| Author: | Cai, Yamei |
| Title: | Reaction mechanism of tricalcium aluminate – seawater systems |
| Advisors: | Poon, Chi Sun (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Calcium aluminate Hydration Seawater Concrete Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxiv, 229 pages : color illustrations |
| Language: | English |
| Abstract: | In concrete production, the possible use of seawater as the mixing water is gaining increasing interest due to scarce freshwater resources, particularly in coastal areas and isolated islands. The main concern in the use of seawater is the reinforcement corrosion caused by Cl- ions in seawater. However, referring to plain concrete and fiber-reinforced polymer (FRP) reinforced concrete, this corrosion problem seems not a limiting factor in applying seawater-mixed concrete. It has been reported that seawater can accelerate cement hydration, shorten setting time, and decrease workability. These fresh properties of cement and concrete are associated with the tricalcium aluminate (C3A) in cement clinkers. However, research on the influence of seawater on the hydration mechanism and hydration products of C3A is still scarce. This thesis aims to fill the research gap, understand the interactions between C3A and seawater, and further explore how these interactions affect the hydration evolution of C3A and the properties of its hydration products. Furthermore, the deterioration degree of freshwater- and seawater-mixed C3A mortars exposed to 50 g/L Na2SO4 and seawater solutions was also compared, and these durability issues were explored from the perspective of aluminate phase in cement clinkers. As for the hydration process of C3A, the results in this study show that, in the absence of gypsum, seawater can retard the C3A hydration and reduce its reaction degree. The reasons could be ascribed to 1) the co-existence or ion pairing of Ca2+ and SO42- onto the surface of C3A to block the reactive sites, which was the determinative role on this retarding effect; 2) the deposition of Mg-based layered double hydroxide (Mg-based LDH) phase on the C3A surface, which prolonged the induction period for another 30 min; 3) an accumulation of Al ions in the pore solution during the formation of Friedel’s salt, which would also hinder the dissolution of C3A. Additionally, the Cl ions in seawater would preferentially react with C3A to form Friedel’s salt, followed by hydrogarnet (C3AH6). This was different from the hydration products formed in the C3A-deionized water (DI) paste, in which the metastable hydroxy-AFm, i.e., C4AH13 and C2AH8, were found at the beginning. Then they were gradually converted into C3AH6. In the presence of gypsum, seawater increased the dissolution driving force of C3A and solubility of gypsum, which accelerated the early hydration of C3A and the formation of ettringite (AFt), leading to a higher hydration degree of C3A at the early age compared to the corresponding DI paste. After gypsum depletion to form AFt, and in the absence of Ca(OH)2, the formation of chloroaluminate hydrates was slower due to the insufficient Ca, resulting in an accumulation of Al in the pore solution. This would delay the subsequent transformation of AFt to monosulphate (SO4-AFm) and the formation of C3AH6, rendering the reduction in the hydration degree of the C3A at later ages. However, in the presence of Ca(OH)2, the sufficient Ca source increased the hydration degree of C3A-gypsum-Ca(OH)2-seawater paste at later ages, enabling it to match the hydration degree of the corresponding DI paste. It can be further inferred that the amount of Ca available in the seawater-mixed cementitious materials could play a dominant role in controlling the hydration rate of C3A after the depletion of gypsum. Regarding the hydration products of C3A, it was found that, whether Ca(OH)2 existed or not in the reaction systems, the AFt formed in seawater had a relatively larger cumulative pore volume and lower packing density compared to the corresponding AFt formed in DI water, which led to a relatively lower elastic modulus and hardness. This could be mainly ascribed to the fact that some Mg ions in seawater entered the sites of Ca ions in AFt, resulting in a relatively unstable structure. When the Mg ions were excluded from seawater, the synthetic AFt samples seemed to have a smaller cumulative pore volume and higher packing density, compared to the corresponding AFt formed in DI water, resulting in a relatively higher micromechanical property. In addition, it was noted that, in the absence of Ca(OH)2, the preferential formation of Mg-Al layered double hydroxide containing Cl- (Mg-Al-Cl LDH) phase promoted the precipitation rate of subsequent AFt crystals. However, in the presence of Ca(OH)2, these Mg ions would form brucite rather than the Mg-Al-Cl LDH phase, and there was no significant acceleration for the precipitation of AFt in the seawater solution, compared to the AFt formed in seawater without adding MgCl2. AFm phase is another main hydration product of C3A. It belongs to the calcium-aluminium layered double hydroxide family, which is composed of positively charged hydroxide layers ([Ca2Al(OH)6]+) with anions occupying the interlayer spaces. It was observed that the basal spacing of the AFm crystals narrowed as the interlayer SO42- in the AFm phase was gradually replaced by Cl- in seawater. This would promote the packing density of nanocrystals and their indentation modulus and hardness, i.e., Cl-AFm (Friedel’s salt) > SO4-Cl-AFm (Kuzel’s salt) > SO4-AFm (monosulphate). Besides, Friedel’s salt formed in seawater featured multiple structural and compositional defects. First, the anions [Cl-, OH-, SO42-] were bound in the interlayer of Friedel’s salt, and the bound OH- contents had negative relations with the Cl- concentration in the seawater. Second, the incorporation of Mg ions in Friedel’s salt barely changed its micromechanical properties based on the experimental data and molecular dynamics simulations. Third, when portlandite was present in the seawater, some Ca vacancies would be formed in Friedel’s salt, which would decrease Young’s modulus significantly. This accounted for the decrease in the indentation modulus of Friedel’s salt, as observed in the experiments. When freshwater- and seawater-mixed C3A mortars were exposed to 50 g/L Na2SO4 and seawater solutions, it was reported that the higher solubility of AFt in the pore solution containing sea salts decreased the precipitation amounts of AFt in the pores of SW-mixed C3A mortars, consequently reducing the expansive damage to the matrix. As a result, the SW-mixed C3A mortar exhibited less mass loss and length decrease compared to the DI-mixed C3A mortar. This suggests that the SW-mixed C3A mortar experienced relatively less degradation by aggressive solution. |
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| Access: | open access |
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