A Review on Polymer-Modified Cementitious Materials for Underwater Repair: Workability, Bonding, Mechanical Performance and Durability

Abstract

Underwater concrete infrastructure is gradually damaged by water scouring, chloride ingress, freeze–thaw cycles, and fatigue loading, so reliable in situ repair materials are increasingly needed. Conventional cement-based repair materials are often unsuitable for underwater use because they disperse in water, bond weakly to wet substrates, and show limited durability. Polymer-modified cementitious materials can reduce these problems by combining cement compatibility with polymer film formation and interfacial strengthening. Water-soluble polymers mainly improve fresh-state cohesion and anti-washout performance through adsorption, bridging, and flocculation regulation. In comparison, polymer emulsions and latexes are more effective after hardening, improving bonding, crack resistance, and durability through polymer films and organic–inorganic networks. For self-leveling underwater repair, the flow spread should reach at least 130 mm. For vertical repair with a 20 mm layer, a yield stress of about 360 Pa is needed to prevent sagging. Therefore, performance should not be judged by strength alone, but by constructability, interfacial water films, and pore connectivity. Future studies should consider responsive polymers, multi-component modification, standardized tests, and low-carbon binders.

1. Introduction

In national economic development and infrastructure operation and maintenance, concrete has become one of the most important and widely used engineering materials in underwater infrastructure construction and repair worldwide, thanks to its high availability, low cost, and mature construction techniques [1]. However, as a heterogeneous, porous, and highly permeable brittle material [2,3,4,5], ordinary concrete suffers from defects such as susceptibility to dispersion underwater, low interfacial bond strength, and insufficient durability in underwater construction environments, making it difficult to meet the stringent requirements for workability and durability demanded of underwater repair materials [6]. A vast number of underwater structures worldwide—including bridge piers, port terminals, hydraulic dams, and offshore wind turbine foundations—are constantly subjected to the effects of flow scouring, water level fluctuations, freeze–thaw cycles, and chloride corrosion [7]. These factors ultimately reduce the durability of concrete structures [8,9,10,11]. It has been reported that the incidence of defects in underwater bridge structures in China exceeds 15% [12]. In 2021, China allocated $10.19 billion for reservoir dam reinforcement, accounting for over 10% of that year’s water conservancy development funds [13]. In the United States, approximately 6.8% (over 42,000) of bridges are currently in “poor” condition, and more than 22,000 bridges face the risk of flooding or foundation scouring during extreme weather events; Furthermore, the average service life of 92,000 dams has reached 64 years, with approximately 15% exhibiting severe structural defects [14]. Given the enormous costs and lengthy timelines associated with such repairs, the trend of deterioration in water-related concrete structures appears difficult to reverse over the coming decades. It is foreseeable that underwater infrastructure repair work will continue worldwide, presenting vast market opportunities for the development and application of underwater repair materials.

Underwater repair materials are material systems designed for the in situ rehabilitation of submerged or water-saturated concrete structures. At present, although inorganic repair systems [15,16,17,18,19] offer advantages in terms of compatibility and cost, they are susceptible to scouring; organic repair systems [20,21,22], while capable of effective curing and bonding underwater, are relatively expensive and have poor compatibility with existing cement-based substrates. Therefore, the practical material options are primarily focused on organic–inorganic composite repair materials [23,24,25]. In this paper, based on different polymer types, these materials are classified into water-soluble polymers (e.g., cellulose ethers, polyacrylamide, etc.), polymer emulsions and latex (e.g., water-based epoxy, SBR, acrylic emulsions, etc.), and nanomaterials (e.g., nanoscale silica). Additionally, this paper provides a systematic characterization and review of the underwater anti-dispersion properties, flowability, bond strength, and durability of these underwater repair materials to clarify the performance requirements for different underwater repair scenarios.

2. Recent Advances in Underwater Repair Materials, Both Domestic and International

To examine how research on underwater repair has developed, a bibliometric search was carried out in the Web of Science Core Collection. The retrieval period covered 2008–2025. Because this section is intended to describe the overall significance and evolution of underwater repair research, rather than to limit the discussion to a single type of repair material, two broad keywords were selected: “underwater” and “repair”. The specific search formula in Web of Science was TS = (“underwater” AND “repair”). The document types were restricted to English-language Articles and Review Articles. This search first returned 630 records. The titles, abstracts, and keywords were checked manually, and duplicate entries as well as papers with no clear relevance to underwater repair were excluded. After screening, 567 papers were retained for publication trend analysis. The retained papers mainly involved underwater repair, underwater concrete repair, hydraulic engineering repair, and marine infrastructure repair. Records were excluded when they were unrelated to underwater repair, were not Articles or Review Articles, were not written in English, or appeared more than once in the dataset.

The annual publication trend shows two stages. From 2008 to 2015, the number of studies increased slowly, indicating that underwater repair had not yet become a major research focus. From 2016 to 2025, however, the growth became faster. This change is closely related to the increasing exposure of underwater infrastructure to complex service environments and to the practical repair problems that have appeared in engineering projects. As a result, more studies have examined underwater concrete from the perspectives of mechanical performance, repair behavior, microstructure, and interfacial adhesion. These topics therefore appear to be central concerns in current underwater repair research (Figure 1).

2.1. Classification and Evolution of Underwater Repair Materials

As discussed above, underwater repair materials can be broadly understood in terms of inorganic [24], organic [26,27,28], and organic–inorganic composite systems [29,30,31]. However, for polymer-modified cementitious underwater repair materials, material classification alone is insufficient to explain their practical applicability. In real underwater repair scenarios, their performance depends not only on chemical composition, but also on their ability to remain stable during placement, resist water scouring, adapt to wet environments, bond with water-saturated substrates, and maintain mechanical integrity and durability after hardening [32,33,34,35,36].

2.2. Control of Underwater Workability in Polymer-Modified Cementitious Materials

2.2.1. Workability of Underwater Repair Materials-Flat Surfaces/Grouting

In underwater surface restoration, repair materials must be able to fully fill defects under their own weight or under pumping pressure. Therefore, they should possess not only sufficient resistance to washout under water flow, but also adequate flowability. Considerable efforts have been devoted to understanding the dynamic balance between anti-washout performance and rheological behavior.

