Advances in Nano-Reinforced Polymer-Modified Cement Composites: Synergy, Mechanisms, and Properties

Abstract

Organic polymer introduction effectively enhances the toughness, bond strength, and durability of ordinary cement-based materials, and is often used for concrete repair and reinforcement. However, the entrained air effect simultaneously induced by polymer and the inhibitory action on cement hydration kinetics often lead to degradation in mechanical performances of polymer-modified cement-based composite (PMC). Nanomaterials provide unique advantages in enhancing the properties of PMC due to their characteristic ultrahigh specific surface area, quantum effects, and interface modulation capabilities. This review systematically examines recent advances in nano-reinforced PMC (NPMC), elucidating their synergistic optimization mechanisms. The synergistic effects of nanomaterials—nano-nucleation, pore-filling, and templating mechanisms—refine the microstructure, significantly enhancing the mechanical strength, impermeability, and erosion resistance of NPMC. Furthermore, nanomaterials establish interpenetrating network structures (A composite structure composed of polymer networks and other materials interwoven with each other) with polymer cured film (The film formed after the polymer loses water), enhancing load-transfer efficiency through physical and chemical action while optimizing dispersion and compatibility of nanomaterials and polymers. By systematically analyzing the synergy among nanomaterials, polymer, and cement matrix, this work provides valuable insights for advancing high-performance repair materials.

1. Introduction

High-performance cement-based materials are crucial for ensuring the structural safety and long-term service life of concrete structures, and represent an effective approach to resource conservation and environmental sustainability. In complex environments such as those with high humidity and high salinity, cement-based materials used in service need to possess high toughness, waterproofing and water resistance, and corrosion resistance. However, ordinary cement-based composite (OCC) may struggle to meet these requirements perfectly [1,2].

Polymer-modified cement-based composite (PMC), formed by mixing polymers, water, cement, aggregates, and admixtures in specific proportions, represent a typical class of organic-inorganic hybrid materials. Common polymers such as polyacrylate emulsions (PA), polyacrylamide (PAM), styrene acrylic (SAE), polyvinyl acetate polymers (PVA), styrene-butadiene rubber polymers (SBR), ethylene-vinyl acetate copolymers (EVA), and epoxy resins (EP) can be incorporated into the binder system to enhance the toughness, impermeability, durability, and interfacial bond strength of the cement-based matrix [3,4]. Benefiting from the above advantages of PMC, it is currently widely used in areas such as concrete structure reinforcement, road repair, and waterproof construction. However, it should be noted that inherent characteristics of polymers—including low elastic modulus, high adsorption capacity, and film formation upon water evaporation—can detrimentally affect cement hydration and lead to a substantial reduction in the mechanical performance of PMC [5]. Kong et al. [6,7,8] observed that certain organic functional groups within polymers can complex with metal ions in the cement paste, inhibiting the formation and early growth of hydration products. Furthermore, Tian et al. [9] found that PA significantly compromises the early-age strength of PMC repair mortars; at a dosage of 10 wt.%, a reduction of approximately 12% in the 28-day compressive strength (f_c^28d) was also observed.

The advent of nanomaterials presents valuable opportunities for enhancing the microstructure and improving the properties of OCC at the nanoscale. It has been demonstrated that the incorporation of nanomaterials such as n-SiO₂, n-Al₂O₃, n-CaCO₃, n-TiO₂, n-Fe₂O₃, carbon nanotubes (CNTs), and graphene oxide (GO) can enhance the overall properties of OCC to varying degrees [10,11,12,13]. If the above nanomaterials are introduced into PMC, they can mitigate the adverse effects of polymers on strength of PMC, resulting in nanomaterial-enhanced PMC (NPMC) with improved overall performance.

The modification effects of diverse polymers and nanomaterials on OCC are multifaceted, and the underlying mechanisms are complex. At present, most review results focus on the influence of polymers or nanomaterials on OCC. The research results of NPMC are still scattered and lack systematic integration. The interaction between nanomaterials, polymers, and cement phases in composite systems has not been fully elucidated. This paper begins by briefly reviewing the modification effects of polymers or nanomaterials on OCC and identifies the performance drawbacks arising from polymer incorporation and the improvement effect and mechanism of nanomaterials on inorganic cement-based matrix. Based upon this, particular emphasis is put on investigating the enhancement effects of nanomaterials on the mechanical performance, bond performance, impermeability/durability, and hydration characteristics of PMC. Compared with previous studies, this review also explores in detail the interactions of various nanomaterials in NPMC and the synergistic effects of composite applications. We further summarize the underlying interaction mechanisms among the nanomaterials, polymers, and the cement-based matrix system.

2. Effect of Polymer on the Properties of PMC

As a result of the significant differences in the properties and interaction of polymers and cement-based materials, the effects of polymers on cement-based materials are also multifaceted.

2.1. Effect of Polymer on the Workability of Cement-Based Materials

The workability of fresh paste is a critical factor influencing the performance and application scope of cement-based materials, with polymers exerting a impact on this property [14]. Ou et al. [15] identified cellulose ethers (CEs) as effective thickening and water-retention agents for cement-based systems; however, their molecular structure, rich in -OH groups, was found to impede cement hydration. Lu et al. [16] demonstrated that the surface characteristics of polymer particles are key determinants of their effect on the rheology of fresh mortar. Polymers bearing abundant -COO⁻ and -SO₃⁻ functional groups readily adsorb onto cement particle surfaces, promoting agglomeration and consequently deteriorating rheological performance. Conversely, polymers containing poly(ethylene oxide) (PEO) long chains exhibit reduced adsorption due to electrostatic repulsion and steric hindrance effects, thereby enhancing paste fluidity (Figure 1). Bessaies-Bey [17] investigated the impact of polyacrylamide (PAM) on the rheological behavior of fresh mortar. The formation of PAM microgels, induced by Ca²⁺ ions, were observed. Most of these microgels adsorb onto cement particles, acting as bridges that increase the macroscopic yield stress of PMC suspensions. As evidenced by the aforementioned studies, an appropriate polymer dosage can improve key workability aspects of OCC, including fluidity, air retention, and water retention. Nevertheless, it is crucial to recognize that polymer incorporation simultaneously affects other OCC properties too.

2.2. Effect of Polymer on the Mechanical Strength of Cement-Based Materials

Mechanical performance, being the fundamental characteristic of cement-based materials, constitute a primary focus in PMC research. Generally, polymers enhance the toughness, bond strength (f_b) and fatigue resistance of cement mortars [18,19,20]. Kim [21] investigated the influence of four polymers—acrylic polymers (AC), PVA, SBR, and EVA—on the mechanical performance of PMC. With the polymer dosage fixed at 3 wt.%, the PMC was compared against a control mortar without polymer. The study revealed that polymers increase the flexural strength (f_f) and f_b of the mortar (Figure 2), with a more pronounced effect on f_f than on f_b. Furthermore, the EVA-modified PMC exhibited the superior performance, achieving a 63% increase in the 28-day flexural strength (f_f^28d). In contrast, the f_f^28d improvements for AC-, PVA-, and SBR-modified mortars were comparatively lower, ranging from 16% to 46% relative to the control. Thus, the EVA-modified repair mortar demonstrated the optimal mechanical performance. Bureau et al. [22] examined the effect of polymers on the elastic modulus of mortar, observing that the modulus decreases progressively with increasing polymer content. This reduction occurs because the inherent elastic modulus of the polymer is lower than that of the cement-based matrix. Consequently, the extent of this reduction is proportional to the polymer dosage.

