PhD Defense | Thi Nhan Nguyen | Physico-chemical evolution of geopolymers in contact with aggressive environments

Physico-chemical evolution of geopolymers in contact with aggressive environments

In recent years, the development of alkali-activated materials (AAMs) including geopolymers, as alternatives to ordinary Portland cement (OPC), has gained significant interest due to their comparatively lower carbon footprint. Next to that, geopolymers exhibit outstanding properties such as high mechanical strength and promising chemical stability, making them increasingly attractive for a wide range of applications beyond construction, including the immobilization of toxic and nuclear waste. Notably, geopolymers have demonstrated superior efficacy compared to OPC in immobilizing hazardous elements, such as ¹³³Cs⁺, ¹³⁴Cs, and Sr²⁺. However, the long-term durability of AAMs, both with and without waste incorporation, when exposed to external chemicals, remains a critical concern. The study herein is addressing that, and in particular, the imperative need to understand and predict the long-term performance of AAMs, especially in the context of nuclear liquid waste immobilization, a relatively novel application.

The present work investigated the performance of AAMs exposed to aggressive environments, focusing on the design of AAMs formulations suitable for nuclear liquid waste immobilization. To achieve this objective, two crucial degradation processes, carbonation and leaching, were analysed, on AAM mortars with high-calcium content, i.e., alkali activated GGBFS (AAS), and metakaolin-based geopolymers, being practically free from calcium. A comprehensive characterization was performed using techniques such as XRD, FTIR, TGA/DSC, (²³Na, ²⁷Al, and ²⁹Si) MAS NMR, ICP-OES, SEM/EDS, nitrogen adsorption, MIP, and micro-indentation. Additionally, water permeability and dissolved gas diffusivity studies were performed to evaluate the transport properties of AAMs. Thermodynamic models of carbonation and leaching were also developed to gain deeper insights into the degradation processes and to predict AAM performance under varying conditions.

The study herein also assessed the immobilization capacity of AASs and metakaolin-based geopolymers with different types of liquid wastes. The waste-forms containing Tributyl phosphate (TBP) and metakaolin-based geopolymer, as well as the waste-forms containing lubricating/motor oils (e.g., Nevastane and Shellspirax) and AAS, were analysed for their fresh properties, mechanical strength, and durability. Results indicated that both carbonation and leaching significantly altered the structure of the C-A-S-H gel in AAS, causing decalcification and dealumination. In contrast, the N-A-S-H gel of the metakaolin-based geopolymers was less affected by these degradation processes. Carbonation-induced microstructural changes were influenced by the water-to-binder ratios, leading to variations in mechanical strength. Leaching, on the other hand, coarsened the microstructure of AAS but reduced the total porosity of metakaolin-based geopolymers due to the formation and precipitation of Al(OH)₃. Consequently, water permeability increased in AAS after leaching, while permeability and diffusivity decreased in the metakaolin-based geopolymer. Notably, an exponential relationship between the total porosity and water permeability was observed in both materials, as well as between the total porosity and diffusivity of metakaolin-based geopolymers, before and after leaching. Furthermore, both AAS and metakaolin-based geopolymers exhibited promising immobilization capacity for various types of wastes when surfactants were employed. Metakaolin-based geopolymers, exhibiting a N-A-S-H network distinctly different from the C-A-S-H network of AAS materials, achieved a better compatibility with liquid wastes compared to AASs. AAS tends to work primarily with high-viscosity liquids such as lubricating oils. Waste loading can reach up to 40 vol.%, with the durability of the waste-forms showing great promise.

This comprehensive study enhances the understanding of AAMs' long-term performance and their potential for nuclear liquid waste immobilization, contributing to the development of sustainable and environmentally friendly materials for critical applications including nuclear waste management.

Promotor:

• Yiannis Pontikes (KU Leuven)

Co-promotor:

• Jan Elsen (KU Leuven)

SCK CEN mentors:

• Quoc Tri Phung (SCK CEN)

• Diederik Jacques (SCK CEN)

IRSN mentor:

• Alexandre Daurezes (IRSN)

Click here for a list of obtained PhD degrees.