RESUMO O processo de carbonatação é um dos causadores da redução do potencial hidrogeniônico (pH) em compósitos cimentícios, o qual pode levar a despassivação das armaduras, dando condições ao início de um processo de corrosão. Neste trabalho, buscou-se acompanhar o avanço da frente de carbonatação conforme a norma europeia EN 14630 paralelamente à medição da variação do pH na superfície do compósito cimentício. Foram utilizados corpos de prova cilíndricos (CP) de argamassa, com dimensão de (2x4) cm, acondicionados em câmara de carbonatação com condições controladas, mantendo-se a umidade em 65 %, a temperatura em 28 ºC e a concentração de CO2 em 5 %. A estrutura de vazios da matriz foi avaliada por meio da técnica de microtomografia computadorizada de raios X (µ-CT). A leitura de pH permitiu um monitoramento do avanço da frente de carbonatação, em função da perda de alcalinidade, antes do processo estabelecido na norma EN 14630. As imagens obtidas por meio da µ-CT não permitiram visualizar a densificação do material presente na camada mais externa do CP.

ABSTRACT The carbonation process is one of the factors that cause a reduction in the hydrogenionic potential (pH) in cementitious composites, which can lead to a depassivation of the reinforcement structures, thus furnishing conditions for a corrosion process to begin. In this paper, the research sought to monitor the progress of the carbonation front according to European standard EN 14630, concurrently with the measurement of pH variation on the cementitious composite surface. Cylindrical mortar specimens with size of (2 x 4) cm were placed in a carbonation chamber with controlled conditions, keeping up the humidity at 65%, temperature at 28 °C and the CO2 concentration of 5%. The void structure of the matrix was evaluated by X-ray microtomography technique (μ-CT). The pH reading enabled the monitoring of the carbonation front progress in function of loss of alkalinity, even before the procedure laid down in standard EN 14630. The images obtained through µ-CT did not enable the visualization of the densification of the material found in the outermost layer of the specimen.

Geometric modeling is an important tool to evaluate structural parameters as well as to follow the application of stereological relationships. The obtention, visualization and analysis of volumetric images of the structure of materials, using computational geometric modeling, facilitates the determination of structural parameters of difficult experimental access, such as topological and morphological parameters. In this work, we developed a geometrical model implemented by computer software that simulates random pore structures. The number of nodes, number of branches (connections between nodes) and the number of isolated parts, are obtained. Also, the connectivity (C) is obtained from this application. Using a list of elements, nodes and branches, generated by the software, in AutoCAD® command line format, the obtained structure can be viewed and analyzed.

In this work we obtained microporous and mesoporous silica gels by sol-gel processing. Tetraethylortosilicate (TEOS) was used as precursor. Nitric acid and hydrofluoric acid were used as catalysts. In order to study the affect of formamide as drying additive, we used a molar ratio alkoxide/formamide of 1/1. The performance of formamide in obtaining crack-free gels was evaluated through monolithicity measurements. The structural evolution occurring in the interconnected network of the gels during thermal treatment was monitored by Fourier transform infrared spectroscopy (FTIR), shrinkage and density measurements and nitrogen gas sorption. We noted that in the presence of formamide, the Si-O-Si bonds are stronger and belong to a more cross-linked structure. The samples obtained in the presence of formamide have larger pore volume and its pore structure is in the range of mesoporosity. The samples obtained without additive are microporous. Formamide allowed the preparation of crack-free silica gels stabilized at high temperatures.

In this work we investigated the influence of formamide on the acid-catalyzed sol-gel process by Fourier transform infrared spectroscopy (FTIR). Three silica sols were studied: Sol catalyzed with nitric acid without formamide, sol catalyzed with nitric acid containing formamide and sol catalyzed with a mixture of nitric acid and hydrofluoric acid and modified with formamide. Following the time evolution of both the Si-(OH) stretching vibration at around 950 cm-1 and the Si-O-(Si) vibration between 1040 cm-1 and 1200 cm-1 we were able to describe the structural evolution of each sol. The curve of evolution of Si-(OH) stretching vibration corresponding to sol A has a simple asymptotic evolution. In the case of formamide containing sol, we observed a two-step structural evolution indicating that for the system containing formamide the polymerization goes through a temporary stabilization of oligomers, which can explain the non-variation of the Si-O(H) bond wavenumber for a certain time. Gelation times were of several days for gels without formamide and few hours for gels containing additive. The presence of additive resulted in a highly interconnected gel.