Guo et al. [23] employed EP to modify OCC for repair purposes. Results indicate that the f_b between EP-modified repair mortar and substrate strengthened progressively with increasing EP content. Compared to OCC, mortar with 5 wt.% EP demonstrated approximately 16.7%, 29.8%, and 6.9% improvements in bond flexural strength, bond shear strength, and bond tensile strength, respectively. Shi et al. [24] found both SBR and EVA emulsions enhanced the f_b of ternary repair mortar containing OPC, calcium aluminate cement, and gypsum. Comparatively, SBR proved more effective for direct mortar modification with an optimal dosage of 20 wt.%, while EVA performed better as an interface agent on damaged concrete substrates at an optimal concentration of 40%. Zheng et al. [25] developed WPU-modified cement-based composites exhibiting significantly higher bond strength than OCC. At a polymer-cement ratio of 5%, the mortar achieved a bond tensile strength of 2.33 MPa, representing an increase exceeding 100%. This enhancement is primarily attributed to the formation of a denser interpenetrating network between WPU and hydration products, which reduces interfacial stress concentration while enhancing overall toughness and adhesion.

While polymers enhance mechanical properties of cementitious materials, such as toughness and bond strength, they concurrently introduce new challenges [26]. Assaad et al. [4] found that both SBR and PVA latex powders can increase the tensile strength (f_t) and f_f of cement-based materials. However, polymers also entrain a volume of air voids within the mortar, leading to a noticeable decrease in the compressive strength (f_c) of PMC. Shi et al. [27] investigated curing regime effects on mechanical properties of PMC. Regardless of curing conditions, both SBR and SAE reduced f_c and elastic modulus. However, curing environments significantly influenced f_f. Specifically, under both water-curing and standard-curing conditions, f_f exhibited an inverse relationship with polymer dosage, analogous to f_c. In contrast, specimens cured in ambient conditions demonstrated increased f_f^28d with higher polymer dosage.

2.3. Effect of Polymer on the Durability of Cement-Based Materials

Incorporation of appropriate organic polymers enhances the durability of cement-based materials, extending their service life in complex environments and broadening application scope. Moodi et al. [28] observed significantly improved impermeability in PMC containing 20 wt.% polyester and with 25 wt.% SBR + 25 wt.% EP, whereas PMC modified with 10 wt.% polyester resin or 15 wt.% SBR exhibited increased permeability. Additionally, certain polymers improved frost resistance, with PMC containing 20 wt.% polyester showing no f_f reduction after 50 freeze-thaw cycles. Yang et al. [29] demonstrated SBR enhances chloride resistance and ion transport impedance in OPC repair mortars. Microstructural analysis revealed improved pore structure and partial pore sealing with increasing SBR content, effectively enhancing impermeability. Łukowski et al. [30] found PA incorporation increases sulfate erosion resistance. Within polymer-cement ratios of 0–0.2, sulfate resistance strengthened progressively with polymer content (Figure 3). For PMC with 0.20 ratio, mass gain after 42-month sulfate immersion remained comparable to water immersion (<5%).

2.4. Effect of Polymer on Hydration Behavior of Silicate-Based Cement

According to Taylor [31], the typical hydration calorimetric curve of OPC hydration comprises five stages: initial, induction, acceleration, deceleration, and prolonged slow reaction. Polymer emulsion influences cement hydration, typically manifesting as retardation. Tripathi et al. [32] contend that polymers, when used as admixtures to enhance workability or durability, generally impair early-age hydration and reduce overall strength. They posit that the retarding effect depends on polymer charge, molecular structure, and dosage. Wang et al. [33] proposed three reaction stages in PA-modified systems: (1) Cement hydration generates substantial Ca(OH)₂, creating an alkaline environment; (2) Strong alkalinity hydrolyzes ester groups in PA chains, forming carboxyl groups; (3) Carboxyl groups react with Ca(OH)₂ to form an organic-inorganic crosslinked network. This third-stage reaction alters ionic concentrations, affecting hydration product precipitation rates (Figure 4). The complexation between polymers and ions essentially affects the dissolution-precipitation process involving coupled chemical reactions in the cement system [34,35,36]. During ionic dissolution and dynamic equilibrium phases, polymer particles with active groups (e.g., hydroxyl, carboxylate) and negative charge encapsulate cement grains and adsorb ions (Ca²⁺, Mg²⁺) [37]. Adsorption prolongs the time required to reach supersaturation around cement particles, reducing pH value of pore solution and extending the induction period [33]. Encapsulation creates steric hindrance between particles, impeding hydration product formation/crosslinking while simultaneously restricting contact of water-cement particle, thereby prolonging hydration.

3. Nano-Modified Cement-Based Composite (NMC)

Organic-inorganic hybridization has proven effective for properties enhancement. Nanomaterials, with their small size, high specific surface area, high chemical activity, and unique quantum properties, have become an important component of cement-based composite systems [38,39].

3.1. Dispersion of Nanomaterials in Aqueous System

Due to the strong van der Waals forces between nanomaterials, they tend to agglomerate easily, hindering their application in aqueous cementitious system. Therefore, proper dispersion is a prerequisite for utilizing the properties of nanomaterials. Gao et al. [40] categorized the current common dispersion processes for nanomaterials into four types: high-shear emulsification, ultrasonic treatment, covalent modification, and non-covalent modification. Their analysis revealed that each dispersion process has distinct principles, advantages, and drawbacks. High-shear emulsification struggles to disperse the core of nanomaterial agglomerates and can cause mechanical damage to the nanomaterials themselves. In contrast, ultrasonic treatment, another physical process, provides more kinetic energy for dispersion while causing less mechanical damage to the nanomaterials. Covalent modification processes can introduce active functional groups onto certain nanomaterials to enhance their dispersibility and hydrophilicity. This method is relatively complex, requiring the use of strong acids or other oxidizing agents and lengthy processing times, and may potentially damage the structure of the nanomaterials. In contrast, non-covalent modification processes are more conducive to the dispersion of nanomaterials. The primary form of non-covalent modification involves modifying the surface of the nanomaterials using surfactants to increase the repulsive forces between them, thereby achieving dispersion. Korayem et al. [41] analyzed the dispersibility of nanomaterials from a geometric perspective. Due to the much lower geometric complexity of 0D nanoparticles compared to 1D and 2D nanomaterials, they are easier to disperse in cement matrices. The geometric complexity of 1D nanomaterials leads to their tendency to entangle. 2D nanoplates have higher surface energy, making their dispersion more complex and exhibiting stronger aggregation tendencies. Additionally, 1D and 2D nanoparticles may also slide against each other within fibrous structures. Therefore, as the geometric structure of the incorporated nanoparticles becomes increasingly complex, the steps required to achieve an ideal dispersion state also become more cumbersome.

