Ageing of clay and clay–tannin geomaterials for building

• Clay geomaterials for building are microcomposites materials with large sand grains embedded in a clay matrix phase.
• The macroscopic strength is from the binding property of the clays matrix and it can be increased with natural tannins.
• Ageing is evidenced by creep behavior under 0.2 MPa during 21 days, and is changed by water and tannin content.
• Ageing is induced by failing of microstructural internal bindings, leading to the weakening of the macroscopic strength.

Clay geomaterials for building are clay sand mixture with controlled microstructural characteristics. The macroscopic strength mostly results from the binding property of clays and can be increased with the addition of various additives as tannins from plants. Such an improvement is due to the formation of chemical complexes tannin–clay–iron hydroxides. In this study, controlled clay–sand mixtures were used to obtain compressed blocks with a composite microstructure. The used clay is mixed with tannin compound that is simply obtained from Parkia biglobosa trees of Burkina Faso.Creep curves during 21 days under a stress level commonly used in building (0.2 MPa) evidence a complex behavior with successive strain stages, depending on water and tannin contents. In general, creep is due to a slow and continuous volumetric deformation of the clay matrix and also to delayed micro-cracks propagation at sharp corners of sand grains. Ageing is evidenced by multi stage creep curves, resulting from local stress distribution within the matrix clay phase, which evolve with clay humidity and tannin content. Under a permanent stress, the microstructural internal bindings evolve with time leading to the weakening of the macroscopic material strength, which progressively tends to failure.