Cellulose ethers are widely used to regulate water retention, pore structure, and rheology in cementitious materials. Pourchez et al. [37] reported that hydroxypropyl methylcellulose (HPMC) can stabilize 50–250 μm air bubbles introduced during mixing, thereby improving water retention while maintaining suitable rheological properties. Chen et al. [38] further showed that 0.45–0.5% medium-viscosity HPMC can form Ca-O bonds with C-S-H, resulting in a flow spread above 145 mm and turbidity below 100 NTU. In polyacrylamide (PAM) systems, Lu et al. [39] identified a lubrication regime at a PAM dosage of 0.3–0.67%, in which non-adsorbed polymer chains reduce flow loss while improving anti-washout performance. Sun et al. [40] synthesized an SA (sodium acrylate):AM (acrylamide) = 7:3 copolymer through in situ copolymerization and used long chains with low charge density to provide dynamic lubrication. This approach limited washout loss to approximately 1% while maintaining a low dynamic yield stress (<150 Pa), thus achieving a favorable balance between flowability and washout resistance. Natural biopolymers also show promise. For example, 0.5% sodium alginate or guar gum reduced washout loss from 15% to approximately 5% while maintaining a slump of about 50 mm [41].

In practical engineering applications, however, superplasticizers are often required to further improve workability. Ma et al. [42] found that competitive adsorption occurs between HPMC and polycarboxylate superplasticizers (PCEs). HPMC can bind with Ca²⁺ to form aggregates, thereby consuming PCE and weakening its dispersing efficiency. At a dosage of 0.2%, high-molecular-weight HPMC sharply increased the yield stress to approximately 50 MPa, significantly reducing the dispersion efficiency of PCE. In addition, Sikandar et al. [43] demonstrated that anti-washout admixtures (AWAs) can enhance matrix cohesion through polymer–water, polymer–polymer, and polymer–particle interactions. As a result, the washout loss of underwater concrete (UWC) was reduced from 16% to 3.1–4.8%, accompanied by substantial improvements in rheological performance, as shown in Figure 2 and Figure 3.

Although water-soluble polymers can effectively improve the anti-washout performance of cementitious materials, their contributions to interfacial bonding and long-term durability remain limited. By contrast, waterborne polymer emulsions can reduce yield stress and improve flowability through particle lubrication, while also forming polymer films and filling pores within the cement matrix and interfacial region. These effects contribute to enhanced bond strength, toughness, and durability. Therefore, waterborne polymer emulsion modification represents an effective strategy for improving the overall performance of cementitious underwater repair materials.

Lu et al. [44] investigated the effects of colloidal polymers with different surface properties on the rheological behavior of fresh cement paste, as shown in Figure 4. They found that polystyrene latex containing carboxyl groups (-COO-) can form complex bonds with Ca²⁺ in the pore solution. Through this “calcium-bridging” effect, polymer particles adsorb onto and bridge adjacent cement particles, forming a stable three-dimensional flocculated network. Although this structure increases yield stress and reduces flowability, it improves the resistance of the system to water-induced washout. Zhang et al. [45] further reported that the bridging-induced thickening effect becomes most pronounced when the polymer surface coverage reaches approximately 20%, at which point the reduction in flowability is also most significant.

Recent studies have also confirmed the value of waterborne polymers in underwater repair systems. Zheng et al. [46] prepared a water–oil gradient composite epoxy resin (DEP)-modified sulfoaluminate cement system by combining waterborne epoxy resin (WEP) with oil-based epoxy resin (EP). Compared with the control group, this system showed a much lower static yield stress, a maximum flow spread of 235 mm, and a washout mass loss of only about 0.01% after 60 min. Its bond strength at the wet underwater interface was also increased by 195.7%, suggesting that flowability, washout resistance, and interfacial bonding were improved at the same time. Han et al. [47] attempted to solve the poor flow retention of underwater epoxy–cement mortar by adding 20% polyacrylate emulsion (PAE). As shown in Figure 5, PAE and WEP formed a compact film on particle surfaces, raising the flow spread to 210 mm and keeping it stable for 30 min. The bond strength of underwater-cast specimens increased by 65.8%, showing improved constructability and mechanical performance.

Overall, a single polymer emulsion is usually insufficient to ensure stable anti-washout performance, especially under flowing-water conditions. Therefore, underwater surface repair materials should be designed as cementitious composites modified by multiple synergistic polymers rather than as single-polymer systems. Low- or medium-viscosity water-soluble polymers can provide washout resistance and suspension stability, whereas waterborne polymer emulsions can compensate for the insufficient interfacial bonding, toughness, and durability of the cementitious matrix. The key optimization target is to determine an appropriate polymer composition and dosage window that simultaneously satisfies the requirements for flowability, underwater washout resistance, interfacial bonding, and long-term durability.

2.2.2. Underwater Repair Materials-Performance on Vertical Surfaces

Unlike underwater construction materials, which mainly require high flowability to achieve self-compaction, underwater repair materials are applied not only on horizontal surfaces but also on vertical and overhead defects. When formwork cannot be used for direct placement, the material must exhibit sufficient plasticity and high yield stress to resist sagging, slumping, and collapse under its own weight. Under these conditions, washout resistance, viscosity, and structural build-up are more critical than high flowability.

High-viscosity cellulose ethers and high-molecular-weight polyacrylamide (PAM) can form dense bridging networks or microgel structures in cement pore solutions, thereby significantly increasing yield stress and improving anti-washout performance [48]. High-viscosity hydroxypropyl methylcellulose (HPMC) at dosages up to approximately 0.7% and PAM at dosages up to approximately 0.5% can generate yield stresses of about 300 Pa [49,50], which is beneficial for sag resistance in vertical repair. However, the use of high-viscosity polymers should be carefully controlled, because excessive thickening may reduce substrate wetting, weaken interfacial contact, and limit local penetration into surface defects.

