Hydromechanical effects: (I) on the Na-smectite microtexture J.-F. ALCOVER, Y. QI, M. AL-MUKHTAR, S. BONNAMY AND F. BERGAYA* CNRS, Universite ´d’Orle ´ans, Centre de Recherche sur la Matie `re Divise ´e, 1B, Rue de la Fe ´rollerie, 45071 Orleans, Cedex 2, France (Received 19 November 1998; revised 16 December 1999 ) ABSTRACT: Changes in particle organization and pore-spaces with applied mechanical and hydraulic stresses were followed using TEM, SAXS mercury porosimetry and gas adsorption for two Na-smectites, Laponite and hectorite, with similar structural formulae but different particle sizes. The TEM images show that hectorite has particles larger and more anisotropic than those of Laponite. The particles order perpendicularly to the direction of axial mechanical stress and become disoriented under hydraulic stress. According to the SAXS results, Laponite is composed of 1 - 3 small layers and hectorite of more compact (10 - 80 layers) particles. In Laponite, mechanical stress strongly reduces the amount of macropores but does not affect micropores and mesopores; hydraulic stress increases the macropores. In hectorite, the pore-volume is lower than in Laponite. The different techniques used yield complementary results andshow the considerable effect of layer dimension on the behaviour and microtexture parameters of smectite submitted to hydromechanical stresses. K EYWORDS: Na-Laponite, Na-hectorite, hydromechanical stress, BET, porosity, TEM, microtexture, SAXS. The determination of the microstructural parameters of clays is a fundamental key to the understanding of mechanisms controlling their macroscopic behaviour. The textural organization, in particular pore-space, strongly influences the rheological and flow properties (van Olphen, 1977). Clay water content evolves continuously from clay suspensions to clay pastes inducing different textural organiza- tion and properties. Using SAXS and TEM techniques, Pons et al. (1981, 1982), Ben Rhaiem et al. (1986, 1987) and Touret et al. (1990) have studied the textural behaviour of Ca- and/or Na- montmorillonite during drying and rewetting cycles of clay pastes in which solid is the main phase. A model has been proposed by Touret et al. (1990) for different types of pores, characterized by their size and their location (between layers, or particles, or aggregates). For Laponite suspensions (with water as the main phase), Morvan et al. (1994) and Mourchid et al. (1995) used SAXS to study the heterogeneous texture which differs from that of montmorillonite because of the limited size of the Laponite platelets. Studies relating microscopic properties to macro- scopic behaviour concerning compacted materials are important for waste disposal site characteriza- tion. They require examination of a large number of parameters: hydro-thermo-mechanical stresses, nature and mineralogical characteristicsof material, cation exchange capacity, specific surfaces, etc., (Pusch, 1982; Pusch & Carlsson, 1985; Tessier et al., 1992, 1998; Vasseur et al., 1995; Al-Mukhtar et al., 1996). Tessier et al. (1992) showed that Na + - smectite, with significant macroscopic swelling and high physical stability, is preferable as regards the performance of engineered barriers. Laponite is a model material for fundamental studies with regard to rheological behaviour, as its composition and layer dimension are well known (Fripiat et al., 1982; Mourchid et al ., 1995; Clay Minerals (2000) 35, 525–536 * E-mail: f.bergaya@cnrs-orleans.fr # 2000 The Mineralogical Society