In addition to the dispersion processes above, that have already been widely adopted, some scholars have proposed more innovative methods. Cui et al. [42] proposed a dispersion method for CNTs based on arc-induced thermal excitation, enabling the dispersion of CNTs in a gas-phase environment. Subsequently, CNTs could be collected in a solvent through appropriate technical methods. The essence of this method is that the high-density energy flow instantaneously input by the electric arc causes the jumping medium to boil instantly, and the high-pressure gradient formed by the instantaneous boiling causes the agglomerates to “explode” and disperse, thereby forming a flocculent CNT dispersion. The jumping medium is a liquid that can rapidly vaporize and expand when subjected to high thermal excitation in an instant, serving as a vaporization and expansion medium throughout the dispersion process without affecting the structure and composition of the final CNT dispersion. He et al. [43] developed a universal and simple ultrasonication treatment assisted by graphene quantum dots (GQDs), which proved effective for exfoliating and dispersing two-dimensional nanomaterials, including GO, calcined layered double hydroxide (CLDH), and g-C₃N₄. Ghavidel et al. [44] proposed a novel dispersion strategy leveraging hydrodynamic stresses exerted on multi-dimensional nanomaterials under an applied electric field. In this process, the nanomaterials were frequently moved by changing the direction of the magnetic field, thereby separating them from agglomerates. The main advantage of this method is that it maintains the structural integrity of the nanomaterials and enables real-time control of dispersion.

3.2. Effect of Nanomaterials on the Properties of NMC

Nanomaterials exhibit particle dimensions at the nanoscale, representing a transitional state between atomic clusters and macroscopic matter. As quintessential mesoscopic systems, their minute scale confers unique properties: Surface atom ratio, specific surface area, and surface energy increase substantially at nanoscale; Energy bands split into discrete levels with spacing widening as particle size decreases; Nanoparticles also have the ability to traverse macroscopic system barriers, a phenomenon known as the macroscopic quantum tunneling effect [45,46]. Leveraging these characteristics, nanomaterials are employed to enhance mechanical performance, interfacial bonding, durability, and hydration processes in cementitious systems.

Beyond the common nanomaterials for enhancing the properties of cementitious composites, recent researches have begun exploring the application of emerging nanomaterials within cement-based systems. Metal-organic frameworks (MOFs) are porous organic-inorganic hybrid materials formed by the interaction of metal ions or ion clusters with organic ligands [47]. Due to their high specific surface area, diversity, and controllability, these porous materials offer numerous possibilities for application in cement-based systems. Zhang et al. [48] found that adding ZIF-8 to Portland cement could increase the f_c^7d and f_c^28d of hardened cement matrix by 31% and 16.2%, respectively. ZIF-8 exhibited high stability in alkaline solutions and acted as a non-reactive filler, accelerating cement hydration and filling pores, thereby enhancing the f_c of cement matrix. Yuan et al. [49] first utilized unreported MOFs to modify cement-based materials, including MOF-5, HKUST-1, ZIF-8, UiO-66, and MIL-68(Al). ZIF-8, UiO-66, and MIL-68(Al) maintained structural integrity in alkaline cement paste, while those of MOF-5 and HKUST-1 were degraded. At a loading of 0.1 wt%, the f_c^28d of MOF-5 and ZIF-8 modified cement-based materials increased by 28.75% and 24.65%, respectively. MXenes are a class of two-dimensional transition metal carbides, nitrides, and carbonitrides with layered structures, which confer self-sensing and self-healing capabilities to NMC and enhance the durability of NMC through their barrier properties. Yin et al. [50] investigated the hydration kinetics and early strength of NMCs doped with MXene (n-Ti₃C₂). Their results showed that MXene did not react chemically with cement particles; however, it helped to promote the formation of crystals in the cement microstructure. The early hydration process was inhibited, while the later hydration process was accelerated by the layered structure of MXene. This was because MXene absorbs water at an early stage and releases it later. Not only that, the incorporation of a small amount of MXene resulted in the formation of a dense network of ettringite (a water-containing calcium-aluminum sulfate mineral) in the cement matrix, which led to a significant increase in early strength. Zhu et al. [51] found that at low MXene dosage, the f_c of NMC got improved, at 0.01 wt.% MXene dosage, the f_c^1d, f^c3d, f_c^7d and f_c^28d of NMC were increased by 20.2%, 45.2%, 32.6% and 27.8%, respectively; while at higher dosage, the effect of MXene was not significant. The dispersed MXene also inhibited the initial hydration process, but the hydration reaction was rapidly enhanced after 10 h. Cao et al. [52] added CNC at a volume fraction of 0.2 to cement paste, resulting in an approximately 30% increase in f_f of NMC. They also found through isothermal calorimetry and thermogravimetric analysis that CNC enhanced the hydration degree of the cement paste. Zheng et al. [53] suggested that the reason CNC improved strength and durability of NMC was that it promoted the growth of the (001) crystal plane of Ca(OH)₂ in cement-based materials and increased the grain size of Ca(OH)₂. Additionally, CNC provided additional nucleation sites, thereby reducing the polymerization degree of C-S-H gel and shortening the C-S-H main chain length. Hydration tests confirmed that CNC could sequentially adsorb Ca²⁺ and SiO₃²⁻ ions, which promoted the formation of a C-S-H gel layer on the CNC surface and enhanced the interfacial forces between CNC and the cement paste.

Above summarizes selected research on nanomaterial-modified cementitious composites. These studies demonstrate multifaceted effects of nanomaterials (n-SiO₂, n-TiO₂, CNTs, GO, MOF, MXene et al.) on cementitious systems. Even minimal additions significantly impact workability, mechanical performance, durability, and hydration characteristics of cementitious system. Regarding workability, most nanomaterials introduction reduces the bulk flowability. This is widely attributed to their high specific surface area and porosity, which adsorb free water from the cement paste. Shang et al. [54] propose an alternative mechanism: certain nanomaterials form aggregates and flocs with cement particles through electrostatic interactions. These flocculated structures immobilize substantial free water, representing another decisive factor affecting rheological performance. Nanomaterials also shorten setting times. This acceleration is primarily attributed to their provision of abundant nucleation sites, which reduce the Gibbs free energy barrier for ion precipitation [55]. Consequently, the hydration and setting reactions of NMC are accelerated.

It’s noting that, minimal nanomaterial incorporation also significantly improves the ITZ and pore structure of cementitious composites (Figure 5), consequently enhancing mechanical performances (modulus, flexural/compressive strength) and durabilities (reduced drying shrinkage, water absorption, corrosion susceptibility). Table 1 reveals multifunctional improvement mechanisms, (1) Nano-filling effect: Pore filling increases matrix compactness; (2) Nucleation promotion: Well-dispersed nanomaterials provide stable nuclei accelerating hydration; (3) Pozzolanic reaction: Reactive nanoparticles (e.g., n-SiO₂, n-Al₂O₃) undergo alkali-activation in cement environments, generating additional hydration products; (4) Templating function: Structurally ordered nanomaterials (CNTs, GO) guide preferential alignment of hydration products; (5) Interfacial bonding: Polar functional groups (e.g., carboxyls on CNTs/GO) react with hydration products (C-S-H, Ca(OH)₂) forming covalent bonds, improving load transfer efficiency and enabling stress-bearing bridging networks.

Table 1. Summary of varied nanomaterials on the properties of common NMC.

