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Title: The influence of gypsum on the hydration kinetics and the microstructure of calcium sulfoaluminate cements in sulfate-rich environmnets
Author: Beltagui, Hoda
ISNI:       0000 0004 6056 8992
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2017
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In an era of concern over carbon emissions arising from the construction industry, calcium sulfoaluminate(C$A) cements are a promising alternative to Portland cements as they offer a reduced carbon footprint. Moreover, there are many speculations that C$A cements could have a better sulfate resistance than Portland cements but there is a lack of solid evidence to support these speculations. This thesis investigates the role of gypsum on the hydration kinetics and microstructure of a C$A cement in sulfate-rich environments and provides a better understanding of the reaction mechanisms occurring. The impact of the gypsum content and of the water to binder ratio on the mechanical properties, hydration kinetics and microstructure was studied, and thermodynamic calculations were carried out to aid in the interpretation of the experimental results. The main hydration product was confirmed by X-ray diffraction to be ettringite, which formed in addition to monosulfate, amorphous aluminium hydroxide (AH3) and amorphous calcium aluminate hydrates (C-A-H) phases within the hydrated phase assemblage. Samples were exposed to 50 g L-1sodium sulfate and magnesium sulfate solutions for up to 365 days according to ASTM C1012. The related length and mass changes and sulfate uptake were investigated, as well as the impacts on the hydration kinetics and microstructure. Upon exposure to the sulfate solutions, sulfate ions were transported inwards, reacting with alumina from monosulfate and amorphous C-A-H phases to form ettringite. The levels of amorphous AH3 determined by thermogravimetric appeared to remain constant in comparison with samples cured in water, indicating that the AH3 was not a source of alumina for ettringite formation. The ettringite levels in the outer 1 mm layer of samples cured in the sulfate solutions were found to be higher than those of samples cured in water; however, no related expansion was recorded. When exposed to the sodium sulfate solution, as the overall mass balance favoured the dissolution of cement phases and the leaching of calcium ions over the precipitation of ettringite, the resulting increase in porosity was able to provide a safety valve for the relief of expansive pressures. Additionally, the precipitation of calcium carbonate on the surface of the samples provided a protective barrier, thus limiting the ingress of further sulfate into the cement matrix. All samples exposed to the sodium sulfate solution remained intact by the end of the exposure period and no detrimental "sulfate attack" was observed. In the case of the magnesium sulfate exposure, the formation of high density phases (hydrotalcite, brucite and gypsum) resulted in a decrease of the overall solid volume and so no expansion was observed. However, while increasing the initial gypsum content of the samples was found to limit the potential amount of ettringite that could form; this caused the detrimental precipitation of gypsum which resulted in spalling of the material. Therefore, the problems associated with C$A cements in sulfate-rich environments are not caused by the formation of ettringite leading to expansion, but rather by the formation of detrimental materials, such as magnesium silicate hydrates, resulting in the weakening of the material and in its spalling. The work presented within this thesis provides evidence in the first instance that C$A binders have the capacity to be durable in sulfate environments, which can enable their development and application in industry. Moreover, the understanding of their hydration kinetics and related mechanical properties, as well as the mechanisms occurring in sulfate rich environments can aid in the design and development of C$A cement compositions.
Supervisor: Not available Sponsor: Gulf Organisation for Research and Development
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Gypsum ; Cement ; Sulfate-resistant concrete