In situ monitoring of pH changes in concrete and lime mortars

This paper describes how a new type of solid state microelectrode based on nanostructured palladium-hydride (PdH) can be used to monitor pH variations during carbonation of calcium hydroxide (Ca(OH)2). The experiments described were carried out under conditions similar to those typical of common porous substrates such as cement and lime mortars. In these tests PdH microelectrodes were employed to record pH transients during the carbonation within a humid fibrous mesh that simulated the porous substrate. To control the carbonation rate, experiments were carried out at different carbon dioxide partial pressures (pCO2) and in each case the experimental results were compared to theoretical calculations obtained using PHREEQC (pH-REdox-EQuilibrium) software. Scanning electron microscopy (SEM) observations and X-ray diffraction (XRD) analysis of the calcium carbonate (CaCO3) crystals deposited on the fibrous mesh were used to evaluate characteristics of the solid phases precipitated during the reaction. Results demonstrate that these pH microsensors can operate reliably in very alkaline environments (pH>12) such as those produced by the dissolution of Ca(OH)2.