Kinetics and Thermodynamics of Iron Sulfide Reactions in Concrete
Restricted (Penn State Only)
- Author:
- Li, Zhanzhao
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 28, 2024
- Committee Members:
- Farshad Rajabipour, Major Field Member
Ismaila Dabo, Outside Unit, Field & Minor Member
Christopher Gorski, Major Field Member
Aleksandra Radli¿ska, Chair & Dissertation Advisor
Farshad Rajabipour, Program Head/Chair - Keywords:
- Concrete Durability
Iron Sulfide
Pyrrhotite
Pyrite
Kinetics
Thermodynamics - Abstract:
- Oxidation of iron sulfide-bearing aggregates in concrete has been found to severely impact the durability and serviceability of concrete structures. The lack of fundamental knowledge of the complex, multi-mechanistic reaction mechanisms, however, has hindered the development of standards and guidelines for the proper use of iron sulfide-containing aggregates and the mitigation of related infrastructure degradation. This dissertation aims to address these challenges by unraveling the reaction mechanisms from the kinetic and thermodynamic perspectives. First, the reaction kinetics of cement mortar containing iron sulfide-bearing aggregates were evaluated under various environmental conditions. When exposed to atmospheric oxygen, length changes of the mortar bar samples were mainly attributed to drying shrinkage within the experimental duration (more than 400 days), which was found to be highly dependent on the relative humidity levels. Additionally, minimal to no expansion was observed. A comparison with existing performance-based tests using more harsh exposure conditions underscores the need for further development of experimental methods that can effectively accelerate the reactions in laboratory settings without generating unrealistic products. Second, the deterioration mechanism of iron sulfide reactions in cementitious systems was investigated using thermodynamic modeling. Calculations revealed that the evolution of phase assemblage is mainly governed by the formation of ettringite, gypsum, goethite, and brucite. Despite reductions in pore solution pH, the reactions may not directly induce rebar corrosion in concrete, and portlandite could serve as a buffer phase to delay the decalcification of C–S–H. Crystallization pressure theory suggested that goethite may be the dominant expansive phase, followed by ettringite. Lastly, we examined the kinetics and mechanisms of iron sulfide dissolution in highly alkaline environments, which is the first step of iron sulfide-induced degradation reactions in concrete. Results revealed that pyrrhotite dissolves orders of magnitude more rapidly than pyrite, with dissolution rates increasing with both pH and temperature. The type of alkali in the solution, whether potassium or sodium, was not found to affect the dissolution behavior. Kinetic modeling and experimental characterization indicated that the dissolution kinetics of pyrrhotite is controlled by a combination of chemical reactions (oxidation of iron and sulfur species) and diffusion (through an Fe(III)-(oxy)hydroxide layer). The research findings of this dissertation advance the scientific understanding of iron sulfide reactions across multiple scales and provide valuable insights into future experimental characterization, test development, and practical solutions to mitigate iron sulfide-induced deterioration. Also included in this dissertation are additional works related to data-driven concrete science as appendices. The rapid development of this field is anticipated to help address grand challenges in the concrete science domain, including concrete durability and beyond.