Efeito da reação álcali-agregado (RAA) associada ao ataque interno por sulfato (isa) na absorção e porosidade de compósito de matriz cimentícia

Abstract

The degradation of the mortar caused by the attack by sulfates and alkali-aggregate reaction occurs by the presence of contaminants in the aggregates that react with the products of the cement hydration, being formed expansive products, causing deleterious effects to the material. This work aims at the isolated and combined analysis of the alkali-aggregate reaction by the presence of deformed quartz in the aggregate and the internal attack by sulphate due to the presence of pyrite, also in the aggregate, in Portland cement mortars, at different ages and mortar cure means, as well as the investigation of the microstructural changes, focusing on the absorption and the change in the porous structure of the material. The experimental program was divided into four stages: (i) characterization of the aggregates; (ii) accelerated tests and natural aging on mortar prisms; (iii) investigation of the behavior, in the natural environment, of concrete specimens with pyrite; (iv) absorption and porosity analysis. The petrographic analyzes, dimensional variations of mortar bars, compressive strengths and tensile strength by diametrical compression were good indicative for the occurrence of RAA and ISA in an isolated and combined manner, where the cures were partially and totally immersed in water maintained at 40 ° C intensified the reactions by the incidence of constant moisture and oxygenation. The porosities, water absorptions by immersion and by capillarity of the test specimens were indicative of the intensity of the alkali-aggregate and sulfate attack reactions, and being that for aggregates containing sulfides the variations could be attributed to the cracking produced by the formation of expansive crystals of sulfates, and for the reactive aggregates the variations could be attributed to the presence of myrhythmic intergrowth and undulating extinction of the silicates, culminating in the formation of expandable gels. In both pathological manifestations, the excessive formation of expansive phases promoted matrix cracking and increased proportion of macropores in the final ages. The analysis of pore diameter by mercury intrusion shows that the RAA occurs first in pores close to 50 nm, due to its character of reaction border, and ISA promotes the refinement and filling of the pores of the whole range of diameters by the formation of sulfate salts, in addition to both processes generating internal stresses and cracking of the matrix. For aggregates with combined effect, these processes occur concomitantly, having influence on the water absorption capacities by immersion and capillarity, besides the mechanical resistances and dimensional variations.