Effect of material age on the adsorption capacity of low-, medium-, and high-calcium alkali-activated materials

Alkali-activated materials (AAMs) are actively studied as adsorbents in wastewater treatment. They are frequently classified as low-, medium-, or high-calcium content since the gel nanostructure of AAMs changes upon the introduction of network-modifying cations (e.g., Ca2+). Furthermore, AAMs undergo changes in the phase composition and pore structure upon aging. In the present study, these two aspects were assessed for the first time in the context of adsorption: three typical mix designs involving metakaolin, blast furnace slag, or their mixture were prepared and studied for ammonium (NH4+) uptake at 7-, 28-, or 90-day age. The adsorption capacity of the “zeolite-like” metakaolin-based geopolymer increased from 34 to 87 mg/g when material was cured up to 90 days at room temperature. However, the adsorption capacity increase appears to be also depending on the metakaolin structure as approx. 1-month and 8-year old samples prepared with another metakaolin grade indicated almost similar adsorption capacity. Adsorption was clearly hindered when blast furnace slag was introduced in the mix designs, which turns the gel into “tobermorite-like” chain structures. Thus, this study demonstrated that using the Ca-content of precursors and material age are significant parameters for the interpretation of the adsorption results and practical development of AAM-based adsorbents.