Otimização da resistência à compressão de pastas cimentícias com superdosagem de aditivo dispersante e adição de sílicas ativa e sol-gel

Abstract

The use of mineral additives, such as active silica, leads to improvements in the properties of cementitious materials, mainly related to the compressive strength of the paste. With additives particles in nanometer scale, the improvement tends to be intensified. Nanostructured silica can be obtained from various techniques, for example by the sol-gel process. In this research, the effects of silica fume and silica sol-gel on the structure and compressive strength of Portland cement paste with high initial resistance using low water/cement factor were compared. The effects of the silica milling, as well as the reduction of the kneading water amount of the samples and the amount of silica added to the paste were evaluated. The hydration of the cement paste was studied by Vicat assay along the cure and by X-ray diffraction after 60 days of cure. The compressive strength of the samples with silica additions was measured. For the addition of silica fume, the compressive strength was evaluated as a function of the water/cement and addition amount (from 10% to 25%) factors. Density and porosity measurements by the Archimedes method were performed in order to discuss the structural variations of the samples. Cement paste samples produced with ground silica fume and low water/cement factor presented mechanical properties of compressive strength superior to the samples produced with as-received silica fume after 28 days of cure. About the amount of silica fume added to the paste, the results showed that the use of 15% in weight presented satisfactory performance, since there was no significant reduction of the compressive strength when compared to the results using the amount of 25%. Thus, this quantity was used to produce cement paste samples with silica sol-gel. Measurements of density, porosity, and compressive strength showed similar structural evolutions along the cure process for the silica fume and silica sol-gel ground or not. The silica sol-gel showed significant pozzolanic activity probably due to its high specific surface area. Cement paste samples produced with silica sol-gel, specially the milled one, showed higher compressive strength than the samples produced with industrialized silica fume.