Wide-range optical pH imaging of cementitious materials exposed to chemically corrosive environments

Abstract

The pH of concrete-based material is a key parameter for the assessment of its stability and durability, since a change in pH is usually associated with major types of chemical degradation such as carbonation, leaching and acid attacks. Conventional surface pH measurements with potentiometric flat surface electrodes have low spatial resolution, whereas optical pH visualization with indicator dyes (phenolphthalein) only indicates the areas with higher or lower pH than the pKa of the indicator. In this regard, it is key to develop wide-range imaging systems, enabling accurate and spatially resolved determination of pH variability for an advanced knowledge of degradation mechanisms. This contribution presents the enhancements made for a high-resolution optical pH imaging system based on fluorescent aza-BODIPY indicator dyes. The measurement range was increased to 6 pH units (pH 6.5 to pH 12.5) by a combination of two indicator dyes. Moreover, background scattering effects were sufficiently eliminated. With the improved sensor foils steep pH gradients (up to 3 pH units within 2 mm) were successfully recorded in various concrete specimens using a macro lens reaching a resolution of down to 35 µm per pixel.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

“…In later work, the same group extended the dynamic range of the sensor to 6 units (pH 6.5–12.6) by using a mixture of two pH indicators with different p K a values. 1249 Also an aza-BODIPY pH indicator was used in a ratiometric approach using a single-lens reflex camera instead of a gated CCD camera. 1250 The system was used to characterize the pH of porewater within cracks of self-healing concrete materials.…”

Optical Sensing of pH and O2 in the Evaluation of Bioactive Self-Healing Cement

“…Imaging pH at highly alkaline conditions found within cement is not trivial and has only recently been demonstrated for concrete surfaces. 23−25 While those studies used a lifetime-based readout, the present work is the first report of the ratiometric pH imaging of cement using a single-lens reflex camera that can be acquired easily and without large investments (approximately $1000). As the applied pH indicator emits light in the near-infrared (NIR) region of the electromagnetic spectrum, it was necessary to remove the NIR blocking filter from the camera prior to the experiments.…”

“…The pH of the well cement was measured in a high resolution of up to 50 μm per pixel with a steep gradient of 1.4 pH unit per 1 mm visually observed, equivalent to more expensive systems. 23−25…”

“…7 Concrete surfaces exposed to accelerated carbonation or biogenic acid attack show the pH values, measured optically, in the range of above 12 to below 10 similar to the results presented here. 23−25 It is therefore not surprising that the alkaliphile B. alkalinitrilicus would metabolize in both concrete and hardened well cement specimens.…”

Near-infrared fluorescent aza-BODIPY dyes for sensing and imaging of pH from the neutral to highly alkaline range

“…38 Here durable and low-cost materials for pH sensing are of the highest interest. 39,40 Reported fluorescent sensors for the alkaline range possess several limitations such as comparably short absorption and emission wavelengths in the case of coumarin 38 and BODIPY 36,37 chromophores and high hydrophobicity in the case of perylene dyes. 22 Tetraaryl-aza-BODIPYs represent a particularly interesting class of fluorescent dyes showing absorption and emission in the red/NIR part of the spectrum, high molar absorption coefficients and extreme photostability.…”