Caracterizações de pastas de cimento Portland com adições de grafite, óxido de grafeno e nanotubos de carbono

Abstract

Materials composed of Portland cement matrix, such as mortars and concretes, are extensively used due to their versatile properties, such as mechanical strengths and durability, as well as the ease of molding parts into various geometries. As such, increasing research efforts focus on developing new applications for cementitious materials, especially with the use of additions. One of the most studied areas is the development of Portland cement matrices with carbon-based additions, such as graphite, graphene, and carbon nanotubes. In this context, it is important to deepen the understanding of the microstructure of Portland cement pastes with the use of carbon-based additions. The objective of this study was to investigate the influence of graphite, graphene oxide (GO), and carbon nanotubes (CNT) additions on the microstructure of hydrated Portland cement paste. For this purpose, 0,01%, 0,03%, and 0,05% of each addition by mass of cement were used, dispersed in pastes at 7, 28, and 90 days of submerged curing. After the curing period, tests were conducted for compressive strength, tensile strength, water absorption by immersion, electrical conductivity, and microstructural characterizations such as XRD, FTIR, Raman, NMR, and SEM/EDS. For compressive and tensile strengths, it was observed that graphite, due to its high deformability, could hinder the strengths, whereas GO and CNT showed more varied results, but with a greater tendency towards stabilization of values compared to the reference paste. Regarding absorption, no significant variations were identified, except for samples with graphite at 90 days of curing, which showed a reduction of 14,4% to 18,0% compared to the reference paste. Electrical conductivity showed gains at 28 days with the use of the additions, but at 90 days, there was a reduction in conductivity. With microstructural characterization, considerable influences of the additions on the formation of hydrated calcium silicate (C-S-H) were observed. Through Raman spectra, alterations in the proportion of portlandite and silicates were verified, and through NMR spectra, specific modifications of C-S-H were identified, optimizing the formation of chains with fewer bridges between oxygen and silicon (Si-O-Si), reducing the average chain length, and increasing the degree of hydration. This indicates an increase in the compaction of this phase due to a closer proximity between calcium silicate lamellae. Thus, it is understood that the results observed in the literature regarding gains in mechanical strength, especially with the use of graphene and carbon nanotubes, can be explained not only by the physical reinforcement of these additions in the cement matrix but also by their influence on cement hydration mechanisms, mainly on the morphology of C-S-H.