Experimental study of the mechanical anisotropy of aerated concretes and of the adjustment parameters of the introduced porosity

The aerated concrete manufacturing process consists in the creation of macro porosity in a micro-mortar matrix with the help of an expansive agent (aluminum powder), which reacts with the water and the lime liberated by the hydration of the binder. Theoretical studies previously conducted on the idealization of aerated concrete have shown that its behavior can present mechanical and thermal anisotropies due to the configuration of the porosity (form, distribution and orientation of pores). Compared with spherical pores, ellipsoidal pores flattened uniaxially, with equal porosity, may improve the mechanical performances in the direction parallel to their long axis and the thermal performances in the perpendicular direction. The aim of this work is to evaluate the development of the macro-porosity by adjusting the different parameters of composition. The quantitative aspect and the qualitative aspect of the macro-porosity were considered at the same time; the former concerns the quantity of pores and the latter their geometrical configuration. Different compositions of non-autoclaved aerated concretes were studied, in which the nature of the binder, the quantity of water, the quantity of sand and the proportioning and the nature of the expansive agent varied. At first the paper presents the study of the influence of each parameter on the introduced porosity. This one is calculated as the difference between the apparent density of the basic matrix and the apparent density of the corresponding lightweight material. Then the incidence of the introduced porosity on the mechanical strengths and anisotropy of the material is studied. The results show a relationship between the quantitative and qualitative aspects of the introduced porosity. This relation is studied with respect to the relativity of the kinetics of setting and expansion, which seems to be a major parameter to optimize properties of aerated concretes.