In this chapter we discuss an optical method used for direct measurement of pH in Portland cement concrete with high spatial and temporal resolution. This approach is designed to better enable the study and analysis of high pH dependent degradation reactions, and hydration reaction in cement-based composites. We demonstrate that the halochromic sensor molecule is suitable for this purpose. Results of experiments showing spatial distribution of early stage carbonation and reaction involving silica aggregates are highlighted at pH levels betweenÂ ~11.0 toÂ ~13.5. Results are compared with the commercially available pH indicator Phenolphthalein.