Effect of Simulated Radioactive Liquid Waste on the Microstructure of Cementitious Materials: Portlandite Orientation and Saturation Factors in the Pore Solution

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 SO42− (0.68M), PO43− (0.89M), and Cl− (0.51M) in one case and SO42− (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.