Adsorption and Desorption Characteristics of K⁺ and Na⁺ Ions in Fly Ash Blended Cement Pastes

Cement pastes containing 0%, 15%, 25% and 35% fly ash were prepared. After being cured for 90 days, all fly ash blended cement pastes were crushed and ground into powders with a particle size less than 80 µm and then the powders were immersed in alkali solutions. Adsorption characteristics of K⁺ and Na⁺ ions in the pastes were investigated. Meawhile, the desorption characteristics of the adsorbed alklai ions and the inherent K⁺ and Na⁺ ions in the pastes were also investigated. Results showed that the contents of K⁺ and Na⁺ ions adsorbed by the pastes increased with increasing the substitution levels of fly ash and/or the concentrations of alkali solutions. Each paste was characterized by having the same adsorption capacity for K⁺ or Na⁺ that was essentially independent of alkali concentration. Adsorption mechanism of K⁺ and Na⁺ ions by the pastes is believed to be an effect of charge compensation of the C-S-H gel. Adsorption-desorption of the adsorbed K⁺ and Na⁺ ions in the pastes is reversible. The inherent K⁺ and Na⁺ ions in the pastes entered rapidly into the deionized water during the first 120 minutes, and then they were released at a relatively slow rate. A steady-state alkali partition was reached at about 720 minutes. Some K⁺ and Na⁺ ions which were originally “bound” by the hydration products were considered to be released into de-ionized water. Leaching tests showed that there was no significant effect of fly ash on the retaining of available alkalis in the pastes. A part of the released alkali ions exists in the pore solutions and the other part may be physically adsorbed by the hydration products.