Preparation and characterization of a novel alkali-activated magnesite cement

Abstract

The present paper focuses on synthesizing an alkali-activated binder, in which magnesium carbonate (MC) was the only precursor. MC was individually activated with sodium hydroxide (NaOH), sodium silicate (Na2SiO3) and sodium aluminate (NaAlO2). Increasing Na2O content (as NaOH) up to 10 wt% enhanced the compressive strength slightly. At constant Na2O content (10 wt%), the sample activated by (Na2SiO3) recorded the highest compressive strength. Although the hardened mixture activated by NaAlO2 showed the higher compressive strength compared to NaOH-activated sample, it still recorded strength lower than Na2SiO3-activated-mixture. XRD, TG/DTG, FTIR, and SEM techniques indicated that phase composition depends mainly on the Na2O source. 29Si MAS-NMR spectroscopy verified the formation of magnesium silicate hydrate with Q1, Q2, and Q3 silicate connectivity, when MC was activated with Na2SiO3. Incorporating lead glass sludge (LGS) as a silicate source, enhances the formation of a cross-like silicate chain (Q3) coinciding with an improvement of the compressive strength and a retardation of Pb-leachability. As proved by 27Al MAS-NMR, Al inside hydrotalcite phase, formed within NaAlO2-activated-MC, has six coordination bonds. Activating MC with NaOH produces magnesium hydroxide (Mg(OH)2) and thermonatrite Na2CO3·H2O phases. Owing to the diversity of the hydration products, a wide range of 90-day compressive strengths (9–53 MPa) was recorded. A preliminary result showed that the CO2-emission generates from optimum mixture containing 30 wt% LGS is lower than resulted from Portland cement manufacturing.