Nano-reinforcement provides an additional strategy for improving the performance of vertical repair materials. Nano-SiO₂, nano-metakaolin, and nano-clay can be incorporated into polymer–cement networks to increase structural build-up, enhance thixotropic recovery, and reduce washout in the fresh state. During hardening, these nanoparticles can refine the pore structure, promote the precipitation of hydration products, densify the interfacial transition zone, and improve mechanical strength and durability [51,52,53]. The practical dosage range of nano-additives is commonly around 1–3%, although the optimal dosage strongly depends on dispersion quality and compatibility with the polymer system (Figure 6 and Figure 7).

Therefore, for vertical and overhead underwater repair, a promising design strategy is to combine high-viscosity water-soluble polymers with well-dispersed nano-reinforcing phases. Such systems can provide sag resistance during placement and improve strength and durability after hardening.

2.2.3. Applicability and Design Indicators

As shown in Table 1, the materials suitable for different underwater repair scenarios are classified, and the corresponding dosage ranges and performance trade-offs reported in existing studies are summarized. It must be noted, however, that key parameters—including flowability, yield stress, washout loss, and turbidity—are highly sensitive to test conditions (e.g., water-to-binder ratio, aggregate content, polymer type and dosage, mixing method, water flow conditions, and testing standards). Therefore, direct numerical comparisons across different studies are not advisable unless the experimental conditions are strictly identical. At present, the open literature does not provide a universally accepted single flow-spread threshold for defining the self-leveling condition required for underwater horizontal repair. As a practical reference, the Chinese industry standard for cementitious self-leveling floor mortars (JC/T 985-2005 [54]) specifies an initial flow spread of not less than 130 mm as the criterion for self-leveling performance.

For vertical repair, yield stress is a key parameter. Theoretically, the yield stress of the repair material should exceed the shear stress induced by gravity. For example, a repair layer with a thickness of 20 mm requires a yield stress of approximately 360 Pa to achieve excellent anti-sagging performance [55]. This provides an important target for practical mixture design. Underwater repair also requires sufficient interfacial bond strength. Waterborne polymer emulsions and latexes can improve both the bonding performance and durability of cementitious materials. However, because fresh waterborne polymer emulsions and cementitious matrices are prone to dispersion in water, multi-polymer synergistic modification is usually required to meet the comprehensive performance requirements of underwater repair.

Overall, mechanisms such as polymer adsorption and bridging, cement-particle flocculation, polymer film formation, and nanoparticle-induced pore refinement have been confirmed by rheological tests, washout tests, and mechanical measurements. However, most studies have verified these mechanisms only in isolated binary systems, such as water-soluble polymer–cement, waterborne polymer emulsion-cement, or nanomaterial-cement systems. Systematic evidence for multi-component polymer systems remains limited. In addition, most conclusions are still derived from laboratory-scale tests under simplified underwater conditions. Field validation under realistic hydrostatic pressure, water flow, substrate roughness, chloride exposure, temperature fluctuation, and construction disturbance remains insufficient and should be prioritized in future research.

Table 1. Classification and design considerations of polymer-modified cementitious materials for different underwater repair scenarios.

Repair Scenario	Major Polymer Systems	Indicative Dosage Window	Key Performance Indicators	Key Considerations
Horizontal patching and self-leveling repair	Low-viscosity water-soluble polymers (e.g., PAM, cellulose ethers, sodium alginate, etc.)	HPMC/CE about 0.1–0.5% binder; PAM about 0.1–0.6%; biopolymers about 0.3–0.5%.	Flowability, self-leveling ability, resistance to dispersion under water, resistance to segregation, turbidity.	HPMC (0.45–0.5%) can balance high flowability (>145 mm) with low turbidity (<100 NTU) and offers excellent resistance to dispersion; the optimal PAM dosage of 0.3–0.6% can address the effects on flowability and viscosity [38,39].
Vertical surface repair	High-viscosity HPMC/CE, high-molecular-weight PAM, and nano-reinforced polymer–cement systems.	HPMC up to about 0.7%; PAM up to about 0.5%; nano-SiO2 or nano-clay commonly about 1–3%.	Yield stress, thixotropy, workability, adhesion to damp surfaces, and resistance to dispersion during placement.	The material’s yield stress must exceed the shear stress caused by gravity (estimated at approximately 360 Pa) to prevent sagging on the facade [55]. While 3% nano-SiO2 can increase thixotropy by approximately 80%.
Scenarios requiring underwater adhesion	Solvent-based epoxies, polyurethanes, etc.; water-based polymers and cement-based composite materials (water-based epoxies, SBR emulsions, water-based polyurethanes, etc.)	WEP/EP about 0.10–0.30 in reported systems; SBR emulsion: Addition level of 5–15%.	Underwater bond strength, early strength, durability, and underwater resistance to dispersion.	Water-based epoxy has virtually no effect on the underwater resistance to dispersion of cement-based materials, whereas WEP/EP (water-based epoxy/epoxy resin) can significantly enhance the cohesion of mortar, thereby improving its underwater resistance to dispersion. In comparison, SBR emulsion yields even better results: at a 15% blend ratio, underwater leaching loss decreased from 6.6% to 3.2% (a 52% reduction), and underwater bond strength increased from 2.57 MPa to 4.22 MPa (a 64% increase) [56].
Long-term marine exposure	WEP/EP, SBR/acrylate, WPU, polymer-nano systems and dense hybrid mineral–polymer binders.	Often P/C about 0.10–0.30 for emulsions/epoxy systems; nano-additions about 1–3%, depending on dispersion.	Chloride diffusion/migration, freeze–thaw, wet–dry cycling, polymer aging and interface durability.	Lower total porosity alone is insufficient; connectivity, tortuosity, ITZ continuity, polymer-film stability and shrinkage/thermal compatibility control service durability. Most long-term conclusions remain based on accelerated tests.

Table 1. Classification and design considerations of polymer-modified cementitious materials for different underwater repair scenarios.

