Carbonatação em compósitos cimentícios: influência do precondicionamento e do uso de sílica ativa e nanosílica

Abstract

Carbonation is one of the main deleterious effects of reinforced concrete structures, characterized by the loss of alkalinity in cementitious composites due to the action of acidic gases present in the atmosphere. This phenomenon results in the destruction of the passivating layer that protects the reinforcement and initiates the corrosive process. Pozzolanic additives, such as silica fume and nanosilica, as well as contribute to increasing the sustainability of the construction industry, interact with the cement through physical and chemical mechanisms, which alter the properties of the composites produced and influence the rate of carbonation front advancement. The present study aimed to assess the influence of silica fume and nanosilica on the carbonation performance of cementitious composites. To achieve this, twelve mix designs were defined for the manufacture of the test specimens, incorporating different levels of these additives and varying the water/binder ratio between 0.45 and 0.55. The accelerated carbonation test was conducted for up to 175 days, during which the advancement of the carbonation front, internal and external pH and mass variation of the samples were monitored. Characterization tests were performed to understand the changes induced by these additives, evaluating workability and entrained air content in the fresh state, and mechanical strength, open porosity, density and volumetric resistivity in the hardened state. The microstructure of the materials was assessed using X-ray computed microtomography. Additionally, a study was conducted to evaluate the influence of the preconditioning stage on accelerated carbonation tests, analyzing drying processes carried out in a vacuum oven or in a laboratory environment. A wide variety of procedures recommended in standards and literature were identified, making this stage a potential source of variability in the results obtained by different researchers. The results indicated a compressive strength increase of nearly 35% at 91 days and a bulk resistivity up to seven times higher, along with reductions in workability, porosity, and density in the mixtures containing the studied additives compared to the reference mix. Nanosilica had a more significant effect on mixtures with a water/binder ratio of 0.45. Despite the observed improvement in material properties, the carbonation front advancement rate was higher in mixes containing the additives, reaching an increase of nearly 92% compared to the reference. This behavior may be associated with cracking caused by carbonation shrinkage and the reduction in alkaline reserve, which exceeded the densification promoted in the material’s porous matrix. Monitoring of pH allowed for the observation of maintenance of internal samples alkalinity and a decrease in surface pH during the initial exposure to CO2. In the preconditioning study, it was found that samples dried in a laboratory environment exhibited a decrease in pH prior to entry into the carbonation chamber. The preconditioning process proposed in this study involves drying in a vacuum oven for a time adjusted according to the mix design, preventing pH reduction and balancing the pore saturation level of the specimens with the internal humidity of the carbonation chamber (65%), thus meeting the expected objectives for this stage.