing of air-voids and in turn the spacing of the air-voids does not have any influence on the average % porosity and 28-day compressive strength.