Influence of the porous network on chloride diffusion in presence of sulphate

The main cause of premature deterioration of reinforced concrete structures is the corrosion of steel bars, induced by chloride ions (for example in marine environment). At the same time, environmental-induced degradations of concrete can also affect the structure, such as external sulphate attack leading to the formation of ettringite, inducing expansion inside the materials and finally degradation. In this paper, chloride ingress are studied for concretes and cement pastes or/and in presence of sulphate in chloride contact solution. The mixtures contain OPC alone or with SCM. SCMs are here pozzolanic materials (Fly Ash or Metakaolin) or alkali-activated materials such as ground granulated blast furnace slag (GGBS). The materials are put in contact with chloride solutions in presence of sulphate. This study focuses on the influence of the porous network on the apparent chloride diffusion coefficients, as well as on chloride binding isotherms which are obtained by the profile method. Various aspects of the microstructure and of the pore structure are investigated, by using Mercury Intrusion Porosimetry and 27Al NMR, in order to better understand the results obtained relatively to the apparent chloride diffusion coefficients and to the chloride binding. Results show the difference between the various microstructures of the materials and their consequences on chloride/sulphate ingress. In addition, chloride ingress increases when sulphates are present in the contact solution for all cement materials tested. It appears that presence of sulphate decrease chloride binding, thus explaining the results

(33)S MAS NMR of a disordered sulfur-doped silicate: signal enhancement via RAPT, QCPMG and adiabatic pulses

Three different signal enhancement techniques have been applied to (33)S magic-angle spinning nuclear magnetic resonance (MAS NMR) of a disordered silicate containing 1.15 wt% (33)S. Partial saturation of the satellite transitions was achieved using a rotor-assisted population transfer (RAPT) pulse sequence, resulting in a signal enhancement of 1.63, albeit with a slight distortion of the line shape due to selective excitation. Adiabatic inversion of the satellite transitions by various amplitude-and frequency-modulated pulse shapes (such as hyperbolic secant and wideband uniform-rate smooth truncation) was also attempted, resulting in a signal enhancement of up to 1.85, with no apparent line shape distortion. Quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) and RAPT-QCPMG sequences were also used, both of which yielded spikelet spectra that accurately reflected the MAS line shape with a greatly improved signal-to-noise ratio. It is hoped that this study demonstrates that (33)S solid-state MAS NMR is now feasible even on disordered, low-sulfur-content systems