The hydration, thermodynamic modelling, mechanical properties, and microstructure of Portland cement in seawater
- Publication Type:
- Thesis
- Issue Date:
- 2023
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Many countries are experiencing freshwater crises due to population growth and infrastructure construction aligned with the enormous freshwater demand. Using seawater (SW) for concrete manufacturing promisingly provides significant economic and environmental benefits, particularly in coastal areas where SW is more accessible. However, the dissolved chemical ions may significantly limit the scope of its application.
In pursuit of a fundamental understanding of SW’s impact on the heterogeneous cementitious material structure and mechanical properties, a series of experimental and modelling studies for the hydration performance of Portland cement in SW was investigated in this thesis. Phase identification of the solid and liquid phases was performed by combining thermogravimetry, X-ray diffraction and inductively coupled plasma optical emission spectrometry technology. The Rietveld refinement approach has been adopted to characterize phase evolution quantitatively, which forms an experimental validation as a reference to develop kinetics modelling for cement hydration with SW. By combining the optimized kinetic and thermodynamic equilibrium models, the simulation and experiment results of hydrate phases achieve a good agreement. The use of SW not only increases the hydration rate of clinker significantly but also affects the evolution of phase assemblage. Both thermodynamic calculations and experimental determinations confirmed that Friedel’s salt (FS) forms instead of sulfo-AFm in hydrated cement by SW. Additionally, The formation of FS indirectly stabilized the ettringite (AFt) due to the higher concentration of sulfate from the conversion of sulfo-AFm.
Based on the characteristics of cement hydration in SW, the practical study of cement mortar with natural SW and sea sand was carried out. Macroscopic properties were also investigated, including heat evolution, compressive strength, and flexural strength. Although SW increases the early strength by its stimulating effect on cement hydration, the slightly lower compressive strength at the late stage may be due to the additional unhydrated cement and pores.
The secondary electron and backscattered electron imaging technology were adopted to characterize the microstructure of cement paste. By combining SE and BSE images, The area for pores in the microstructure can be accurately segmented with lower data errors. Quantifying hydrated and unhydrated phases was performed by EDS mapping after the data processing of denoising, region segmentation, manual identification, and imaging calibration. The comparison between multiple results shows that the detailed distribution of the phases can be obtained with low data error. Therefore, it is feasible and promising to use EDS mapping to quantify the various cement phases for submicroscopic and microscopic characterization.
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