Estudo da carbonatação em compósitos cimentícios produzidos com o emprego de aditivos redutores de permeabilidade por cristalização capilar

Abstract

The reaction of carbonic acid (H2CO3), released during dissolution of atmospheric carbon dioxide (CO2), with alkalis derived from hydration of Portland cement is the main cause of pH reduction in interstitial liquids existing within pores of cement-matrix composites. The forward speed of the carbonation front depends on factors that are directly related to its composition, exposure conditions, and microstructural characteristics. The system of pore distribution in Portland cement composites, in turn, is strongly influenced by dosing parameters. Among these dosing parameters, the use of additives and water/cement ratios bear great significance. The proposed work was, therefore, sought to verify the effects of two additives, of the capillary crystallization permeability reducer type, measured in two different water/cement ratios in order to monitor the hydrogenionic potential (pH) variation of cylindrical and prismatic mortar specimens (CPs). These specimens measured 5 × 10 cm each, were prepared using six different traits, and subsequently conditioned in carbonation chambers under controlled conditions of humidity, temperature, and CO2 concentration. The forward speed of the carbonation front was monitored using, as indicator, a phenolphthalein solution prepared in accordance with ISO 1920-12 (2015) standards. Simultaneously, we measured surface pH variation of the cement composite by means of equipment specific to this purpose. In order to evaluate changes in the pore microstructure of the cementitious material matrix, we tested a 2 × 4 cm specimen, extracted from each of the six traits investigated, which were maintained under 56 days of accelerated carbonation, using the scanning electron microscopy (SEM) technique. Results of these tests demonstrated that use of the above equipment for pH determination facilitates efficient monitoring of pH variation as a function of carbonation time. This also helped facilitate the tracking of alkalinity reduction in regions not disclosed by the phenolphthalein indicator, since the pH values exceeded the turning range of the sprayed solution. From images obtained via SEM, it was possible to envision the densification of the material present on the edge and in the most central layer of CPs. The results helped disclose great variation in the forward speed of the carbonation front, along with pH reduction of CPs, as a function of the additive and of the water/cement ratio used when preparing the mixture, as well as the geometry established for CP. Thus, a direct relationship between the forward speed of the carbonation front and microstructural properties of each tested trait was revealed