MgO-induced phase variation in alkali-activated binders synthesized under hydrothermal conditions

The effects of magnesium content in the precursors on the phase formation and microstructural variation of alkali-activated binders under hydrothermal conditions are investigated. One-step hydrothermal treatment method was applied on all samples synthesized from industrial by-products (e.g., fly ash and slag), MgO powders, and alkali activator, and they were characterized by compressive strength tests, XRD, SEM, N₂ sorption, and FTIR analyses. The samples containing zeolite Na-P1 and hydrotalcite crystals were obtained until the Mg/(Al + Si) molar ratio was higher than 0.58 with a 20 wt.% slag content in the precursor, however, the samples with 40 wt.% slag content possessed both of these phases even at lower Mg/(Al + Si) molar ratio of 0.39. Specifically, increasing the Mg/(Al + Si) ratio significantly reduced the formation of zeolite Na-P1, while promoted the formation of hydrotalcite. Furthermore, the extended hydrothermal treatment promoted the formation of zeolite Na-P1, but simultaneously reduced the formation of hydrotalcite. All samples exhibited mesoporous characteristics having major sorption behaviors of multilayer physisorption and capillary condensation.