Repair Scenario	Major Polymer Systems	Indicative Dosage Window	Key Performance Indicators	Key Considerations
Horizontal patching and self-leveling repair	Low-viscosity water-soluble polymers (e.g., PAM, cellulose ethers, sodium alginate, etc.)	HPMC/CE about 0.1–0.5% binder; PAM about 0.1–0.6%; biopolymers about 0.3–0.5%.	Flowability, self-leveling ability, resistance to dispersion under water, resistance to segregation, turbidity.	HPMC (0.45–0.5%) can balance high flowability (>145 mm) with low turbidity (<100 NTU) and offers excellent resistance to dispersion; the optimal PAM dosage of 0.3–0.6% can address the effects on flowability and viscosity [38,39].
Vertical surface repair	High-viscosity HPMC/CE, high-molecular-weight PAM, and nano-reinforced polymer–cement systems.	HPMC up to about 0.7%; PAM up to about 0.5%; nano-SiO2 or nano-clay commonly about 1–3%.	Yield stress, thixotropy, workability, adhesion to damp surfaces, and resistance to dispersion during placement.	The material’s yield stress must exceed the shear stress caused by gravity (estimated at approximately 360 Pa) to prevent sagging on the facade [55]. While 3% nano-SiO2 can increase thixotropy by approximately 80%.
Scenarios requiring underwater adhesion	Solvent-based epoxies, polyurethanes, etc.; water-based polymers and cement-based composite materials (water-based epoxies, SBR emulsions, water-based polyurethanes, etc.)	WEP/EP about 0.10–0.30 in reported systems; SBR emulsion: Addition level of 5–15%.	Underwater bond strength, early strength, durability, and underwater resistance to dispersion.	Water-based epoxy has virtually no effect on the underwater resistance to dispersion of cement-based materials, whereas WEP/EP (water-based epoxy/epoxy resin) can significantly enhance the cohesion of mortar, thereby improving its underwater resistance to dispersion. In comparison, SBR emulsion yields even better results: at a 15% blend ratio, underwater leaching loss decreased from 6.6% to 3.2% (a 52% reduction), and underwater bond strength increased from 2.57 MPa to 4.22 MPa (a 64% increase) [56].
Long-term marine exposure	WEP/EP, SBR/acrylate, WPU, polymer-nano systems and dense hybrid mineral–polymer binders.	Often P/C about 0.10–0.30 for emulsions/epoxy systems; nano-additions about 1–3%, depending on dispersion.	Chloride diffusion/migration, freeze–thaw, wet–dry cycling, polymer aging and interface durability.	Lower total porosity alone is insufficient; connectivity, tortuosity, ITZ continuity, polymer-film stability and shrinkage/thermal compatibility control service durability. Most long-term conclusions remain based on accelerated tests.

2.3. Interface Bonding Mechanisms of Polymer-Modified Cement-Based Underwater Repair Materials

2.3.1. Challenges and Failure Mechanisms in Underwater Interface Bonding

In underwater repair scenarios, such as hydraulic, port, and underground engineering, the existing concrete substrate is typically water-saturated. Free water, capillary water, and strongly adsorbed water films on its surface [57,58] hinder effective wetting and physical contact between the repair material and the substrate, thereby increasing interfacial defects and pore connectivity. Meanwhile, flowing water can wash out ordinary cementitious repair materials, leading to the loss of cement paste at the new-to-old concrete interface, exposure of aggregates, and the formation of a weak interfacial transition zone (ITZ), which severely compromises early-age bond strength and long-term service performance [59] (Figure 8). In addition to the water film, which represents the primary barrier, the actual bonding performance is also affected by the substrate surface condition and contaminants such as oil stains, laitance, and biofilms, as summarized in Table 2. Therefore, the key to underwater in situ repair lies in how to penetrate, displace, or exploit the water film [60,61,62,63,64]. A comprehensive consideration of substrate conditions and environmental factors, together with an in-depth understanding of the bonding behavior of polymer-modified materials under different service conditions, is essential for providing a theoretical basis for reliable repair.

2.3.2. Bonding Mechanisms of Underwater Repair Materials at the Water Interface

To overcome the barrier posed by water films and re-establish high-strength underwater interfacial bonding, researchers primarily employ polymer physics and chemistry to actively displace, penetrate, or utilize interfacial water. Based on the core physicochemical mechanisms by which polymers act at saturated interfaces, the bonding mechanisms relevant to cement-based underwater repair can be broadly classified into the following three categories:

The first mechanism departs from the conventional drainage-based approach and directly employs an organic binder phase with high polarity and strong hydrophilicity. This mechanism relies on the exceptionally high thermodynamic affinity between polar functional groups, such as carboxyl, amino, and epoxy groups, and cations on the concrete surface. Through competitive adsorption, these groups can directly penetrate and replace the layer of bound water molecules on the substrate surface, thereby enabling in situ electrostatic adsorption or covalent bonding [65,66]. When the bonding force exceeds the intrinsic strength of the material, cohesive failure occurs. Du et al. [67] conducted molecular dynamics simulations and observed that Ca²⁺ in C-S-H gels migrates to the interface and forms strong electrostatic bonds with hydroxyl groups (-OH) in epoxy resin. This electrostatic interaction contributes up to 126.5 kJ/m² of adhesion energy to the system and serves as the fundamental origin of high-strength bonding at the macroscopic interface. In addition, Kim et al. [68] incorporated cresyl glycidyl ether into pure bisphenol A as a reactive diluent to improve wettability and promote long-term curing (Figure 9). In a real marine environment, the bond strength showed a continuous increase after 91 days of exposure, rising from approximately 2.0 MPa to 2.71 MPa, indicating excellent long-term interfacial adaptability. Xie et al. [69] found that low viscosity enhanced interfacial penetration, whereas high polarity promoted hydrogen bonding with cement hydrates. The negative Gibbs free energy confirmed the spontaneous nature of adhesion, which was mainly driven by entropy gains from OH-π interactions and further stabilized by hydrogen bonding, van der Waals forces, and electrostatic interactions. The contribution of hydrates followed the order C-S-H > AFt > Ca(OH)₂ > CaCO₃, indicating that polymer bonding is governed by interfacial free-energy regulation and specific molecular interactions rather than simple wetting or mechanical anchoring.

Figure 8. Relationship between the relative bond strength of UWC and the underwater mass loss ratio. Reproduced with permission from [70], Elsevier, 2026.

