Relation between the macroscopic behavior of clays and their microstructural properties

Abstract This paper shows the basis for establishing a relation between the mineralogical characteristics and the macroscopic properties of clays for radioactive waste disposal. Clays prepared as a paste were submitted to decreasing suction pressures to obtain the curves for the retention of water and changes in volume. The powders from these samples were compacted from 30 to 120 MPa. Comparison of the porosities and the water contents obtained shows that the compaction ability of materials depends on their mineralogical properties. The cation exchange capacity (CEC) of a clay and its specific surface area determined by ethylene glycol monoethyl ether (EGME) are the two best indicators of the mineralogical properties. They provide data on the texture and indicate an order of magnitude of the extent of desiccation required to reach the air-entry point. The existence of a three phase water-air-solution system in the clay matrix plays a decisive role in the physical stability of clays. For clays with low CEC (illites), rapid wetting produces a complete disintegration of the material. For smectites a high physical stability is confirmed, but significant macroscopic swelling is obtained only with dilute wetting solutions and with a monovalent cation such as Na + . It is concluded that the choice of smectitic materials is prefereable to clays that are partly illitic, like the mixed layer clays, where the poorer physical stability appears to be detrimental to the performance of engineered barriers.