Self-compacting concretes (SCC) represent a move toward a sustainable material since they encourage the use of waste and recycled materials. The high volume of very fine powder necessary to achieve deformability and passing ability properties, in fact, permits SCC to consume large amount of fly-ash, very fine particles generated by the recycling of demolished concrete structures, and huge amount of calcareous filler available from the marble quarries. Moreover SCC turn out to be materials with an extended durability with respect to conventional concretes. Since fresh properties of self-compacting concretes (SCC) are significantly different from those of conventional concretes (CC) durability can be significantly improved when a SCC is used due to a modification of the microstructure of the interfacial transition zone between aggregates and cement matrix. This paper presents results of an experimental study carried out to evaluate changes in microstructure of interfacial transition zone (itz) and of bulk paste for both SCC and CC. Data on the influence of the calcareous filler, a fundamental ingredients to achieve self-compactability, on the hydration process of cement are also presented. Data indicate that the decrease in internal bleeding, when self-compacting concrete is used, seems to favour the formation of a stronger transition zone characterized by a less porous structure and with a limited amount of microcracking responsible for higher compressive strength values for SCC with respect CC. No differences were detected by EDAX analysis in the chemical nature of itz with respect the bulk matrix both for SCC and CC. Finally, observations of the cement hydration by analysis of the temperature profile vs time seem to indicate the calcareous grains promote formation of heterogeneous nucleation responsible for the increased crystallinity of ettringite, for a shorter normally dormant period and, hence, for higher strength values at early ages, when the calcareous filler is used.
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