Refs.	Nano Material	Body Material	Particle Size and Dosage	Consequences	Mechanisms
Senff [56]	n-SiO2	P.O 52.5R	9 nm, 2.5 wt%	The flow expansion diameter of mortar decreased by 19.6%, the yield stress increased by 157%.	High specific surface area, adsorbs large amounts of free water.
Beigi [57]	n-SiO2	P.O II	15 nm, 6 wt.%	The slump decreased by 10%; fc28d, 28-day split tensile strength, and ff28d increased by 17.9%, 35%, and 39.5%, respectively.	1. Nano-filling effect; 2. Volcanic ash reaction, generating Ca(OH)2, filling pores; 3. Acting as a nano-core and bonding with C-S-H to improve the strength of the cementitious system.
Khaloo [58]	n-SiO2	P.O II	100 nm, 1.5 wt.%	Slump decreased by 29.2%, fc28d and splitting tensile strength increased by 9.0% and 10.7% respectively.	1. SiO2 has a large specific surface area and high nanopore porosity; 2. Nanoscale nucleation promotes C-S-H growth and restricts the growth of weak crystal types such as AFt [59,60].
Supit [61]	n-SiO2	P.O I	25 nm, 4 wt.%	Slump decreased by 60%, fc28d increased by 75.8%, water absorption and porosity decreased by 20–40% and 25% respectively.	1. SiO2 nano-filling; 2. Volcanic ash is highly active and undergoes secondary hydration to form C-S-H, filling pores.
Li [62]	n-SiO2	P.O 52.5	20 nm, 1 wt.%	Liquidity decreased by 20%, ff28d increased by 41%, and fc28d increased by 31%.	1. Nano-filling; 2. High volcanic ash activity; 3. Nano-nucleation promotes hydration.
Zhang [63]	n-SiO2	P. I 42.5	30 nm, 2 wt.%	The proportion of Ca(OH)2 in the hydration products decreased, while C-S-H increased (Figure 5c,d).	1. Volcanic reaction and nano-filling; 2. Improved microstructure of hydration products.
Shang [54]	GO	P.O 42.5R	0.08 wt.%	The yield stress increased from 25.6 Pa to 105.3 Pa, and the plastic viscosity increased from 0.84 Pa·s to 1.95 Pa·s.	The electrostatic interaction between GO and cement promotes the formation of flocculation structures.
Liu [64]	GO	P.O 42.5	L * < 10 μm, 0.03 wt.%	fc28d increased by 12.4%, water resistance increased by 80%, and sulfate erosion after 3 months of fc increased by 11.3% compared to OCC.	1. Nano-filling; 2. Nano-nucleation; 3. Nano-templating, i.e., two-dimensional layered structures regulate the distribution and morphology of hydration products.
Yan [65]	GO	P.O I	L: 3–10 μm, T * < 5 nm, 0.04 wt.%	fc3d and fc28d increased by 14.4% and 3.1%, respectively, and ITZ density increased (Figure 5a,b).	1. High specific surface area increases interface roughness; 2. GO surface functional groups complex with Ca2+ in ITZ, enhancing interface forces; 3. Nanoscale nucleation.
Pan [66]	GO	P.O I	L: 1–14 μm, T: 1 nm, 0.05 wt.%	fc28d and ff28d improved by 15–33% and 41–59%, respectively.	GO surface groups can react with C-S-H or Ca(OH)2 to form strong covalent bonds at the interface, thereby improving the load transfer efficiency between the cement matrix and GO.
Manzur [67]	CNTs	P.O II	D *: 10–20 nm, 0.1 wt.%	Setting time reduced by approximately 25%; bond shear strength at 3, 7, and 28 days increased by 20%, 23%, and 22%, respectively.	1. Nano-nucleation accelerates hydration; 2. The carboxyl groups on the surface of CNTs react chemically with calcium silicate hydrate (C-S-H) or Ca(OH)2; 3. Nano-filling; 4. CNTs act as a bridge between cracks and voids, ensuring load transfer.
Alafogianni [68]	CNTs	P.O I	D: 20–45 nm, L > 10 μm, 0.4 wt.%	Large pores reduced by 47.9%; water absorption rate reduced by 50%.
Li [69]	CNTs	P.O 42.5	0.5 wt.%	fc and ff increased by 19% and 25%, respectively, the total porosity decreased by 64%, and pores larger than 50 nm decreased by 82%.
Nasibulin [70]	CNTs	P.O 42.5	In situ synthesis	fc28d increased by 2 times, and conductivity increased by 40 times.	CNTs act as a bridge between cement particles and hydration products.
Metaxa [71]	CNTs	P.O I	L: 30–100 μm, D: 100–150 nm, 0.048 wt.%	ff28d and Young’s modulus increased by 50% and 75%, respectively.	1. Nano filling; 2. Bridging effect.
Wu [72]	n-CaCO3	P.O II 42.5	15–105 nm, 3.2 wt.%	Liquidity decreased by 15.9%, while fc28d and ff28d increased by 9.9% and 25%, respectively.	1. Nano-filling; 2. Providing nucleation sites to refine pores.
Li [69]	n-CaCO3	P.O 52.5	15–80 nm, 3 wt.%	Liquidity decreased by 34%, while ff28d and fc28d increased by 40% and 17%, respectively.
Vitharana [73]	TiO2	P.O 42.5	20–30 nm, 1 wt.%	Fluidity decreased by 11.1%, fc7d increased by 43.7%, ff28d increased by 25%, and the weight loss rate due to reinforcement corrosion decreased by 10.54%.
Hou [74]	nano-clay	White cement	L: 1.5 μm, D: 3 nm, 1wt.%,	fc7d and ff28d increased by 9.10% and 69.84% respectively, with short-term shrinkage increasing and long-term shrinkage decreasing by 3% to 6%.	1. Accelerates early hydration and consumes a large amount of free water; 2. Nano-filling in the later stage.
Morsy [75]	nano metakaolin (NMK) & CNTs	P.O I	200 × 100 × 20 nm 6 wt.%; D: 3–8 nm, 0.02 wt.%	fc28d increased by 29%.	1. NMK undergoes an alkaline activation reaction; 2. NMK can inhibit CNT aggregation; 3. CNTs act as nucleation sites to promote hydration.

* Note: the “L”, “T”, “D” denotes length, thickness, diameter, respectively.

Table 1. Summary of varied nanomaterials on the properties of common NMC.

