Cementi contenenti fumi di silice e polimeri ibridi POSS: caratterizzazione delle propietà fisiche e microstrutturali.

In this thesis have been characterized the physical and microstructural properties of cement modified with the addition of microsilica (silica fume), replacing the cement in percentages of 5, 10 and 15%, and organic-inorganic hybrid polymers (Phenylic-POSS) with percentages of coating of 10, 30 and 50%; with water/cement ratio 0.5. The materials used have been characterized before being used with X-ray diffraction techniques, with laser granulometery and BET for the microsilica (for the activation). The hydration process of cement pastes has been observed within the first 24 hours, and the trend of the ratio CH/C3S after 1,7 and 28 days. The microstructures at 28 days of hydration have been analyzed by SEM and with the X-ray microtomography by techniques of image analysis; the information obtained on the porosity have been implemented with the helium pycnometry. The physical properties observed in mortars were the rheometry (workability) within the first hour of hydration, the uniaxial compressive strength at 28 and 60 days and tests of water absorption at 60 days. The presence of microsilica, in case it has not been activated with the POSS, has the effect of reducing the percentage of portlandite at 28 days through the pozzolanic reaction, in function of the percentage of addition. The activation leads to a more consistent development of the pozzolanic reaction at 5% of replacement, but the trend is reversed at 10 and 15%. This for the lack of correspondence between theoretical particle size distribution (between 0.5 and 20 μm) and the real (even several tens of microns) with consequent inerting of the admixture for excess POSS. The viscosity increases with the increase of microsilica; the POSS acts as a dispersant improving the workability. The distribution of porosity (between 0.5 and 160 μm) undergoes an increase in percentage between 1 and 20-30 μm in the pastes with microsilica compared with those without microsilica, because of the tendency of the latter to agglomerate. The helium pycnometry has however pointed out a reduction of open and total porosity with the increase of the microsilica. A greater reduction of open porosity seems to be present with a 10% of coating. The development of compressive strength at 28 days is accelerated compared to mortars without silica fume, then at 60 days the differential thins. The microsilica reduces the ability of the mortars to absorb water at 60 days with a decrease of up to almost 50% with 15% replacement.