Figure 8. Relationship between the relative bond strength of UWC and the underwater mass loss ratio. Reproduced with permission from [70], Elsevier, 2026.

Figure 9. Effect of exposure time on (a) coating thickness under TW conditions; (b) coating thickness under RS conditions; (c) bond strength under TW conditions; and (d) bond strength under RS conditions [68].

Figure 9. Effect of exposure time on (a) coating thickness under TW conditions; (b) coating thickness under RS conditions; (c) bond strength under TW conditions; and (d) bond strength under RS conditions [68].

The second mechanism employs an amphiphilic composite system containing both nonpolar hydrophobic segments and polar reactive groups [71,72,73]. The hydrophobic phase displaces free interfacial water and creates a locally dry microenvironment, while the polar phase bonds with the substrate. In this way, the interfacial contact quality is improved, water-film-induced interfacial defects are eliminated, and loads can be effectively transferred throughout the composite system, thereby inducing cohesive failure. Zheng et al. [46] designed a gradient network in which WEP served as a hydrophilic transition phase and cross-linked with hydrophobic EP. At P/C = 0.3, the 28-day underwater wet bond strength reached 2.75 MPa, representing a 295% increase compared with the control, and the failure mode shifted from interfacial delamination to cohesive failure within the old concrete (Figure 10 and Figure 11). Zhang et al. [74] developed a barnacle-inspired amphiphilic high-strength adhesive, PUU-FPD. This material repels interfacial water molecules through the self-assembly of bulky hydrophobic fluorinated side chains (FPD), while exposing free urea/carbamate groups to form high-density hydrogen-bond bridges with the substrate. As a result, its adhesive strength increased from an initial value of 308 kPa to approximately 1.62 MPa after 24 h of immersion in water, and further exhibited an anomalous increase to 3.72 MPa at a low temperature of −50 °C. Pang et al. [24] further confirmed, using Flory-Huggins-theory-based simulations, that low-density C-S-H can act as a hydrophilic–hydrophobic bridge, promoting uniform polymer dispersion and the formation of an interpenetrating polymer network (IPN).

The third mechanism relies on polar polymers to provide initial interfacial adhesion and tack, while inorganic mineral phases, such as cement clinker and sulfoaluminates, absorb residual free water at the interface. As the mineral phases hydrate, calcium silicate hydrate (C-S-H) gel or ettringite crystals penetrate the pores and microcracks of the old substrate and form deep physical and mechanical interlocking with the polymer film [75]. This markedly increases the crack propagation path and energy dissipation, forcing fractures to occur within the substrate. Ma et al. [76] found that hydroxyl groups in EVA chains can form hydrogen bonds with interfacial water and generate a dense polymer film that fills pores during cement hydration. The incorporation of 3% EVA increased the 28-day underwater bond strength from approximately 1.5 MPa to 2.8 MPa, corresponding to an 87% increase, while 6% EVA reduced the thickness of the interface zone (IZ) from 90 μm to 30 μm. Pan et al. [77] developed a biomimetic underwater adhesive using a PAM hydrogel and nanocrystals. In this system, the PAM precursor absorbs surface water, whereas anhydrous calcium sulfoaluminate rapidly hydrates to form nanocrystals that precipitate within micropores, thereby providing physical anchorage. The adhesive achieved a bond strength of approximately 3.3 MPa on aluminum substrates after 3 days, with continued strength development (Figure 12). Assaad et al. [78] reported that SBR-modified cementitious grout performs well under humid conditions. SBR particles swell and entangle in the alkaline pore solution and adsorb cement particles to form a viscous gel, thereby blocking interfacial water and reducing grout loss. At an SBR content of 15%, the underwater pull-out bond strength of anchor rods increased from 2.32 MPa to 5.26 MPa.

In recent years, numerous advanced and biomimetic studies on underwater adhesion have emerged. Strong underwater adhesion has been achieved through biomimetic coordination bonding [79,80,81], water-triggered/water-consuming mechanisms [82,83,84], and physical adsorption based on topological microstructures [85], providing important insights for the design and practical application of underwater repair materials. Overall, interfacial bonding in underwater repair, particularly on water-saturated concrete substrates, is highly complex and strongly dependent on material composition, interfacial regulation mechanisms, and service conditions. Reliable underwater adhesion is generally not achieved by a single component, but rather through the synergistic effects of multiple components in water displacement, wetting, bonding, hydration, and interfacial filling. Among these strategies, organic–inorganic hybrid systems exhibit significant advantages in enhancing the bond strength and durability of water-saturated concrete surfaces. However, current methods for evaluating underwater bonding performance remain insufficient. Existing tests mainly include direct tensile bond tests, slant shear tests, and flexural bond tests, which are typically conducted by underwater casting or molding followed by conventional bond strength testing procedures. Standardized methods for evaluating interfacial bond strength under complex service conditions, such as flowing-water scouring, marine erosion, and long-term immersion, are still lacking. This limitation hinders reliable performance comparison among different material systems and the assessment of their engineering applicability.

Table 2. Practical factors affecting underwater interfacial bond strength and corresponding treatment or material-design strategies.

Practical Factor	Effect on Bond Strength	Recommended Treatment or Material-Design Strategy
Surface roughness [86]	Moderate roughness increases contact area, enhances mechanical interlocking, and promotes the penetration of hydration products into pores and microcracks. Excessive roughness may trap water, sediment, or air bubbles, causing local interfacial defects.	Use controlled roughening, such as sandblasting, high-pressure water jetting, scabbling, or grinding, and remove loose particles before repair.
Laitance and weak surface layer [87]	Long-term service or water scouring may leave laitance, loose hydration products, powdery deposits, or deteriorated mortar layers on the old concrete surface, weakening interfacial bonding.	Remove weak layers by grinding, high-pressure water jetting, sandblasting, or wire brushing to expose a sound and stable substrate.
Biofouling [88]	Algae, microbial biofilms, shellfish, soft organic deposits, or other biological fouling may hinder wetting and direct contact between the repair material and substrate.	Remove biofouling by scraping, high-pressure water jetting, brushing, or eco-friendly biological treatment, and repair promptly after cleaning.
Flowing water/hydraulic pressure	Static water, slow flow, tidal fluctuation, or flowing-water scouring may disturb placement, increase washout, and weaken early interfacial contact.	Use anti-washout admixtures, thixotropic agents, or multi-polymer systems to improve cohesion, build-up, and washout resistance.