Refs.	Nano Material	Body Material	Particle Size and Dosage	Consequences	Mechanisms
Senff [56]	n-SiO2	P.O 52.5R	9 nm, 2.5 wt%	The flow expansion diameter of mortar decreased by 19.6%, the yield stress increased by 157%.	High specific surface area, adsorbs large amounts of free water.
Beigi [57]	n-SiO2	P.O II	15 nm, 6 wt.%	The slump decreased by 10%; fc28d, 28-day split tensile strength, and ff28d increased by 17.9%, 35%, and 39.5%, respectively.	1. Nano-filling effect; 2. Volcanic ash reaction, generating Ca(OH)2, filling pores; 3. Acting as a nano-core and bonding with C-S-H to improve the strength of the cementitious system.
Khaloo [58]	n-SiO2	P.O II	100 nm, 1.5 wt.%	Slump decreased by 29.2%, fc28d and splitting tensile strength increased by 9.0% and 10.7% respectively.	1. SiO2 has a large specific surface area and high nanopore porosity; 2. Nanoscale nucleation promotes C-S-H growth and restricts the growth of weak crystal types such as AFt [59,60].
Supit [61]	n-SiO2	P.O I	25 nm, 4 wt.%	Slump decreased by 60%, fc28d increased by 75.8%, water absorption and porosity decreased by 20–40% and 25% respectively.	1. SiO2 nano-filling; 2. Volcanic ash is highly active and undergoes secondary hydration to form C-S-H, filling pores.
Li [62]	n-SiO2	P.O 52.5	20 nm, 1 wt.%	Liquidity decreased by 20%, ff28d increased by 41%, and fc28d increased by 31%.	1. Nano-filling; 2. High volcanic ash activity; 3. Nano-nucleation promotes hydration.
Zhang [63]	n-SiO2	P. I 42.5	30 nm, 2 wt.%	The proportion of Ca(OH)2 in the hydration products decreased, while C-S-H increased (Figure 5c,d).	1. Volcanic reaction and nano-filling; 2. Improved microstructure of hydration products.
Shang [54]	GO	P.O 42.5R	0.08 wt.%	The yield stress increased from 25.6 Pa to 105.3 Pa, and the plastic viscosity increased from 0.84 Pa·s to 1.95 Pa·s.	The electrostatic interaction between GO and cement promotes the formation of flocculation structures.
Liu [64]	GO	P.O 42.5	L * < 10 μm, 0.03 wt.%	fc28d increased by 12.4%, water resistance increased by 80%, and sulfate erosion after 3 months of fc increased by 11.3% compared to OCC.	1. Nano-filling; 2. Nano-nucleation; 3. Nano-templating, i.e., two-dimensional layered structures regulate the distribution and morphology of hydration products.
Yan [65]	GO	P.O I	L: 3–10 μm, T * < 5 nm, 0.04 wt.%	fc3d and fc28d increased by 14.4% and 3.1%, respectively, and ITZ density increased (Figure 5a,b).	1. High specific surface area increases interface roughness; 2. GO surface functional groups complex with Ca2+ in ITZ, enhancing interface forces; 3. Nanoscale nucleation.
Pan [66]	GO	P.O I	L: 1–14 μm, T: 1 nm, 0.05 wt.%	fc28d and ff28d improved by 15–33% and 41–59%, respectively.	GO surface groups can react with C-S-H or Ca(OH)2 to form strong covalent bonds at the interface, thereby improving the load transfer efficiency between the cement matrix and GO.
Manzur [67]	CNTs	P.O II	D *: 10–20 nm, 0.1 wt.%	Setting time reduced by approximately 25%; bond shear strength at 3, 7, and 28 days increased by 20%, 23%, and 22%, respectively.	1. Nano-nucleation accelerates hydration; 2. The carboxyl groups on the surface of CNTs react chemically with calcium silicate hydrate (C-S-H) or Ca(OH)2; 3. Nano-filling; 4. CNTs act as a bridge between cracks and voids, ensuring load transfer.
Alafogianni [68]	CNTs	P.O I	D: 20–45 nm, L > 10 μm, 0.4 wt.%	Large pores reduced by 47.9%; water absorption rate reduced by 50%.
Li [69]	CNTs	P.O 42.5	0.5 wt.%	fc and ff increased by 19% and 25%, respectively, the total porosity decreased by 64%, and pores larger than 50 nm decreased by 82%.
Nasibulin [70]	CNTs	P.O 42.5	In situ synthesis	fc28d increased by 2 times, and conductivity increased by 40 times.	CNTs act as a bridge between cement particles and hydration products.
Metaxa [71]	CNTs	P.O I	L: 30–100 μm, D: 100–150 nm, 0.048 wt.%	ff28d and Young’s modulus increased by 50% and 75%, respectively.	1. Nano filling; 2. Bridging effect.
Wu [72]	n-CaCO3	P.O II 42.5	15–105 nm, 3.2 wt.%	Liquidity decreased by 15.9%, while fc28d and ff28d increased by 9.9% and 25%, respectively.	1. Nano-filling; 2. Providing nucleation sites to refine pores.
Li [69]	n-CaCO3	P.O 52.5	15–80 nm, 3 wt.%	Liquidity decreased by 34%, while ff28d and fc28d increased by 40% and 17%, respectively.
Vitharana [73]	TiO2	P.O 42.5	20–30 nm, 1 wt.%	Fluidity decreased by 11.1%, fc7d increased by 43.7%, ff28d increased by 25%, and the weight loss rate due to reinforcement corrosion decreased by 10.54%.
Hou [74]	nano-clay	White cement	L: 1.5 μm, D: 3 nm, 1wt.%,	fc7d and ff28d increased by 9.10% and 69.84% respectively, with short-term shrinkage increasing and long-term shrinkage decreasing by 3% to 6%.	1. Accelerates early hydration and consumes a large amount of free water; 2. Nano-filling in the later stage.
Morsy [75]	nano metakaolin (NMK) & CNTs	P.O I	200 × 100 × 20 nm 6 wt.%; D: 3–8 nm, 0.02 wt.%	fc28d increased by 29%.	1. NMK undergoes an alkaline activation reaction; 2. NMK can inhibit CNT aggregation; 3. CNTs act as nucleation sites to promote hydration.

* Note: the “L”, “T”, “D” denotes length, thickness, diameter, respectively.

Figure 5. GO and n-SiO₂ modified cement-based material: (a,b), GO; (c,d) n-SiO₂ (In (d₁), the yellow, green, and red areas represent Ca/Si ratios of 1.75, 3.82, and 5.05, respectively; In (d₂), the yellow, green, and blue areas represent Ca/Si ratios of 1.4, 2.62, and 3.54, respectively; A higher Ca/Si ratio indicates a higher proportion of Ca(OH)₂ and a lower proportion of C-S-H.), adapted from [63,65], and copyright (2018) Elsevier.

Figure 5. GO and n-SiO₂ modified cement-based material: (a,b), GO; (c,d) n-SiO₂ (In (d₁), the yellow, green, and red areas represent Ca/Si ratios of 1.75, 3.82, and 5.05, respectively; In (d₂), the yellow, green, and blue areas represent Ca/Si ratios of 1.4, 2.62, and 3.54, respectively; A higher Ca/Si ratio indicates a higher proportion of Ca(OH)₂ and a lower proportion of C-S-H.), adapted from [63,65], and copyright (2018) Elsevier.

4. Properties of NPMC

In PMC, polymers partially substitute ordinary cement as binders. The properties of PMC—including workability, mechanical performance, and durability—are affected to varying degrees by polymer characteristics and distribution, with certain properties potentially compromised. The problems introduced by polymers—such as delayed early hydration, restricted long-term strength development, and increased material creep—constrain the application scope and service life of PMC. There is an urgent need to explore novel materials and methods to address these limitations.

