Desempenho de argamassas para concretos autoadensáveis com baixos teores de cimento Portland

Abstract

The search for sustainability in the concrete industry involves an increasingly intensive use of Portland cement replacement materials. These materials, commonly called mineral additions, when used in large quantities can reduce the water alkalinity of the concrete pores, which can favors the corrosion of reinforcement in reinforced concrete. This work seeks to make a contribution in this field, having studied self-compacting mortars in which Portland cement has been replaced by 40%, in mass, by sugarcane bagasse ash (SCBA), rice husk ash (RHA) and metakaolin (MK), in addition to binary mixtures between additions. The effect of replacing an additional 10% of cement by lime was also verified. Thus, in addition to the reference mortar, 12 compositions were made involving the mentioned materials, six without lime and six with lime. The binder: sand ratio was 1: 1.64, and the water / binder ratio was 0.44. The percentage of additive was optimized through measurements of the spreading of the mine-cone and the content of incorporated air of the mixtures in the fresh state was measured. In the hardened state, compressive strength was obtained at 3, 7, 28 and 70 days and water absorption by capillarity at 28 days. The effect of accelerated carbonation on different compositions was studied through cubic specimens immersed in a carbonation chamber with a concentration of 10% CO2. Through the spraying of phenolphthalein, the carbonation depths were obtained at 3, 7, 14 and 28 days after the specimens were placed in the chamber. The microstructure of the thirteen compositions was analyzed using X-ray diffraction (DRX) and thermogravimetry (TG) tests, highlighting the presence and content of calcium hydroxide and calcium carbonate. The analyses were done before the specimens entered the carbonation chamber and after 28 days inside it. The results indicated that all mixtures with additions significantly reduced the absorption of water by capillarity, the metakaolin showing the best performance. The binary mixtures when SCBA was present led to higher values of compressive strength. The incorporation of lime interfered little in this property. The reference mortar showed a higher resistance value, reaching 66 MPa at 70 days, and two others mixtures had strength higher than 60 MPa. Concerning penetration of the carbonation front, the reference was the one with the lowest value. Binary mixtures led to a greater depth of carbonation. The presence of lime did not reduced this effect. In the XRD and TG tests, the additions reduced the amount of calcium hydroxide available, before accelerated carbonation. After carbonatation, calcium hydroxide is not present. Although in some parameters the performance of mortars with additions is lower than the reference, the mortars developed here can have practical application in the manufacture of selfcompacting concretes, with remarkable environmental benefits.