Table 2. Practical factors affecting underwater interfacial bond strength and corresponding treatment or material-design strategies.

Practical Factor	Effect on Bond Strength	Recommended Treatment or Material-Design Strategy
Surface roughness [86]	Moderate roughness increases contact area, enhances mechanical interlocking, and promotes the penetration of hydration products into pores and microcracks. Excessive roughness may trap water, sediment, or air bubbles, causing local interfacial defects.	Use controlled roughening, such as sandblasting, high-pressure water jetting, scabbling, or grinding, and remove loose particles before repair.
Laitance and weak surface layer [87]	Long-term service or water scouring may leave laitance, loose hydration products, powdery deposits, or deteriorated mortar layers on the old concrete surface, weakening interfacial bonding.	Remove weak layers by grinding, high-pressure water jetting, sandblasting, or wire brushing to expose a sound and stable substrate.
Biofouling [88]	Algae, microbial biofilms, shellfish, soft organic deposits, or other biological fouling may hinder wetting and direct contact between the repair material and substrate.	Remove biofouling by scraping, high-pressure water jetting, brushing, or eco-friendly biological treatment, and repair promptly after cleaning.
Flowing water/hydraulic pressure	Static water, slow flow, tidal fluctuation, or flowing-water scouring may disturb placement, increase washout, and weaken early interfacial contact.	Use anti-washout admixtures, thixotropic agents, or multi-polymer systems to improve cohesion, build-up, and washout resistance.

2.4. Mechanical Properties and Microstructural Mechanisms of Polymer-Modified Underwater Repair Materials

2.4.1. Mechanical Properties

The effect of polymers on the mechanical properties of cement-based materials varies depending on environmental conditions, polymer type, and polymer dosage [89,90,91,92]. In terms of compressive strength, polymer modification does not always produce a beneficial effect. In some cases, polymers can enhance compressive strength by reducing underwater washout, retaining cementitious particles, improving matrix cohesion, and refining the pore structure. Song et al. [93] applied anionic anti-dispersants, including CMS and PAC, to underwater repair materials. At a dosage of 0.6%, the 28-day compressive strength of underwater concrete reached 37 MPa, representing a 151% increase compared with the control group without anti-dispersants. This improvement was mainly attributed to the “point-adsorption” mechanism of anionic anti-dispersants, which preserves the hydration activity of cement particles. By contrast, non-ionic anti-dispersants tend to form an “enveloping adsorption” layer around cement particles, resulting in much lower compressive strengths of only 16–18 MPa. Wang et al. [94] developed a magnetically driven cementitious grouting material incorporating Fe₃O₄ magnetic powder. When the Fe₃O₄ content increased from 10% to 20%, the 3-day compressive strength increased by 33.40%, while the 7-day compressive strength increased by 5.67%. This improvement was mainly associated with the filling effect of Fe₃O₄ during the early hydration stage, which reduced voids, increased the solid volume fraction, and promoted early matrix densification. However, these beneficial effects are conditional. Excessive polymer content may reduce compressive strength because of air entrainment [95,96], delayed cement hydration, increased capillary porosity, or the formation of polymer films that partially hinder the continued hydration of cement particles. Therefore, the compressive strength of polymer-modified underwater repair materials should be regarded as a dosage-dependent property rather than a property that is inevitably improved by polymer addition.

Compared with compressive strength, polymer modification usually shows a more direct and stable beneficial effect on flexural strength. This is because flexural failure is closely related to crack initiation, crack propagation, and matrix continuity, all of which can be improved by polymer film formation and the development of organic–inorganic networks. Under conventional curing conditions, Tian et al. [97] systematically investigated the effects of four types of polymer emulsions, namely silicone-acrylic, styrene-acrylic, waterborne epoxy, and acrylic emulsions, on the mechanical properties of cement-based materials. They reported that styrene-acrylic emulsion increased the flexural strength by 9.7% at a polymer-to-cement ratio of 0.15, while waterborne epoxy increased the flexural strength by up to 16.7% at a polymer-to-cement ratio of 0.2. These results indicate that an appropriate polymer dosage can improve the flexural performance of cementitious materials by enhancing matrix continuity and delaying crack propagation. Nevertheless, excessive polymer addition may interfere with cement hydration, form polymer films with non-uniform thickness, weaken the bonding between cement particles, and eventually reduce the overall mechanical strength [98].

With respect to dynamic mechanical performance, polymer modification can improve crack-bridging capacity and energy dissipation under rapid loading. Yu et al. [99] systematically investigated the interfacial mechanical behavior between underwater-cast alkali-activated mortar and ordinary Portland cement substrates. Under dynamic impact loading at high strain rates of 5.9–9.9 s⁻¹, the energy absorption capacity of PAM-modified specimens in dynamic splitting tensile tests increased by approximately 17.4%. This result indicates that the polymer network can delay crack propagation and improve resistance to tensile-type failure under dynamic loading.

These results suggest that the main mechanical benefit of polymer modification is not always the maximization of compressive strength [100], but the improvement of toughness, crack-bridging capacity, and interfacial integrity under underwater curing and loading conditions. Beyond dosage, polymer Tg and surface charge critically govern mechanical performance [101]. Low-Tg polymers form films that enhance flexural/tensile strength via crack-bridging, while high-Tg polymers remain as particles unless post-heated. For underwater repair, low curing temperatures necessitate proper Tg selection to ensure film formation and effective reinforcement (Figure 13, Figure 14 and Figure 15).

2.4.2. Modification Mechanisms of Polymer-Modified Cement-Based Underwater Repair Materials

The evolution of macroscopic mechanical properties resulting from polymer modification stems from fundamental changes in the system’s internal pore structure, hydration kinetics, interface transition zone (ITZ), and molecular-scale interactions.

