Influência da adição de sílica ativa e coloidal na durabilidade de materiais cimentícios submetidos a ataque por sulfatos

Abstract

A recurring challenge in civil construction is the durability of concrete structures inserted in aggressive environments. It is common in these environments the presence of sulfate ions, which react with the hydrated compounds of the hardened cement paste. These deleterious reactions produce expandable compounds as a product, which can generate internal tensions, cracking the cementitious matrix, allowing the entry of new aggressive agents. One of the solutions found to mitigate these effects was the use of mineral additions in cement. This research aims to compare the behavior of mortars with the partial replacement of Portland cement by silica fume and/or colloidal, against the degradation caused by the sulfate attack, for an exposure period of 140 days. For this, the dimensional variation, the mass variation, the surface electrical resistivity, the volumetric resistivity and the mechanical properties of the tested specimens were analyzed and evaluated. In this work, four mortar mixtures were prepared: the reference one T-REF (CPV - ARI), the T-10 mixture where Portland cement was replaced by silica fume at a content of 10% in relation to the mass, T-10-1, 5 (10% silica fume + 1.5% colloidal silica) and T-10-2.5 (10% silica fume + 2.5% colloidal silica). The composition of the mortar mix was based on current standards NBR 13583 (ABNT, 2014), that is, produced in a mass of 1,0:3,2 (binder: regular Brazilian sand); with a water/binder ratio of 0.60. The specimens were classified into two groups: group A (square section of 25 mm on a side and 285 mm in length) and group B (square section of 40 mm on a side and 160 mm in length). Samples from group A were evaluated for linear and mass dimensional variations. The samples from group B were evaluated for electrical resistivities, compressive strength and tensile in flexion strength. To obtain the necessary data, 96 specimens were needed, 24 for each type of mixture, 12 for each group. The samples were exposed to three different solutions: Ca(OH)2 + H2O, Na2SO4 + H2O e MgSO4 + H2O. The results demonstrated that only through an extended exposure period (20 weeks), it was possible to correctly evaluate the behavior of cements against sulfate attack. The partial replacement of Portland cement by silica fume and/or colloidal was effective in mitigating the attack by sodium sulfate (Na2SO4). The same did not occur in the case of exposure to (MgSO4), where samples T-10-1.5 and T-10-2.5 showed expansions greater than 0.03%. All samples with partial replacement of Portland cement suffered high mass losses. The volumetric variation test reinforced the data obtained in the mass variation test. All samples suffered significant losses in surface and volumetric electrical resistivities when exposed to aggressive sulfate solutions. The compressive strength was influenced by the replacement by pozzolanic additions, though the flexural tensile strength did not suffer significant losses. This research demonstrates that replacing Portland cement with sílica fume and/or colloidal silica was beneficial in the case of sodium sulfate, however, when suffering the more aggressive effects of magnesium sulfate, the samples proved to be unviable. Furthermore, the complementary tests and the complementary solution (MgSO4) proved to be essential for the correct assessment of the cement's resistance to sulfate attack.