Synthesis of Inorganic Polymers Using a CaO-Al<sub>2</sub>O<sub>3</sub>-FeO-SiO<sub>2</sub> Slag

Abstract

The focus of the present paper is to investigate the effect of the activating solution on the structure and mechanical properties of inorganic polymers synthesised from a slag resembling the vitrified residue from a Waste-to-Energy plasma installation. The slag consists of (in wt.%) 22 CaO, 12 Al2O3, 34 SiO2 and 20 Fe2O3 and the activation solution was 50:50 mass ratio NaOH and sodium silicate, with the NaOH solution molarities varying from 2 M to 10 M. The synthesised slag was almost completely amorphous due to the rapid cooling, with only traces of magnetite and quartz. The inorganic polymers were prepared by mixing the slag, sand and activation solution. In all cases, heat was generated during sample preparation and its amount increased with the activating solution strength. After 90 days, the compressive strength of the samples activated with 6 M or higher NaOH solutions was similar, approximately 88 MPa. For NaOH activation solutions with molarities lower than 6 M, the compressive strength was lower, both at early as well as late curing times. SEM and EPMA analysis revealed-between undissolved particle remnants-a distinct binder phase, composed of (in wt.%) 18.9±2.5 CaO, 11.5±0.1 Al2O3, 40.3±2.1 SiO2, 15.8±1.2 FeO, 5.1±1.9 Na2O and 3.7±0.6 MgO. In conclusion, the present study showed that the CaO-Al2O3-FeO-SiO2 vitrified residue could be converted into a stable inorganic polymer having reasonably high mechanical strength, when activated with a mixture of sodium silicate and sodium hydroxide solution with a molarity of at least 4 M.