The evolution of macroscopic performance resulting from polymer modification can be understood through the chain of polymer adsorption and bridging, fresh-state flocculation, hydration kinetics, film formation or IPN development, pore-structure evolution, and ITZ densification.

Pore Structure

Pang et al. [24] identified the threshold conditions for the formation of an interpenetrating polymer network (IPN) between water-based epoxy (WEP) and C-S-H gel: when the degree of polymerization is ≥6 and the polymer-to-ash ratio is >10%, low-density C-S-H acts as a “hydrophilic–hydrophobic bridge,” promoting the formation of a continuous network of WEP that penetrates the matrix. This bridging effect generates a composite coating less than 1 μm thick on the surface of inorganic fibers, significantly enhancing interfacial sliding friction (Figure 16).

Zheng et al. [102] characterized the pore structure and phase distribution of gradient epoxy (CEP)-modified cement paste. The results indicated that the total porosity increased from 1.53% to 4.19% with the increase in CEP/C from 0 to 0.4 (Figure 17), and the proportion of capillaries and macropores larger than 0.1 μm increased significantly, which was attributed to the air-entraining effect of the polymer. The X-CT three-dimensional reconstruction, however, showed (Figure 18) that the CEP phase was uniformly distributed in the cement paste. As the CEP content increased, the volume fraction of the CEP phase increased from 27.57% to 46.31%, forming an interpenetrating continuous network (IPN) with the cement hydration products. Although this IPN structure did not significantly reduce the total porosity, it effectively slowed down the transport of water and corrosive media through encapsulation and bridging of the pores by the polymer membrane, while at the same time increasing crack deflection and energy dissipation pathways and improving the material toughness and durability.

Jeon et al. [103] found through quantitative analysis using the mercury intrusion porosimetry (MIP) method that the addition of nano-SiO₂ reduced the total porosity from 38.1% to 19.5%, with a significant increase in the proportion of gel pores; SEM images revealed that brucite (Mg(OH)₂) and hydrated magnesium carbonate crystals filled the capillary pores in rose-like or needle-like morphologies, resulting in a denser microstructure after carbonization.

Hydration Reactions and Microscopic Mechanisms

The effect of polymers on cement hydration is a key factor in the determination of the microstructure and macroscopic properties of the material. Different polymers have different mechanisms of action, but most commonly include a retardation effect [104,105,106,107,108,109,110]. The extent and stage of action vary according to the type of polymer. Yuan et al. [111] studied the heat of hydration and found that anionic PAM (APAM) greatly delayed the exothermic peak of hydration (from about 12 h to more than 17 h) and reduced the total heat released, while cationic (CPAM) and nonionic (NPAM) PAMs slightly increased the exothermic rate. The fitting of the Krstulovic-Dabic kinetic model revealed that APAM primarily retarded the crystallization nucleation and phase boundary reactions. Thermogravimetric analysis (TGA) showed that APAM significantly inhibited the formation of Ca(OH)₂ in the first 12 h, but the CH content was higher than that of the blank group afterwards. Quantitative X-ray diffraction (XRD) analysis showed that APAM inhibited the dissolution of C₃S in the early stage and promoted the dissolution of C₂S, while CPAM and NPAM only enhanced the early dissolution of C₃S (Figure 19).

Zheng et al. [112] demonstrated that the incorporation of waterborne polyurethane (WPU) delays the hydration process of cement and postpones the peak of hydration heat, primarily because WPU particles encapsulate cement particles, increasing steric hindrance and thereby inhibiting ion dissolution and crystallization. At the same time, microstructural characterization confirmed that the addition of WPU did not generate new crystalline phases, but significantly affected the yield of existing hydration products, forming a three-dimensional polymer network in conjunction with hydration products such as C-S-H gel (Figure 20).

Pang et al. [90] concluded through comparative analysis of FTIR, Raman spectroscopy, and XRD that at low blending ratios (<20%), water-based epoxy resin promotes the early formation of CH and C-S-H, but at high blending ratios, physical encapsulation inhibits later-stage hydration; more importantly, non-emulsion epoxy resin (NEP) promotes the microcrystallization of CH (fine grains), whereas emulsion-type (EEP) primarily promotes the coarsening and growth of CH grains; this change in crystallization behavior directly affects the density and mechanical toughness of the interfacial transition zone.

3. Durability in Complex Hydraulic Environments

Underwater repair structures are continuously exposed to water-saturated environments and face multiple severe challenges, including chloride attack, flowing-water scouring, fatigue loading, and freeze–thaw cycles [113]. Therefore, polymer-modified cementitious materials should not only possess good early-age performance and mechanical properties, but should also maintain long-term durability under complex service conditions, which is critical for determining the service life of repair projects.

Zhi et al. [114] found that cellulose ether (CE) can increase Cl⁻ adsorption sites and improve chloride-binding capacity. However, it also increases the proportion of capillary pores and macropores and reduces pore tortuosity. As a result, higher CE contents lead to lower overall resistance to chloride ingress. In contrast, polyacrylamide (PAM) can enhance the physical chloride-binding capacity of C-S-H gel by reducing the alkalinity of the system. Although PAM increases the total porosity, the increase is mainly associated with gel pores, thereby effectively improving the overall resistance to chloride ion penetration [115].

Fu et al. [116] investigated the effect of waterborne epoxy resin (WEP) on the sulfate resistance of sulfoaluminate cement (SAC) repair mortar under sodium sulfate wet–dry cycles. The results showed that, with changes in the polymer-to-cement ratio (P/C), the corrosion resistance coefficient and relative dynamic elastic modulus of the mortar changed only slightly, indicating that WEP dosage had little influence on sulfate resistance. This was mainly because the SAC-based mortar had intrinsically low porosity and formed a dense IPN structure, which limited the ingress of SO₄²⁻. Even when a small amount of SO₄²⁻ entered the matrix, the AFt and CaSO₄ produced by its reaction with cementitious components preferentially filled the pores and temporarily increased the compactness of the structure. This behavior contrasts with OPC mortar, which has higher porosity and is more sensitive to sulfate attack.

