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Author ORCID Identifier

https://orcid.org/0000-0002-0800-1556

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Civil and Environmental Engineering

Year Degree Awarded

2020

Month Degree Awarded

May

First Advisor

Guoping Zhang

Subject Categories

Geotechnical Engineering

Abstract

The characteristics of underwater cast and cured geopolymers were experimentally studied to understand the interactions between geopolymer slurry and curing saline water and pertinent effects on its strength development. Geopolymers were synthesized from a mixture of Class C fly ash and metakaolin, and the influences of two different Si/Al molar ratios (1.78 and 2.00) and four different Na/Al (0.67, 0.80, 0.84, and 1.00) molar ratios were investigated, respectively. The geopolymer slurry was prepared and then cast into porous molds, followed by immediately submerged into 0, 15, and 35 ppt saline solutions (prepared with commercial seasalts) for 28-days’ curing in ambient laboratory environment. Parallel experiments were also performed on Class G oil well cement. The mineralogy, chemical composition, microstructure, and mechanical properties of the cured geopolymers were characterized by X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, and unconfined compression testing, respectively, together with measurements of the pH and salinity of all the curing solutions. The oil well cement exhibits a decreased compressive strength in high-salinity solutions, while geopolymers display the distinguishable performance. For the two geopolymers with different Si/Al ratios, strength increases with increasing salinity, and a higher Si/Al ratio brings greater strength. For geopolymers with four Na/Al ratios, the results are relatively complicated. The strength of the geopolymers with Na/Al ratios of 0.67 and 0.80 increases with increasing salinity, while opposite to that with Na/Al ratios of 0.84 and 1.00. Under the same curing condition, a higher Na/Al ratio results in a decrease of strength. The results of X-ray fluorescence, pH, and salinity measurements show that the important alkali ions (e.g., Na+, OH-) not only leach out from the geopolymer slurry cured in relatively low-salinity solutions but also intrude into the slurry cured in high-salinity ones, thus leading to the change in the mechanical performance. Moreover, Na content of geopolymers as a direct indicator reflects the strength’s variation trend. The underlying mechanism is the chemical exchanges, particularly the leaching or ingress of the alkali ions via dispersions, between the curing solution and being-cured geopolymer slurry, affecting the development of geopolymeric structure, which mainly contributes to the development of strength.

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