A Comparison of Methods for Determining Carbonation Depth in Fly Ash-Blended Cement Mortars.

Several methods were compared for measuring depth of carbonation in 50.8 x 101.6 mm (2 x 4 in.) mortar cylinders that had been prepared from ordinary portland cement, with 0 to 40% Type C coal fly ash (FA) substitution, and reacted for up to 28 days in an atmosphere of 50% CO₂ at 62% relative humidity (RH). Freshly split sections were analyzed by traditional phenolphthalein staining, digital image analysis (stained and unstained), pH profiling of aqueous digests, and Fourier-transform infrared spectrophotometry. Advancement of the carbonation front was modeled using the diffusion-limited unreacted core model, illustrating that partial substitution of portland cement by FA increased the rate of sample carbonation. The traditional phenolphthalein method significantly underestimated the advancement of the carbonation front's leading edge. The other techniques were all in reasonable agreement with one another. However, of these, digital image analysis was the fastest and least expensive method to carry out. [ABSTRACT FROM AUTHOR]