Shizhe Zhang

Master thesis (2)

2 records found

Immobilization of heavy metals in hazardous fly ashes

Investigation on the feasibility of increasing immobilization efficiency by carbon mineralization

Master thesis (2024) - M.E. Biagini (author) , Guang Ye (graduation committee member) , Julia Gebert (graduation committee member) , Shizhe Zhang (graduation committee member)

This study investigated the possibility of enhancing immobilization of heavy metals in hazardous fly ashes by mineral carbonation. The research initially investigated five different fly ashes for their chemical composition and carbonation potential. The highest lime content resulted in the highest carbonation potential, but mineral carbonation occurred also in other fly ashes with very limited free lime. One of the municipal solid waste incineration (MSWI) fly ash (FA) with a high heavy metal content and the highest carbonation potential was selected for further carbonation and immobilization investigations.
The selected MSWI FA was carbonized following the first three out of four possible different routes:

1.Pre-carbonation of fly ash and casting with binder
2.Pre-carbonation of fly ash-binder mixture and casting after carbonation
3.Curing of the sample in the carbonation chamber
4.Carbonation of fresh mixture during mixing

The materials were tested for their crystalline composition before and after curing as well as for effective carbon-uptake. The second route resulted in the highest effective carbonation for the MSWI FA but also in the lowest compressive strength. The third route showed an increase in strength compared to the reference sample and effective reduction of final pH on the leachate and electrical conductivity. A fast shake leaching test was used to determine the leachability of relevant anions and cations from the mix designs. The results were compared with a reference sample prepared following a procedure used in industrial-scale applications and with the Dutch Soil Quality Decree and Regulation (Ministerie van Infrastructuur en Waterstaat, 2022).
The leaching tests resulted in an increased immobilization of lead, copper, and zinc for both a MSWI FA and a biomass (BM) FA also selected for its high carbonation potential. The highest total immobilization was reached for the MSWI FA through pre-carbonation of fly ash-binder mixture as the high reduction of pH of the sample cured in the carbonation chamber increased the solubility of other heavy metals. The immobilized BM FA through carbon-curing resulted in the optimal immobilization solution, followed by the pre-carbonation of the mixture of binder and fly ash. For both tested materials, the addition of carbonation outperforms the reference samples.
The comparison with the Dutch soil decree showed promising outcomes for future development of mineral carbonation for immobilization of hazardous materials. The carbonized specimens met most of the requirements for construction materials, which were expected to be more stringent than those for landfill use. The materials were evaluated under a worst-case scenario, showing the effective reduction of lead leachability. The carbonation of hazardous materials offered potential for application in highly lead-polluting waste streams to reduce carbon emissions and contribute to a cleaner environment.

A study on the use of aluminium etching solution as an alkali activator

Master thesis (2024) - S. Hesselmans (author) , G. YE (graduation committee member) , Shizhe Zhang (graduation committee member) , M Luković (graduation committee member)

The construction industry faces significant environmental challenges due to its reliance on Portland cement, which accounts for up to 7% of global CO2 emissions (Benhelal, Zahedi, Shamsaei, & Bahadori, 2013). In the Netherlands, initiatives like the ”Betonakkoord” aim to mitigate these emissions. Geopolymer concrete, which utilizes alkali-activated binders derived from industrial byproducts, presents a promising alternative. However, the carbon footprint associated with conventional activators remains a concern. This research investigates the

viability of using waste aluminium etching solutions as a sustainable source of alkali activators, with the goal of minimizing the environmental impact of geopolymer concrete and providing a practical recycling pathway for waste aluminium etching solutions.

The research begins with the characterization of the aluminium etching solution received from APT Extrusions B.V., which serves as the basis for thermodynamic modelling to assess the influence of activator composition on reaction products. This modelling informs the development of six distinct mixture designs based on a BFS precursor, each varying in activator composition. The experimental study is organized into three sections: the first investigates the reaction process through dissolution tests, isothermal calorimetry, and pore solution chemistry; the second examines the fresh and hardened properties, including setting time and strength; and the third presents an analysis of the reaction products using techniques such as TGA, XRD, FTIR, and SEM-EDS.

The results indicate that aluminium-containing activators significantly influence reaction kinetics, initially delaying strength development. However, mixtures with higher aluminium content exhibit enhanced long-term strength due to increased geopolymerization and the formation of more cross-linked C-(N-)A-S-H gel phases. The delayed reaction kinetics are attributed to the passivation of Si sites and the formation of metastable phases,

which temporarily impede the dissolution of slag particles. Although aluminium etching solutions can improve the mechanical performance of the mixtures, optimizing aluminium concentration is critical to avoid shrinkageinduced cracking. To validate this optimization, a new mixture was developed to prevent crack formation while enhancing strength development using the aluminium etching solution.

This study underscores the potential of aluminium etching solutions as eco-friendly activators in the production of sustainable cement alternatives. Given the promise of this byproduct, future research should focus on the dissolution behavior of the precursor in the new activator, detailed investigations of the reaction kinetics within the early age, and an in-depth exploration of metastable phases. Additionally, a comprehensive understanding of fresh and hardened properties, with a particular emphasis on autogenous shrinkage, will be essential for practical applications.