Section 2 and Section 3 introduce the properties of polymer- or nanoparticle-modified cement-based materials. When nanoscale materials are well introduced into PMC, NPMC is obtained. Its internal structure and properties differ significantly from those of PMC. Incorporating inorganic nano-particles into PMC networks provides rigid reinforcement while forming densely cross-linked hybrid systems, representing a viable approach for material improvement. Consequently, researchers are introducing nanoparticles into PMC to mitigate the reduction in mechanical performance, enhance durability, and refine internal microstructure. Current understanding of NPMC remains confined to individual effects of nanomaterials on polymer and cement phases. This section reviews nanomaterial-induced modifications in PMC to examine enhancement patterns and mechanisms.

4.1. Effect of Nanomaterials on the Workability of NPMC

Organic polymers significantly alter PMC workability (flowability, setting time) through air-entrainment and electrostatic adsorption. Nanomaterial incorporation similarly exerts non-negligible effects. Shen et al. [76] encapsulated n-SiO₂ in a polyacrylate-acrylamide network via physical adsorption and chemical grafting. They found that the incorporation of n-SiO₂ enhanced the water absorption capacity of polymer in alkaline environments, leading to a decrease in the flowability and an increase in the viscosity of NPMC. Adding 0.2 wt.% n-SiO₂ required an additional 22% water content to maintain the same flowability. Chen et al. [77] utilized the novel 2D nanomaterial MXene to enhance the durability of NPMC. First, MXene was functionalized with vinyl triethylsilane (V-MXene), followed by the synthesis of V-MXene-modified polyacrylate emulsion via in situ polymerization. The shear stress and plastic viscosity of NPMC gradually increased with the addition of V-MXene, as V-MXene had a large surface area that could absorb a significant amount of water from NPMC, thereby increasing the friction between the polymer and cement. Naseem et al. [78] observed ≈11% higher flowability in EVA-modified cement (5 wt.%) versus OCC, attributable to reduced flocculation [79] enabling ball-bearing action. Conversely, GO-modified composites (GOMC) exhibited 24% lower flowability than OCC. Notably, introducing GO-polymer composite phases (GOP) yielded GO-reinforced PMC (GOPMC) with ≈20% reduced flowability versus PMC (Figure 6a). The flowability reduction of GO stems from its high specific surface area promoting agglomeration [80]. Positively charged EVA particles coating GO surfaces suppress agglomeration and prevent flocculation [81]. UV spectrophotometry confirms this: GOP (5 wt.%) demonstrated ≈67% higher dispersion intensity than GO in alkaline pore solution within 0–30 min (Figure 6b). Visual observation further verified rapid GO agglomeration versus GOP stability (Figure 6c).

Naseem et al. [82] used GO nanosheets to weaken the gas-entraining effect of polymers in NPMC, thereby improving its mechanical properties. SEM analysis revealed rough, heterogeneous EVA surfaces (Figure 7a₁,a₃), whereas GO transformed EVA morphology into smooth, uniform GOP composites (Figure 7a₂,a₄). TEM characterization further elucidated GOP morphology: EVA particles exhibited rough and significant agglomeration (Figure 7b₁); Corresponding selected area electron diffraction (SAED) patterns displayed amorphous halo patterns (Figure 7b₂), confirming agglomeration and inhomogeneous distribution, factors stabilizing air bubbles in PMC [83]. By contrast, GO converted agglomerated EVA into ordered assemblies with uniform distribution (Figure 7b₃). Distinct hexagonal diffraction spots confirmed the crystalline nature of GO nanosheets within GOP composites (Figure 7b₄). These findings demonstrate nanoparticles’ capacity to improve polymer dispersion and establish continuous network structures [84]. Morphological analysis of fresh mortar reveals abundant stabilized air bubbles atop PMC (Figure 7c₁), whereas bubbles rapidly evacuate from GOPMC without stabilization (Figure 7c₂). The defoaming mechanism of GO originates from its well-dispersed particles inhibiting EVA agglomeration, thereby altering the composite medium’s surface tension and reducing bubble stability. Relative to PMC, GOPMC exhibits increased density, enhanced mechanical performance, and substantially reduced water absorption (Figure 7d–g).

Figure 8 illustrates mechanisms governing bubble stability in cement pastes modified by polymers and GO. Polymer-induced air entrainment primarily operates through two mechanisms: (1) Hydrophobic polymer groups trap air, generating microbubbles within the cement-based matrix; (2) Surfactant polymers drastically reduce system surface tension by altering adsorption kinetics and molecular diffusion rates to interfaces [85]. Consequently, polymer particles migrate to solution surfaces, stabilizing bubbles in cement paste. Conversely, well-dispersed GO interacts with vinyl acetate groups in EVA while its hydrophilic domains engage water molecules (Figure 8b). This dual interaction modifies the composite medium’s surface tension, destabilizing air bubbles.

4.2. Effect of Nanomaterials on the Mechanical Properties of NPMC

Nanomaterials enhance mechanical performance of cement-based composites through microstructural refinement. Idrees et al. [86] observed that SBR emulsion-induced air entrainment significantly compromises mechanical performance. Incorporation of n-SiO₂ or n-TiO₂ nanoparticles counteracts these detrimental effects via nano-filling and nucleation mechanisms. Fan et al. [87] demonstrated superior interfacial adhesion and load transfer in systems containing CNT/SAE mixtures versus individual CNTs or SAE. Mortars with 0.1 wt.% CNTs + 15 wt.% SAE exhibited 21% and 25% higher f_f than CNT-only and SAE-only systems, respectively. Wetzel et al. [88] incorporated n-TiO₂ or n-Al₂O₃ into EP to toughen epoxy mortar composites. Unlike conventional tougheners (e.g., glass beads, rubber particles), these nanoparticles simultaneously enhance stiffness, strength, and toughness of EP without compromising thermomechanical performance.

As NPMC is frequently employed for concrete repair, bond strength constitutes a critical performance metric. Studies confirm nanomaterials effectively enhance PMC bond strength. Guo et al. [89] incorporated n-TiO₂ (0.1 wt.%) into epoxy mortar (4.5 wt.% EP), observing ≈9% and ≈5% increases in f_c^28d and f_b^28d respectively, alongside improved ITZ. This enhancement is attributed to robust EP adhesion to high-stiffness n-TiO₂, enabling stress transfer under load—higher n-TiO₂ content yields greater ultimate strength. Lin et al. [90] identified synergistic bond enhancement between EVA and CNTs. Carboxyl groups prevalent in both components induce like-charge repulsion, promoting CNT dispersion. Well-dispersed CNTs amplify nucleation/bridging effects, accelerating hydration and densifying ITZ. Li [91] utilized n-SiO₂ to augment bond strength in polyacrylate-modified mortar. At 4 wt.% n-SiO₂, f_b increased by 13.0% with 5–10 μm ITZ width reduction. Whereas conventional PMC exhibited interfacial debonding failure (Figure 9a), n-SiO₂ incorporation transformed failure mode to cohesive failure (Figure 9b) through microstructural modification: Oriented CH crystals at bond interfaces (reducing adhesion) form under hydration and local stress; Nanoparticles react with CH to form expansive C-S-H nanocrystals [92]; This consumes interfacial CH while increasing C-S-H bonds, strengthening the interface (Figure 9c).

