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

Abstract

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.