Takahashi et al. [117] investigated the fatigue behavior of bridge bearing mortar submerged in water using compression fatigue tests, in which the maximum stress was 60% of the static compressive strength. Fluorescence tracing showed that water-dispersible polyurethane ether (PUE) effectively suppressed the dynamic ingress and egress of water under fatigue loading by forming hydrophobic or dense structures from the nanoscale to the micrometer scale. Consequently, the fatigue life under saturated conditions increased by 4.7 times. This finding is highly relevant to underwater repair materials subjected to wave action, traffic vibration, and flow-induced cyclic loading.

Yang et al. [118] evaluated the durability of silica nanoparticle (SN)-reinforced underwater repair materials in terms of impermeability, corrosion resistance, and freeze–thaw resistance. The results showed that SNs act as microstructural densifiers through pore filling, pozzolanic reaction, and nucleation effects. Their practical application in dam impermeabilization repair confirmed simultaneous improvements in workability, impermeability grade up to Grade 8, and underwater bond strength. Zheng et al. [119] systematically evaluated anionic waterborne polyurethane (WPU)-modified repair mortar and found that WPU formed an interpenetrating polymer network (IPN) with cement hydration products, reducing the volume of large pores of 1–100 μm by 68%. This significantly reduced drying shrinkage and provided good resistance to acid corrosion and 200 freeze–thaw cycles.

Felekoglu [120] evaluated the deformation compatibility between a conventional repair mortar (RM), a high-strength micro-concrete (MiC), and a substrate mortar (SM). Although MiC had higher strength, it showed much poorer compatibility: its 28-day drying shrinkage reached −1930 × 10⁻⁶ mm/mm, about 6.5 times that of RM, and its coefficient of thermal expansion was 15.5 × 10⁻⁶ °C⁻¹, more than twice that of RM. Accordingly, the calculated thermal stress under a 50 °C temperature differential was 27.0 MPa for MiC, compared with only 7.4 MPa for RM. These results indicate that high-strength repair materials may be unsuitable for moderate-strength substrates when they exhibit high shrinkage and large thermal expansion mismatch.

Overall, the durability of polymer-modified cementitious underwater repair materials should be evaluated from transport-controlled and interface-controlled perspectives. Lower total porosity does not necessarily indicate better durability. Pore connectivity, pore tortuosity, pore-size distribution, interfacial transition zone stability, transport-path blocking, polymer-film integrity, IPN stability, and deformation compatibility with old concrete are more directly related to long-term service performance. Existing studies have shown that polymer modification can improve durability indicators such as chloride resistance, impermeability, and freeze–thaw resistance. However, most results are still based on single-factor or accelerated laboratory tests. Future studies should focus on durability evaluation under coupled actions, including chloride–sulfate attack [121], wet–dry cycling, flowing-water scouring, fatigue loading, and polymer aging.

4. Summary and Prospects

This review summarizes the effects of polymer-modified cementitious materials on workability, interfacial bonding, mechanical properties, and durability in underwater repair applications. Existing studies indicate that polymers do not merely act as thickeners or reinforcing agents. Instead, they improve the construction adaptability and long-term service performance of cementitious materials in underwater environments by regulating the fresh-state slurry structure, interfacial water-film behavior, pore connectivity, and organic–inorganic composite networks. The main mechanisms can be summarized as follows.

(1)

Regulation of underwater workability. Polymers reconstruct the flocculated structure of fresh cementitious materials through adsorption, entanglement, bridging, and steric hindrance, thereby improving slurry cohesion and washout resistance. The key challenge is to balance flowability, anti-washout performance, and shape stability according to different repair scenarios, such as grouting, horizontal repair, and vertical repair.

(2)

Enhancement of underwater interfacial bonding. Polymers can weaken the interfacial water-film barrier through competitive adsorption, hydrogen bonding, electrostatic interactions, or coordination involving polar groups. Hydrophobic segments can also displace free water at the interface. Meanwhile, cement hydration products can penetrate pores and microcracks in the old concrete, forming mechanical interlocking. Therefore, reliable underwater interfacial bonding arises from the synergy among interfacial water-film regulation, chemical interactions, and physical interlocking.

(3)

Improvement of mechanical properties and durability. Polymer film formation or the development of organic–inorganic interpenetrating networks can optimize the pore structure, enhance crack-bridging capacity, and improve interfacial continuity. Long-term durability depends not only on the reduction in total porosity, but also on the blocking of connected pores, the extension of transport pathways for aggressive media, the stability of the polymer phase, and the long-term integrity of the repair–substrate interface.

It should be noted that existing studies still focus mainly on single-polymer or binary systems, whereas practical underwater repair materials are usually multi-component organic–inorganic composites. In addition, performance evaluation methods and standard testing protocols for underwater repair materials remain insufficient, which limits reliable comparisons among different material systems and assessments of their engineering applicability (

Table 3. Test methods for the underwater performance of polymer-modified cementitious materials.

Performance Metrics	Test Methods/Standards	Testing and Adaptation Methods for Underwater Repair Materials
Compressive/Flexural strength	ASTM C109/C109M-21 [122,123]	Test specimens were cast underwater, cured underwater, or formed underwater and then subjected to standard curing.
Bond strength	Direct tensile bond tests, slant shear tests, and flexural bond tests [26]	Test specimens were cast underwater, cured underwater, or formed underwater and then subjected to standard curing.
Underwater Dispersion Resistance	Place the freshly mixed material in a water column to allow it to fall freely, and test for parameters such as dispersion loss, turbidity, and pH in the water.	Mass loss rate [124], turbidity, pH [125], slurry loss
Durability	SL/T 352-2020 [126], DL/T5126-2021 [127], ASTM C 1202 [128], ASTM C 666 [116,129]	Test specimens were cast underwater, cured underwater, or formed underwater and then subjected to standard curing.

).

Overall, future research should focus on smart responsive underwater repair materials, self-healing and self-reinforcing underwater interfaces, multi-component organic–inorganic synergistic design, testing methods closer to real underwater service conditions, coupled models for durability and service-life prediction, and low-carbon environmentally friendly material systems. These directions are expected to provide more durable, adaptive, low-carbon, and field-reliable repair solutions for marine, hydraulic, and offshore concrete infrastructure.