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Durability in aggressive environments is an important factor for extended the service life of concrete and the use ternary blended cements (limestone filler + calcined clays) can contribute for this purpose. In sulfate environments, the effects of supplementary cementing materials depend on the level, Portland cement and the progress of hydration reactions. Low level of limestone filler influences on the stabilization of AFt due to formation of monocarboaluminate, but high replacements increase of the effective w/c and the capillary porosity favouring penetration of the sulfates. The use of active pozzolans suppress the sulfate attack due to minimise both ettringite and gypsum formation. It is generally assumed that previously curing should be extended for the pozzolanic reaction progress to reduce the portlandite content and refine the pore size structure, reducing permeability, and thereby improving resistance. The question is if an illitic-calcined clay are effective, when it is used with limestone filler and the cement is exposed immediately to aggressive environments.In this paper, the external sulfate resistance of blended cements containing 30% replacement of limestone filler and/or calcined clay (C30F, C30CC and C15F15CC) are analysed. Two different calcined clays from Buenos Aires, Argentina were selected. Comparison of sulfate resistance was based on the expansion, mass gain or loss, visual appearance and compressive strength of mortar (w/cm = 0.485) exposed to sodium sulfate solution up to 6 months but without previous curing that the standards commands. Furthermore, the evolution of microstructure of blended cements exposed to sodium sulfate solution was analysed using XRD on the exposure surface and core of cement-blended pastes.