My PhD project has the purpose to develop glass-ceramic materials with novel functionalities made with materials originated from the processing and conversion of the landfill waste, which are delivered by Work Package 2. The outputs from my PhD will be the production of building materials such as dense and porous alkali activated materials with a low carbon footprint and chemically stable.
Problem statement & objectives
Vitrification of waste allows the immobilisation of pollutants in the glass structure and reduction of waste volume, producing a glass that can be safely disposed or applied as aggregates (Colombo et al., 2003). However, as this process is very expensive due to high energy consumption, vitrification is only economically viable and environmentally sustainable if the vitrified residue is further upcycled into high-added value products, such as glass-ceramics and alkali-activated materials. According to the composition of the glass, the vitrified residue can be converted into safe and strong building materials, such as foams, tiles and aggregates (Rawlings et al., 2006; Rincón et al., 2016). Besides the economic benefits of commercialising waste-derived products, the landfilling of vitrified residues and the mining of natural raw materials could be also avoided (Rincón et al., 2016).
Based on this, the research carried on during the PhD project focused on valorising vitrified residues produced within NEW-MINE (e.g. plasmastone and conditioned bottom ash) into building materials with high-added values, such as dense glass-ceramics and porous materials (lightweight aggregates and panels). Furthermore, the formation of magnetite in plasmastone-based glass-ceramics was also investigated in order to obtain panels with novel functionalities.
The dense glass-ceramics were made by cold-pressing and sinter-crystallisation of mixtures of fine powders of plasmastone and soda-lime glass. Different compositions, firing temperatures, heating and cooling rates and holding times were applied in order to produce tiles with water absorption below 2%.
The porous glass-ceramics were developed through alkali activation, gel-casting and sinter-crystallisation. After hardening, a surfactant was added to the partially gelified slurry, which was then submitted to intensive mechanical stirring. The foamed structure was then dried and fired in order to stabilise the foams (Rincón et al., 2017).
Porous alkali-activated materials were also developed with vitrified bottom ash. The glass was mostly dissolved in a strong alkaline solution (5 M Na-silicate) and transformed into an alkali-activated foam by addition of metallic aluminium powder followed by curing. Part of these foams were converted into porous glass-ceramics, by firing at 1000 °C.
The lightweight aggregates were produced by alkali activation, gelation, granulation and fast firing of 70 wt% soda-lime glass/30 wt% plasmastone mixtures. Furthermore, during the granulation step, fine soda-lime glass powder was poured in the mixture in order to cover the aggregates with a ‘shell’ of soda-lime glass. During firing, compounds formed by alkali activation were decomposed, leading to the self-foaming of the material.
Results obtained during the reporting period
Dense glass-ceramics with high specific bending strength were obtained from mixtures of 45 wt% plasmastone, 45 wt% soda-lime glass and 10 wt% kaolin clay. The firing treatment was performed at 1000 °C for 30 min with heating and cooling rates of approximately 40 °C/min, mimicking that of industrial ceramic tiles. The fast treatment prevented extensive crystallisation directly upon heating and promoted viscous flow. In this way, dense glass-ceramics with water absorption below 0.7% could be achieved. The produced tiles exhibited mechanical properties comparable to those of commercial ceramic tiles and natural stones: the elastic modulus was equal to 76.8 ± 2.5 GPa, whereas the bending strength σ04pt (obtained by a four-point bending test with 32 mm outer span, 8 mm inner span) was equal to 69.8 MPa. By scaling the bending strength for bigger tiles (cross-section of 8 mm x 300 mm and loading span of 300 mm), it was possible to estimate the equivalent strength σeqL, which was above the lower strength limit (35 MPa) for tiles of the BIa group. Furthermore, the specific bending strength considering σ04pt and σeqL were equal to 27.7 MPa cm3/g and 16 MPa cm3/g, respectively, which highly exceed the value required by the milestone. Finally, the environmental impact assessment performed on these materials showed that the leaching of hazardous elements was particularly limited. The results show that plasmastone, combined with recycled soda-lime glass, may be converted in safe building materials, with a possible commercial exploitation.
