Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Civil and Environmental Engineering | en_US |
| dc.contributor.advisor | Dai, Jian-guo (CEE) | en_US |
| dc.contributor.advisor | Dong, You (CEE) | en_US |
| dc.creator | Hayat, Umar | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/14642 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | en_US |
| dc.rights | All rights reserved | en_US |
| dc.title | Degradation of seawater-mixed cement paste : MD simulation of C-S-H behaviour and experimental study | en_US |
| dcterms.abstract | Concrete is the most widely used man-made construction material, serving as the backbone for infrastructure such as buildings, bridges, pavements, pipelines, and dams. However, its substantial demand places unsustainable pressure on freshwater resources, a challenge that is particularly critical in regions where freshwater is already scarce, such as the Middle East, North Africa, and remote islands. In such coastal and island environments, the limited availability of freshwater makes the use of seawater as a mixing water for concrete construction a practical and often necessary alternative. However, the use of seawater as mixing water introduces high concentration of seawater ions into the concrete. While numerous studies have established that seawater typically expedites setting and improves early compressive strength, its long-term durability remains a primary concern for structural service life. This concern primarily arises from the chemical interaction between seawater and concrete due to the presence of multiple aggressive ions in seawater, such as chloride, sulphate, magnesium, and carbonate, which can lead to complex degradation mechanism. Despite extensive research, the mechanistic understanding of these processes, particularly ion transport, interfacial phenomena, and phase degradation at the atomic and macroscale levels, is still inadequate. To address these fundamental knowledge gaps, this thesis employs an integrated approach of molecular dynamics (MD) simulations and long-term experimental studies to elucidate the multiscale mechanisms of ion transport, water stability, and phase degradation in seawater-mixed cement paste. The investigation is structured in four parts. | en_US |
| dcterms.abstract | First, MD simulations used to investigate the atomistic-level ion transport of NaCl within the pores of calcium silicate hydrate (C-S-H) gel, the primary binding phase of hydrated cement paste. The influence of pore size (35-95Å) and temperature (275-350 K) on ionic transport was thoroughly investigated using a tobermorite-based model. The results indicate that NaCl transport is significantly accelerated by the increase of both pore size and temperature. The smaller pores and high temperature create a filtration effect, facilitating deeper water penetration while limiting the entry of Na⁺ and Cl⁻ ions. Moreover, increased pore size and elevated temperatures accelerate Na-Ca cation exchange, resulting in the release of Ca²⁺ from C-S-H surface. | en_US |
| dcterms.abstract | Building on these transport mechanisms, the second part investigates how the retained water inside C-S-H pores responds to drying, as water stability has a direct impact on ionic mobility and cohesion. This section investigates the desorption of water from NaCl solutions confined within C-S-H gel nanopores, a mechanism that governs drying shrinkage and salt crystallisation in cement-based materials. Both canonical (NVT) and isothermal-isobaric (NPT) ensembles were employed to investigate the underlying mechanisms of water desorption, considering various Ca/Si ratios (1.2, 1.4, 1.6, 1.8, and 2.0). The desorption mechanism was found to be largely independent of the Ca/Si ratio. | en_US |
| dcterms.abstract | To understand how this interfacial ion accumulation directly impacts mechanical performance, the third section investigates the effect of seawater ions on the nanoscale cohesion and mechanical properties of C-S-H, relating transport and desorption to structural performance. MD simulations were used to investigate how interfacial distance and cation type (Na⁺, K⁺, Mg²⁺) affect shear cohesion and tensile properties of C-S-H. Results show that pure C-S-H exhibits the highest strength, while partial substitution reduces performance in the order: CSH ≈ CSH_Mg > CSH_Na > CSH_K. Shear strength decreases sharply with increasing interfacial distance, vanishing beyond 0.32 nm when water transitions from bilayer to trilayer and becomes mobile. Tensile tests reveal consistent trends, with pure C-S-H displaying superior strength, stiffness, and ductility. | en_US |
| dcterms.abstract | Finally, to understand the degradation mechanism at larger scale, a complementary year-long experimental study was conducted in fourth part that explain how these mechanisms emerge in real seawater cement pastes under aggressive ions exposures. In this study, seawater-mixed cement pastes were exposed for one year to 5% chloride, 5% sulphate, and combined chloride-sulphate solutions containing Na⁺, K⁺, Ca2⁺, and Mg²⁺ cations. The phase assemblage, microstructure, and micromechanical properties were systematically investigated using complementary analytical techniques. The results show that chloride attack primarily promotes Friedel's salt formation without severe decalcification, whereas MgCl₂ causes significant portlandite (CH) depletion, brucite formation, and C-S-H degradation. Sulphate attack results in extensive formation of ettringite and gypsum, accompanied by calcium leaching, with MgSO₄ identified as the most aggressive among the sulphate solutions. The exposure to combined chloride-sulphate solutions intensified these processes, especially the exposure to MgCl₂+MgSO₄ system causes the most pronounced deterioration. | en_US |
| dcterms.abstract | An improved understanding of seawater-cement interactions across multiple scales will enable the development of predictive models for long-term durability, supporting the design of sustainable concrete for marine infrastructures and contributing to resource efficiency in water-stressed regions. | en_US |
| dcterms.extent | xxvii, 259 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2026 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.accessRights | open access | en_US |
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