Organic-inorganic hybrid sol–gel membranes for pH sensing in highly alkaline environment

Abstract

Carbonation and corrosion of reinforced concrete structures (RCS) are the two main phenomena that more often affect civil engineering structures. The main consequence of carbonation is the pH reduction of concrete to values between 9 and 6 which may compromise the RCS durability. Thus, the development of sensors that would predict concrete carbonation is of utmost importance. Fiber optic sensors (OFS) are commonly used for concrete applications due to their high sensitivity and resistance to external interference. This work reports the synthesis of organic-inorganic hybrid (OIH) materials for possible functionalization of OFS and application in concrete and RCS. As precursors, 3-glycidyloxypropyltrimethoxysilane (GPTMS) and Jeffamine® THF-170 were used. The OIH sol-gel materials were doped with phenolphthalein (phph) for pH sensing proposes and cetyl trimethyl ammonium bromide (CTAB) as surfactant. OIH with pH sensing ability properties were successfully obtained and the doping of the OIH matrix did not significantly changed the dielectric properties of the resultant material. Moreover, the results obtained in simulated concrete pore solutions, obtained for the OIH produced, showed to be promising for application in fresh concrete. The transmittance of the synthesized OIH materials decreased after the doping. The matrix shows intrinsic fluorescence and doping it only led to small deviations in the wavelength of the peak of maximum emission. No significative changes were obtained on the thermal profile of the OIHs. The pH variation was shown by the OIH material color change. Stability studies regarding the release of phph from the OIH matrix indicates that the proposed system reveals long-term stability.