Efeito da nanossílica em matrizes cimentícias de ultra alta resistência

Abstract

Ultra high performance concretes are estrutural composites, which exhibit a higher structural efficiency, allowing the implementation of lighter structures. Recent researchies have demonstrated that the use of nanossílica contributes to the gain on mechanical performance of these cementiteous materials. This thesis evaluated the effect of nanossílica in ultra high strength matrix, for which mechanical properties were measured, such as: compression strength, flexura strength and dynamic elastic modulus. The reaction kinetics was analyzed using adiabatic calorimetry and rheological behavior of suspensions was evaluated throught rotational rheometry and mini-slump tests. The pore size distribution was measured by mercury intrusion porosimetry. And the hydration products were identified and quantified by X-ray diffractometry and thermogravimetry. Scanning Electron Microscopy was employed in order to evaluate the microstrucuture of matrix and chemical analysis was obtained using backscattered electron probe. The results show that nano-silica changes the reaction kinetics of mixtures, reducing induction period and accelerating the hydration process according to the content of nano-silica. The rheological behavior of mixtures was also influencied by the use of nano-silica, reducing the paste spread due to the increased viscosity and yield stress of suspensions. For mixtures containing 0,2% of nano-silica, gains ranging from 12 to 16% were obtained on compression strength. However, for nano-silica content exceeding 0,8%, a reduction in mechanical performance was achivied. It was found that nano-silica increases the open porosity of the mixtures. Hydration of cimentiteous matrix also changes due the incorporation of nano-silica, increasing the consumption of di and tri-calcium silicates of Portland cement, measured through X-ray diffraction. As consequence, an increasing on aluminates and silicates calcium hydrates, and calcium hydroxide can be observed for mixtures formulated with nanoparticles of silica. It can be conclued that, depending on content used, nano-silica contributes to the resistance gain, without compromissing rheological behavior and reaction kinetics of cementitious matrix. Nuccleation and pozzolanic are the main effects of nano-silica on microstructure of ultra high strenght cementiteous matrix.