MgO induced nano-structural evolution in alkali-activated binders and their durability performances

Abstract

In recent decades, the content of MgO in the normal precursors of alkali-activated binders was realized generally significant to affect their hydration properties. Thus, the roles of MgO on mechanical properties, structural evolution, and durability performances of alkali-activated binders have attracted increasing attention. This dissertation mainly focuses on the properties of the synthetic alkali-activated binders incorporated with reactive MgO. In particular, MgO at different reactive levels were generated through a calcination of magnesium carbonates, which was then incorporated in alkali-activated fly ash/slag designed under widely-adopted activator modulus and water-to-binder ratios. MgO at the moderate reactivity induced the highest compressive strength (more than 50 MPa), whereas higher reactive MgO contributed to a higher hydration degree. The hydration mechanism associated with hydrolysis of MgO and CaO, and geopolymerization of aluminosilicate was studied. After aging for 90 days, these MgO modified binders were treated under an accelerated carbonation for 28 days. After carbonation, the compressive strengths all increased. Decalcification of C-S-H type gels was observed as the main carbonation behavior, along with the formation of aragonite and magnesium carbonate. Nonetheless, the accelerated carbonation benefited the formation of layered double hydroxides, increasing the carbonation resistance of the synthesized samples. In the third study, a one-step hydrothermal treatment was applied to synthesize the binders incorporated with moderately reactive MgO, in order to give rise to generate zeolite Na-P1 and hydrotalcite crystals in a single binder system. The binders with mesoporous characteristics and well-defined pore structures, which are promising for water purification, were successfully synthesized.