The aerated concrete is a fully mature technology. It is a dynamic, single component building material system that is a mixture of Portland cement, sand, aluminum powder, and water. In the early 1920's Dr. Axel Eriksson an Assistant Professor for Building Techniques at the Royal Institute in Stockholm, discovered that by adding aluminum powder to cement, water, and finely ground sand caused the mixture to expand dramatically.
This is a four part articles series which will touch on some of the most common properties of aerated concrete. In Part 1 of this series, I will discuss about the density and compressive strength of aerated concrete.
Density and Compressive Strength
Aerated concrete is a material with good mechanical strength, together with a high insulation value over a wide range of densities. The density of aerated concrete is affected by the water cementitious ratio; because the amount of aeration depends on the water cementitious ratio. However, when pozzolans is used, water solids ratio is more important than water cementitious ratio. In determining the water solid ratio sand also will be included. For gas formed concrete, a lesser water-solids ratio would lead to insufficient aeration, while a higher water solid ratio will results in rupture of the voids. However, in both conditions, some increase can be expected in density. According to Neville (1973), aerated concrete can be produced in any required density. This is due to the fact that the density directly related to the gas forming admixtures (aluminum powder). Besides that, there has been general rule that compressive strength increases linearly with the density.
Compressive strength is one of the most important characteristics of concrete. It has been used as a yardstick to determine the quality of concrete, and this is not an exceptional for aerated concrete. The specimen size and shape, method of pore-formation, direction of loading, age, water content, characteristics of raw material, and method of curing have been reported to influence the strength of aerated concrete. Pore structures of the air pores and mechanical condition of pore shells have marked influence on the compressive strength of aerated concrete. The strength of non-autoclaved increases 30 to 80 percent between 28 days and 6 months, but only marginally beyond this period. A portion of this increase is attributed to the process of carbonation. Compressive strength variations inversely with moisture content, and this could have due to the water held in the pore structure acted as a lubricant in the microstructure of the material. On drying to equilibrium with normal atmosphere, there is an increase in strength and an even larger increase on complete drying out.
The part 2 of this article series will discuss on the drying shrinkage and water absorption properties of aerated concrete.