| Author: | Sun, Yanjie |
| Title: | Effects of seawater and seawater-abundant salt solutions on the hydration behaviours and mechanical properties of alite pastes |
| Advisors: | Poon, Chi Sun (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Concrete Seawater Reinforced concrete -- Corrosion Reinforced concrete construction Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xix, 188 pages : color illustrations |
| Language: | English |
| Abstract: | Concrete is one of modern society's most used construction materials due to its low price, high durability, easy-preparedness, etc. Within the preparation process of concrete, mixing water is one of the main components. Massive concrete usage in modern structures requires a considerable amount of water. Usually, freshwater is the first choice for the water resource since there is no or limited amount of hazardous ions causing the corrosion of steel bars. However, even though freshwater is an easily obtained resource, the shortage problem has become increasingly significant, especially in places such as the Middle East, North Africa, remote islands, etc. In contrast, seawater is the most abundant water source in the world. So the application of seawater in concrete mixing can be quite promising. In fact, seawater concrete is not a new invention, and its early application can be traced back to 2000 years ago, known as ancient Roman concrete. The longevity of such structures demonstrated the feasibility of seawater concrete used in simple concrete structures. However, for modern concrete structures, the application of seawater in concrete mixing is limited or forbidden because of the wide application of steel reinforcement. The abundant chloride ions in seawater would cause severe corrosion problems to steel, degrading the durability of reinforced concrete. Thus, for a long time, seawater has been removed from the choice of mixing water for concrete. The situation has remained a challenge for the application of seawater concrete until the invention of fibre-reinforced polymer (FRP) and its application as reinforcement components in concrete structures. Thanks to the high corrosion resistance of FRP in marine environments. The possibility of seawater mixing becomes more and more visible. Because of the limitation of seawater concrete in the past, only a limited amount of studies focused on this field. More comprehensive studies are necessary to provide a better understanding of seawater concrete. Since seawater and cement are complex systems, simplifying the study methods is necessary. This thesis tried to separately study the effects of single salts and their combinations in seawater (NaCl, MgCl2, Na2SO4) on the hydration behaviour of cement. Alite, the most abundant phase in ordinary Portland cement, was also chosen as the representative of the OPC binder. To understand the hydration kinetics and micro-mechanical properties of alite pastes, the alite was mixed with the salt solutions at a low solution-to-binder ratio (0.5) and high solution-to-binder ratio (10). Based on these prepared alite pastes, various experimental and simulation methods were used to study the hydration and mechanical properties. The hydration kinetics and micro-mechanical properties were studied for the low solution-to-binder ratio samples. The isothermal calorimetry and in-situ XRD tests were adopted to study the hydration kinetics. The results showed that both seawater and salt solutions would accelerate the hydration rate of the alite pastes. The acceleration effect was more significant with increasing concentrations of the single salts. The effect of MgCl2 was the most severe. The boundary nucleation and growth (BNG) model was used to simulate the heat release data, and it showed that these salt solutions increased the nucleation rate of the hydration products, which was regarded as the main reason for the acceleration effect. The nanoindentation technique was adopted for the micro-mechanical properties of the alite pates. The modulus and hardness of the alite pastes were improved when the alite was mixed with NaCl solutions at varying concentrations. However, adding MgCl2 and Na2SO4 decreased the micro-mechanical properties of the alite pastes. The effect of MgCl2 was also more significant than that of Na2SO4. The nanoindentation data were further analyzed to get the packing density of each phase. The packing density distribution tendency was quite similar to the micro-mechanical properties. The comparison of the effects of common cations was also subjective in this thesis. The micro-mechanical properties and the compositions of C-S-H were the two main focuses. Based on the nanoindentation and EDX analysis, CaO/SiO2 and H2O/SiO2 ratios were obtained to help explain the micro-mechanical property variation after incorporating each cation. It was found that the presence of Mg2+ in seawater was the main reason for the strength degradation of the alite pastes. For the dilution study, the main reason for the study is to give a mechanism for the acceleration effect. A high solution-to-binder ratio of 10 was able to amplify the dissolution process of alite. This study mainly studied the dissolution process of alite in different solutions. The ion variation results showed that the dissolution rate of alite was accelerated. There were several reasons. For all salt, the addition increased the ionic strength of the solutions, which increased the undersaturation status of alite. For MgCl2 solutions, the quick formation of brucite consumed the OH- in the solution, accelerating the dissolution process. The formation of gypsum due to the addition of Na2SO4 decreased the Ca2+ concentration in the solution. That was another reason for the acceleration. Based on the results of this thesis, some basic understanding of seawater concrete can be obtained. First, the acceleration effect of seawater mainly happened during the early hydration process. The effect can be considered in two aspects: dissolution and precipitation. For the dissolution, the increasing ionic strength, and formation of brucite and gypsum accelerated the dissolution. The precipitation process was accelerated due to the increased nucleation rate of hydration products due to the high calcium concentration in the solution. The mechanical properties of the alite pastes were degraded due to the presence of Mg2+ and SO42-, which meant that when seawater is used as the mixing water of concrete, the concentrations of these two ions need to be carefully evaluated. |
| Rights: | All rights reserved |
| Access: | open access |
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