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The microstructural implications of the preferred orientation of portlandite crystals and portlandite saturation factors in the pore solution of a fly-ash-pozzolanic-cement mortar that is used in Spain for the disposal of low-and medium-level radioactive waste have been discussed. Changes in the saturation of portlandite, which were promoted by the interaction of two types of simulated radioactive liquid waste (SRLW) with the mortar, were evaluated during a period of 365 d at a temperature of 40°C. The mortar was immersed in two SRLW materials, whose main ions were SO 4 2؊ (0.68M), PO 4 3؊ (0.89M), and Cl ؊ (0.51M) in one case and SO 4 2؊ (2.05M) and Na ؉ (4.1M) in the other case. The diffusion of those ions through the porous mortar microstructure and related dissolution-precipitation reactions influenced both the quantity and the degree of orientation of portlandite crystals at the paste/sand and paste/ fly-ash interfaces. All these effects will lead to a pore refinement of the mortar that improves the strength and durability of a mixture of fly ash and cement mortar.

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The influence of NaCl (3% of Cl" by weight of cement) on the reactivity of High Alumina Cement (HAC) in water has been studied over a period of one month. The changes in microstructure were monitored by x-ray diffraction and scanning electron microscopy. The pore-solution, extracted by the application of high mechanical pressure (500 MPa), was studied for the chemical composition and changes caused by chloride and sodium ions.