Gao et al. [93,94] employed GO to enhance durability and bond strength in PA/EVA-modified repair mortars. Their research demonstrates GO significantly refines PMC pore structure (Figure 10a–d), increases compactness and the degree of hydration (Figure 10e) thereby improving mechanical and durability properties. Microstructural analysis of bond interfaces revealed: Polymer films form within pores and at interfaces, providing pore-filling effects that reduce porosity and even diminish bond interface width (Figure 10f); However, EVA encapsulation isolates unhydrated cement clinker from free water, impeding hydration.

4.3. Effect of Nanomaterials on Durability of NPMC

Benefiting from microstructural refinement by nanomaterials, the durability of NPMC is also significantly enhanced [95]. Muthalvan et al. [96] observed that sodium polyacrylate-modified cement mortar (SPCM) exhibited reduced resistance to acid and sulfate attack compared to OCC. To address this limitation, they introduced n-SiO₂. With increasing n-SiO₂ dosage, both mass loss and strength degradation of SPCM specimens exposed to magnesium sulfate solution were substantially mitigated. SPCM reinforced with 2 wt.% n-SiO₂ demonstrated approximately 33% and 25% reductions in mass loss and strength deterioration, respectively. Li et al. [97] reported 7.1× and 2.8× extensions in chloride resistance lifespan for PMC coatings with 0.5 wt.% n-SiO₂ and n-TiO₂, respectively. After 650 h: PMC with 0.2 wt.%/0.5 wt.%/0.8 wt.% n-SiO₂ or n-TiO₂ reduced water absorption by 30%/50%/16% or 60%/45%/20% (Figure 11a). Coulomb electric flux, inversely related to chloride resistance, decreased significantly with n-SiO₂/n-TiO₂ incorporation (Figure 11b). NPMC coatings further reduced chloride content at equivalent depths versus unmodified PMC (Figure 11c), confirming enhanced chloride penetration resistance. Chen et al. [77] found that the introduction of V-MXene into NPMC significantly improved its UV resistance, saltwater immersion resistance, and heat resistance. V-MXene reduced the amount of UV radiation energy absorbed by PCC and created a “maze effect,” prolonging the penetration path of water and corrosive ions. V-MXene also increased the cross-linking density of the organic-inorganic interpenetrating network, thereby enhancing the thermal stability of NPMC.

The effects of nano-polymer composites (NPC) on mechanical and durability properties of cement-based materials are intrinsically linked to their influence on cement hydration [98]. Zabihi et al. [99] demonstrated that n-SiO₂ counteracts polymer-induced hydration retardation through acceleration effects. EVA (20 wt.%) and SBR (17 wt.%) reduced hydration rates by 13% and 62% respectively, while subsequent 0.5 wt.% n-SiO₂ addition restored 23% of hydration rate. Meng et al. [100] introduced n-SiO₂ into EVA-modified systems to mitigate adverse pore structure development. They found EVA inhibits hydration, increases harmful pore volume, and compromises strength. Nanoparticles not only provide nucleation sites in pore solution—promoting uniform hydration product distribution—but also refine pore structure by filling inter-hydrate voids [101].

Table 2. Properties contrasts between NPMC and PMC dosed with varied nanomaterial or/and polymer.

Refs.	Body Material	Polymer	Nano Material	Changes in Workability	Changes in Mechanical Properties	Changes in Durability
Naseem [78]	OPC	EVA, 3–5 wt.%	GO, 0.05 wt.%	Fluidity decreased by 5–15%.	The yield stress increased by 15% to 400%. The higher the EVA codntent, the stronger the effect.	-
Naseem [82]	OPC	EVA, 5 wt.%	GO, 0.05 wt.%	Slump reduction of 15%	fc28d and ft28d improved by approximately 26% and 32%, respectively.	The water absorption rate decreased by approximately 21%.
Idrees [86]	P.O I	SBR, 5 wt.%	n-TiO2, 2 wt.%	Fluidity decreased by 9.3%.	fc28d increased by 20%.	Water absorption decreased by 14.2%.
			n-SiO2, 2 wt.%	Fluidity decreased by 8.0%.	fc28d increased by 48%.	Water absorption decreased by 8.6%.
			Zinc-Stearate, 0.5 wt.%	Fluidity decreased by 20.0%.	fc28d increased by 24%.	Water absorption decreased by 5.7%.
Fan [87]	P.O I	SAE, 15 wt.%	CNT, 0.1 wt.%	-	fc3d, fc7d, fc28d increased by 15%, 14% and 12%; ff3d, ff7d, ff28d increased by 22%, 16% and 15%.	The capillary water absorption rate and adsorption rate decreased by 48.5% and 41.1%, respectively.
Guo [89]	P.O 42.5	EP, 4.5 wt.%	n-TiO2, 2 wt.%	No significant change in rheology	fc28d, ff28d, fb28d increased by 8.8%, 7.5% and 4.5%	-
Gao [94]	P.O 42.5	EVA, 8 wt.%	GO, 0.03 wt.%	Fluidity decreased by 15% and setting time was reduced by 21%.	fc28d, ff28d, fb28d increased by 8.3%, 19.1% and 36.6%.	Dry shrinkage was reduced by approximately 30%, water absorption was reduced by approximately 40%, and resistance to chloride ion permeability was also significantly improved.
Li [97]	P.O I	AC emulsion, 50 wt.%	n-TiO2, n-SiO2, 0.5 wt.%	-	-	Improved resistance to chloride ion penetration.
Chen [77]	P. I 42.5	PA, 10 wt.%	MXene, 0.15 wt.%	Curing timel and rheology are reduced by approximately 21% and 15%.	ft and fb increased by 15%, and 10%.	Water absorption decreased by 40%.

summarizes the performance changes of NPCM after modification with nanomaterials. As shown in Table 2, compared with PMC, NPMC with an appropriate amount of nanomaterials has lower rheological properties, faster curing, and significantly improved mechanical properties and durability. The mechanisms by which nanomaterials improve the mechanical properties and durability of NPMC is multi-faceted, (1) Nano-filling effect: Pore filling increases matrix compactness; (2) Nucleation promotion: Well-dispersed nanomaterials provide stable nuclei accelerating hydration; (3) Pozzolanic reaction: Reactive nanoparticles (e.g., n-SiO₂, n-Al₂O₃) undergo alkali-activation in cement environments, generating additional hydration products; (4) Templating function: Structurally ordered nanomaterials (CNTs, GO) guide preferential alignment of hydration products; (5) Interfacial bonding: Polar functional groups (e.g., carboxyls on CNTs/GO) react with hydration products (C-S-H, Ca(OH)₂) forming covalent bonds, improving load transfer efficiency and enabling stress-bearing bridging networks.

4.4. Interactions Between Nanomaterials and Polymers in NPMC

In addition to the advantages of NPMC mentioned above, the interaction between nanomaterials and polymers is also a key factor in the rapid development of properties of NPMC.

