A novel approach for preparing glass ceramic foams from MSWI fly ash: foaming characteristics and hierarchical pore formation mechanism

Abstract

Powder sintering and in–situ foaming are the primary treatments for the preparation of glass ceramic foams from solid wastes. However, variables such as crystallization and viscosity, which are difficult to regulate at high temperatures. Herein, a novel technology based on alkali activation–crystallization strengthening was developed for the fabrication of glass ceramic foams with hierarchical pores. Vitrified municipal solid waste incineration (MSWI) ashes as the main calcium and aluminum source and secondary aluminum ash (SAA) as a foaming agent were foamed in NaOH solution at room temperature. The effects of solid–liquid ratio and NaOH concentration on the pore structure, individual foam force, phase evolution and physical properties were investigated, including crystallization kinetics and hierarchical pore formation mechanism. The results indicated that the solid–liquid ratio decreased from 60:40 to 40:60, making the pore wall thin and eventually disappear. The pore size in the range of 0–1 mm reaches 99.5% in 3 mol/L NaOH sample, with the lowest mean size of 0.34 mm. The crystallization kinetics parameters of the basic glass at 5–20 K/min showed that grains were primarily precipitated as three–dimensional at 5 K/min, resulting in micropores. The prepared samples exhibit high porosity (79.23–88.35%), adequate compressive strength (0.36–5.55 MPa) and a bulk density of 0.68–0.76 g/cm3. This work develops the function of SAA as a foaming agent at room temperature, as well as a new idea for the preparation of glass ceramic foams from solid waste containing glass phase or needing vitrification.