Regarding the lightweight aggregates, the developed materials presented particle density below 2 g/cm3, as specified by the Norm EN 13-055 for lightweight aggregates. In addition, the samples presented a large particle size distribution (between 8 and 18 mm) and low leaching of heavy metals. Finally, the developed lightweight aggregates were applied to produce lightweight mortars. These mortars presented low thermal conductivity, comparable to lightweight mortars produced with granulated foam glass. The results indicate that the developed lightweight aggregates could be potentially applied in cement-based products in order to increase the thermal insulation in buildings.
The results of this work demonstrated the feasibility of producing ceramic foams by alkali activation, followed by gel-casting and sinter-crystallisation. In specific conditions, soda-lime glass was also added in order to improve hardening and sintering by viscous flow, as in the case of glasses prone to crystallisation (e.g. plasmastone) and when recycling vitrified bottom ash-based foams into new foams. Furthermore, the introduction of soda-lime glass has also demonstrated to enable the decrease in molarity of the alkaline solution, without affecting the hardening step.
The leachability has proven to be a critical point for the panels, due to a higher surface area. In the case of plasmastone-based foams, this could be adjusted ‘a posteriori’ by the addition of boro-alumino-silicate glass, which presents higher stability than soda-lime glass. Furthermore, the stability of foams made with iron-rich glasses (e.g. plasmastone) mixed with soda-lime glass could be finally achieved by firing in nitrogen, due to the phase assemblage formed.
In general, the firing treatment or increase in firing temperature enhanced the stabilisation of pollutants, with some exceptions. This was highlighted by the study comparing porous alkali-activated materials and porous glass-ceramics. This study demonstrated that crystallisation increased the chemical stability of foams, whereas strength was enhanced by sinter-crystallisation. On the other hand, this research also showed the impact of the firing treatment in the life cycle assessment (performed by ESR13) of ceramic foams. Indeed, this study evidenced that waste-derived glass-ceramics foams should present multiple functionalities in order to justify the firing treatment in terms of costs and environmental impact.
Multifunctional waste-derived glass-ceramics could be produced by firing plasmastone-based foams at lower temperature or in nitrogen. The different functionalities were related to thermal and acoustic insulation (due to high porosity presented by the foams), combined with shielding properties (especially for foams fired at lower temperature in nitrogen) or dielectric characteristics (for foams fired at higher temperature in nitrogen).
Finally, the milestones required by the NEW-MINE project were achieved: dense glass-ceramics with specific bending strength above 2 MPa cm3/g were obtained by cold pressing and sinter-crystallisation of fine powders of plasmastone, soda-lime glass and kaolin clay. Highly porous glass-ceramics with specific compressive strength above 5 MPa cm3/g could be achieved by valorising vitrified bottom ash into porous glass-ceramics. Finally, glass-ceramics with magnetic functionality could be established by firing iron-oxide rich glasses at low temperature. The formation of magnetite was maximised by firing at low temperature, especially when coupled with firing in nitrogen.
Conclusion & Outlook
In this work, residues were converted into safe building materials with high-added values, by sinter-crystallisation. Dense glass-ceramics (with properties comparable to those of commercial tiles) as well as lightweight aggregates (developed to increase the thermal insulation of cement-based products) were produced with mixtures of plasmastone with soda-lime glass. Furthermore, highly porous inorganic materials for thermal and acoustic insulation were developed by alkali activation, followed by gel-casting and sinter-crystallisation. Interestingly, the addition of waste glass enabled to recycle powders of vitrified bottom ash-based glass-ceramics into even stronger foams. In addition, the chemical stability of plasmastone-based foams was considered a critical point, due to high leaching of heavy metals. This was adjusted using boro-alumino-silicate glass instead of soda-lime-glass or by firing in nitrogen. Besides stabilisation of heavy metals, firing in an inert atmosphere also promoted functionalities in plasmastone-based porous glass-ceramics: foams fired at low temperature exhibited shielding properties, whereas foams fired at a higher temperature presented dielectric characteristics. A waste-derived glass-ceramic exhibiting multifunctionalities (e.g. insulation coupled with shielding properties) is essential to justify costs and environmental impact of the firing treatment.
In order to achieve a full circularity envisaged by Enhanced Landfill Mining projects, the vitreous by-products have to be valorised into building materials with high-added value. Therefore, the overall research carried on could be applied as a guideline regarding the different waste-derived products that could be developed with these residues, as well as techniques to increase the stability and the products value.