Measuring concrete air-entraining admixture adsorption on coal ash using three-phase equilibrium and fluorescence-based methods

This manuscript presents two novel methods of evaluating the adsorption of air-entraining admixtures (AEAs) by coal ashes used in cementitious mixtures. A developed three-phase equilibrium (TPE) method measures fly ash adsorption capacity accounting for carbon adsorption, Ca²⁺ interactions with AEA, and the equilibrium between the two. A fluorescence-based method (FBM) was also developed, utilizing a non-ionic NP-10 surfactant as a representative for AEA. This study verifies the applicability and accuracy of the TPE and FBM methods using seven class C and F coal ashes with a wide range of loss on ignition values, varying from 0.2 to 15.6 %, and three commercial AEAs. Verified with foam index test results, the TPE method was applicable to all tested AEAs and coal ashes. The results were consistent between the FBM and TPE methods when comparing rosin- and fatty acid-based AEAs, but less consistent when using a sulfonate-based AEA. These findings help us understand the applicability and limitations of the TPE and FBM methods and provide two methods for quantifying adsorption in fly ash samples.