Search

Secondary aluminum ash (SAA) is a hazardous solid waste generated in the process of recycling aluminum. The preparation of glass ceramic foams from high content of SAA, used as nucleating and foaming agent, was proposed to achieve green and comprehensive utilization of SAA. With the help of thermodynamic phase diagram and viscosity calculation, a mixture of SAA and waste glass (WG) were transformed, at 1200 °C for 30 min, into glass ceramic foams with porosity of 45.49%–59.45%, respectively. The mean pore size of the samples decreased from 2.89 to 1.53 mm with the decrease of SAA addition. Through the adjustment of temperature, it was found that the mixture had a microporous region at 1100 °C. The expansion region was observed at 1180 °C and 1200 °C, because of the oxidation of AlN in SAA. The porosity of the samples could be raised to 64.51%, with 5–9% B2O3 as a supplement, which contributed to the development of the liquid phase and a decrease in the melt's viscosity. The mechanism of in-situ isothermal foaming, nucleation, and crystallization of SAA and WG was clarified. The crystal intensity dropped and the amorphous phase became the primary phase at 800 °C. Al2O3, sodium silicate and sodium aluminum silicate precipitated at 1000 °C. Al2O3 and spinel with high melting point were wrapped by silicate. Silicate turned into the melt again as the temperature continued to rise. With the further reduction of viscosity, the gas diffused in crystalline and amorphous phases, and finally forms glass ceramic foams with hierarchical pore structure.

In 2020, the municipal solid waste removal and transportation volume reached 235.117 million tons, of which 146.076 million tons were incinerated in China. Because it can reduce the harmfulness of waste and recycle energy, municipal solid waste incineration (MSWI) technology has become the primary method for the disposal treatment of urban domestic waste in China. However, this method produces MSWI fly ash, which is defined as a hazardous waste rich in dioxins and heavy metals. Calculated based on 5% (mass fraction) of the original waste, the output of MSWI fly ash in China nearly reached 7.304 million tons in 2020. Moreover, the stockpile management and treatment capacities are seriously out of balance. At present, the main disposal method of MSWI fly ash is landfilling, which consumes land resources and poses an environmental hazard. As a result, the harmlessness and recyclability of MSWI fly ash have become a bottleneck for green development. In this review, the harmless melting and recycling of MSWI fly ash are introduced in detail. The mechanisms of heavy metal solidification and dioxin degradation during MSWI fly ash melting have been explained. MSWI fly ash can be transformed into glass slag containing CaO−SiO2−Al2O3 after co-melting with other solid wastes rich in silicon aluminum oxide. Heavy metals in MSWI fly ash can be solidified at the atomic scale in the silicate network of glass. More importantly, as the temperature increases beyond 800 °C, dioxins undergo dechlorination and degradation, reducing the harmfulness and revealing the harmlessness of MSWI fly ash. This review also describes how to deal with the glass slag that forms because of co-melting. The glass slag has low added value and poor mechanical properties. Future disposal trends for vitrified slag from MSWI fly ash, including glass–ceramic, glass–ceramic foam, and cementitious materials, have been proposed. Given that vitrification can solidify heavy metals in the process of subsequent resource usage and product service, the migration and leaching characteristics of heavy metals need to be further investigated. This study provides a reference for the comprehensive usage of MSWI fly ash.

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.

Safety and efficient dispose of municipal solid waste incineration (MSWI) fly ash with high toxicity has emerged as a worldwide challenge. Vitrification provides the advantages of capacity reduction, detoxification, and solidification of heavy metals, which has the potential to dispose of hazardous waste on a large scale. Herein, co–vitrification of MSWI fly ash and bottom slag has been accomplished based on the characteristics of calcium and silicon composition. A novel approach for producing glass ceramic foams by alkaline activation–crystallization was developed to realize the disposal of the obtained glass. The effect of MSWI fly ash/bottom slag ratios on the glass network, crystallization ability of the basic glass, pore structure, and physical properties of the porous green body was investigated. The results revealed that with increasing MSWI fly ash proportion, the Si–O of [SiO4] in the basic glass changed significantly and the crystallization ability steadily reduced. Si–O and Al–O in basic glass are easy to corrode under alkaline conditions, releasing Ca2+ and forming a low solubility product, calcium silicate hydrate. When the crystallization temperature increases from 950 ℃ to 1150 ℃, it is more conducive to the precipitation of the gehlenite phase. Extending the crystallization time promotes three–dimensional growth of crystals that are coupled with each other to form a network structure and a multi–stage pore structure. The pore structure was developed with the help of NH3 and H2 generated by the secondary aluminum ash (SAA). Through the preparation of glass ceramic foams, the raw materials were detoxified. The toxic heavy metals showed extremely low leaching concentrations, which were smaller than the limit of TCLP. The prepared samples had 70.22–80.61% of porosity, 0.78–1.19 g/cm3 of low bulk density, and 0.54–7.86 MPa of compressive strength.

The search for the possibility of expanding the resource base through the use of local rocks, as well as reducing the cost of final product, is one of the scientific research areas in the field of obtaining foam glass-based building materials. The aim of the research is the development of compositions and recommendations for the production of silicic rock-based glass ceramic foams. These studies will allow to create strong and durable building materials with low density and thermal conductivity, as well as increased biological stability. The results of studying the phase transformations occurring in the charge (tripoli : soda ash) during heating, obtained by thermal analysis methods are presented as well as the production technology, physico-mechanical and thermophysical properties of the developed glass ceramic foams. As a result, construction materials resistant to aggressive media with a density of 200 to 600 kg/m3, thermal conductivity from 0.053 to 0.115 W/m∙°C, compressive strength from 1.2 to 9.8 MPa, have been developed. Due to its properties, developed glass ceramic foams will be used primarily as insulants for the construction of nuclear power plants, in the gas and oil industries, industrial and civil engineering.