Micromechanics-guided development of strain-hardening alkali-activated composites

Micromechanics-guided development of strain-hardening alkali-activated composites: Towards a low-carbon built environment

Zhang, Shizhe (TU Delft Materials and Environment)

van Breugel, K. (promotor)
Ye, G. (promotor)

Delft University of Technology

2022-12-01

Alkali-activated Materials (AAMs), including those classified as geopolymer, are obtained through the reaction between a solid precursor and an alkaline solution. Compared with ordinary Portland cement (OPC) binders, these materials maintain comparable mechanical properties but have the advantage of reducing greenhouse gas emissions and utilization of industrial by-products and residuals which helps to meet sustainability goals. AAMs are thus considered an environment-friendly construction material with great potential for next-generation concrete.
AAMs are inherently brittle. The low ductility of AAMs makes them prone to cracking and corresponding performance degradation, which is detrimental to their durability. Based on the concept of strain-hardening cementitious composite (SHCC), one solution relates to a family of fiber-reinforced composites that have high tensile ductility and multiple-cracking characteristics, i.e., strain-hardening geopolymer composite (SHGC). While much effort has been taken to develop conventional SHCC, scientific and technical knowledge of SHGC is still in the very early stage of development. This PhD project deals with the development of a cement-free strain-hardening geopolymer composite (SHGC) as a high-performance construction material using industrial wastes and by-products through alkaline activation technology:
The fracture properties and other mechanical properties of the alkali-activated slag/fly ash (AASF) paste as the matrix for SHGC were experimentally tested. The microstructure and chemistry of the reaction products were investigated to understand the fracture mechanism. It was found that the fracture properties of pastes are strongly related to the chemical composition (Ca/Si ratio) of the main reaction product, i.e., C-(N-)A-S-H gel. The fracture properties were also found to be dominated by a cohesion/adhesion-based mechanism. Furthermore, the compressive strength of AASF paste is primarily determined by its capillary porosity.
The fiber/matrix properties, including chemical bonding energy, initial frictional bond, and slip-hardening behavior of fiber during the pullout process were also experimentally studied. The chemistry and microstructure of the reaction product in the fiber/matrix interfacial transition zone (ITZ) were characterized. Their influence on the interface bonding properties was also investigated. It is found that the chemical bonding between PVA fiber and AASF matrix increases with increasing Ca/Si and Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Hence, changing the slag content and the alkali activator Ms appears to be an effective way to modify chemical bonding. Unlike the formation of portlandite near the PVA fiber surface in conventional SHCC, a high-Ca C-(N-)A-S-H phase was formed in the fiber-matrix ITZ of SHGC. This explains the higher chemical bonding energy found in SHGC compared to that in conventional SHCCs. Furthermore, the adhesion mechanism of the PVA molecule in reaction products was studied using MD simulation. The study suggests that the adhesion between PVA fiber and C-(N-)A-S-H gel is primarily due to electrostatic interactions rather than van der Waals interactions.
Based on the result of fracture properties of the matrix and fiber/matrix interface properties, the SHGC is then systematically developed following a micromechanics-based design approach. The experimentally-attained matrix and interface properties served as input for the numerical micromechanics model to simulate the crack bridging behavior. Through the micromechanical modeling, the optimal fiber length and volume were selected and the behavior of mixtures with different fiber/matrix combinations was predicted. With this approach, researchers and materials engineers can design and tailor future SHGC more efficiently than by using the commonly used trial-and-error method.
Finally, the environmental impact of the SHGC with the most promising performance was also evaluated. This evaluation was conducted using a cradle-to-gate life-cycle assessment (LCA) of SHGC compared to that of conventional SHCC materials. The developed SHGC demonstrates a very promising environmental profile. It has a significant reduction of the global warming potential (GWP) and a lower or similar total environmental impact compared to conventional SHCC materials. In addition, the results also provide recommendations for further improvements in mixture design for the future development of SHGC.
This study successfully developed a sustainable slag/fly ash-based SHGC with a lower carbon footprint than conventional SHCC. It is considered a good example to utilize industrial by-products as secondary resources and at the same time contribute to a circular economy. Furthermore, this study helps to understand the fracture properties of AAMs. It also clarifies the adhesion mechanism of PVA fiber in AAMs. All of these give promising guidance for researchers and engineers to design fiber-reinforced AAMs with required fracture properties and interface bonding properties. In particular, it contributes to the design and tailoring strategies for high-performance composite, for instance, SHGC, through proper mixture design.

Alkali-activated materials
Strain-hardening
Slag
Fly Ash
Polyvinyl alcohol (PVA)
Fiber-reinforced composites
Micromechanics
Fracture behaviour
Interface
Bonding
Adhesion
Molecular dynamics (MD)
Life-cycle assessment
Sustainable construction
CO2 reduction

https://doi.org/10.4233/uuid:cdea2e9d-e069-4683-a8ca-c436b43fa64c

978-94-6421-961-6

2022-12-01

Institutional Repository

doctoral thesis

© 2022 Shizhe Zhang