Zabihi et al. [99] found that the addition of n-SiO₂ reduced the zeta potential of cement paste, indicating that n-SiO₂ particles adsorb onto the positively charged surfaces of hydration products. In contrast, EVA and SBR tended to shift the zeta potential toward the positive direction, with SBR showing a more pronounced effect (Figure 12a,d). The opposite effects of nanomaterials and polymers on the cement paste potential suggested a possible competitive interaction between the two. Pourjavadi et al. [102] found that when n-SiO₂ and acrylic acid-crosslinked acrylamide (AA-co-AM) were used together, in addition to performing their respective functions, AA-co-AM might also generate Ca (OH)₂ in the paste, thereby enhancing the pozzolanic reaction of n-SiO₂ (n-SiO₂ can convert Ca(OH)₂ into C-S-H gel through the pozzolanic reaction, thereby making the pore structure of the paste more compact). As mentioned in Section 2.4, agglomeration of nanomaterials has been a key issue limiting their application. The interaction between polymers and nanomaterials may serve as an effective means to address this issue. Zhang et al. [103] investigated the effect of polymer concentration on the dispersion of CNTs through experiments and molecular dynamics (MD) simulations. MD results showed that the agglomeration of CNTs depends on their interaction energy in the solution; higher energy led to stronger agglomeration forces. The interaction energy wass related to the type of CNTs and the introduced polymer. As shown in Figure 12e–g, CNTs with -OH or -COOH groups on their surfaces (OH-type or COOH-type CNTs) exhibited significantly lower interaction energy than CNTs without these functional groups (C-type CNTs). However, introducing polymers into the solution significantly reduced the interaction energy between CNTs, facilitating their dispersion. Atomic trajectory analysis (Figure 12h) showed that polymers tend to adsorb onto the surfaces of CNTs and envelop them, thereby exerting steric hindrance. To verify the underlying mechanism of this phenomenon, they used radial distribution functions (RDF) to study the interactions between CNTs and CNTs/polymers. In alkaline cement slurry solutions, interactions between CNTs primarily fell into three categories: π-π, coordination bonds, and hydrogen bonds. For π-π interactions, the C-C distance between OH/COOH-type CNTs was greater than that between C-type CNTs, resulting in weaker interaction strength compared to C-type CNTs (Figure 12i). For coordination bond interactions, only COOH-type CNTs exhibited a Ca-O(C=O) coordination bond, with Ca²⁺ acting as a bridge connecting two COOH-type CNTs, resulting in weaker dispersion compared to OH-type CNTs (Figure 12j). Hydrogen bonds were present between OH/COOH-type CNTs, and the formation of hydrogen bonds also promoted CNT adsorption (Figure 12k). In addition to the above conclusions, analysis of the interactions between polymers and C-type CNTs revealed a peak at 3.57 Å between the benzene ring (SB) and CNTs, indicating a possible π-π interaction. However, no such interactions were observed between EVA/SAE and C-type CNTs (Figure 12l). Additionally, the polar functional groups in all three polymers formed hydrogen bonds with water molecules, and the bond length between O (COO-) and H (water) (1.65 Å) was significantly shorter than that between O (C=O) and H (water) (1.77 Å), indicating that the hydrogen bonds formed by O (COO-) and H (water) were stronger than those formed by O (C=O) and H (water) (Figure 12m). Therefore, compared to the mutual attraction between CNTs, polymers were more likely to adsorb onto the CNT surface due to stronger interactions with CNTs, thereby promoting the separation and dispersion of CNTs in NPMC.

By the interactions between nanomaterials and polymers, some researchers have proposed new strategies for modifying PMC with nanomaterials. Yin et al. [104] successfully prepared zirconium phosphate-acrylamide (ZrP-AM) composite materials by inserting part of the AM monomer into the interlayer of zirconium phosphate (ZrP). During the cement hydration process, AM undergone in situ self-polymerization and copolymerization with ZrP-AM, formed a complete intercalation network with ZrP distributed in the polymer network (Figure 13a). Meanwhile, due to the formation of chemical bonds, ZrP-AM was tightly bonded to the cement matrix. In the optimal formulation design, the f_f of the ZrP-PAM modified sample containing 1 wt.% ZrP and 3 wt.% AM was 105% higher than that of the cement paste. Furthermore, due to the reinforcing effect of ZrP on both the cement matrix and the polymer network, the f_f and f_c of the modified samples were increased by 28.5% and 17%, respectively, compared to samples modified with AM alone. This process significantly addressed the issues of uneven polymer distribution and structural incompleteness in composite materials [105,106,107]. Wang et al. [108] utilized the polymers to facilitate the dispersion of nanomaterials, mixing vinyl trimethoxysilane, AC, agent isobutyl polyethylene glycol ether (molecular weight 2400), and GO in a pre-polymerization process to obtain silane and copolymer-modified GO (P-S-GO), which could serve as a water-reducing agent for cement paste. When GO dispersion was added to cement paste, GO formed flocculation structures with cement particles (Figure 13b₁). These flocculation structures encapsulated part of the free water, thereby reducing the rheological properties of the paste [109,110,111]. When the same amount of P-S-GO dispersion was added to the slurry (as shown in Figure 13b₂), the P-S-GO composite particles did not form significant aggregates in the cement solution, and the cement slurry maintained superior dispersion and rheological properties.

5. Conclusions and Outlook

5.1. Conclusions

To enhance the repair and protective capabilities of cement-based materials as building repair material and extend its service life in complex environments, admixtures such as polymer and nanomaterials are commonly employed for modification of cement-based material. This work introduces the effects of organic polymers on the workability, mechanical performance, impermeability/durability, and hydration characteristics of OCC, while analyzing current limitations of PMC. Subsequently, it systematically reviews state-of-the-art research on NPMC and summarizes the underlying enhancement mechanisms.

Polymers modify OCC properties through dual physical-chemical mechanisms. Physical encapsulation directly governs OCC rheology and hydration kinetics. Post-formation of crosslinked networks with the cement-based matrix, low elastic modulus of polymer reduces bulk strength of PMC, while their low permeability concurrently enhances durability. Properties modifications also depend on complexation reactions between organic functional groups and metal cations in pore solution. These reactions alter ionic concentrations and precipitation kinetics, thereby affecting macroscopic properties and hydration processes. Compared to PMC, NPMC exhibits improvements in mechanical performance, durability, bond strength, and hydration degree. Nanomaterials enhance OCC properties through inherent characteristics: nano-filling effects, nanoscale nucleation, high reactivity, templating capabilities. Beyond individual enhancements, nanomaterial-polymer interactions within NPMC systems: Modulate mutual spatial distribution in cement paste, improving dispersion stability; Regulate polymer-matrix compatibility, thereby governing composite system performance.

5.2. Outlook

The enhancement of NPMC strength and long-term durability is not merely an improvement in material performance but also a guarantee of the long-term safety and stability of cement-based structures. The safe and long-term service life of cement-based materials contributes to resource conservation, environmental protection, and the sustainable development of the industry. However, the application and development of NPMC still face numerous challenges that require urgent resolution.

• One of the key issues constraining the application of nanomaterials in cement-based structures is high cost (e.g., GO, CNT, etc., have high production costs). Developing low-cost processes for large-scale production of nanomaterials is a prerequisite for NPMC application. Low-cost industrial solid waste can be used as raw materials for nanomaterial production; new processes can be developed or improved, such as microwave synthesis, in-situ crystallization of gel glass, and ton-scale production of water treatment composite nanomaterials.
• Improving the dispersion process of nanomaterials in cement-based systems. While existing dispersion processes can effectively disperse nanomaterials, they may damage the structure or functionality of the nanomaterials. The synthesis and optimization of new high-efficiency water-based dispersants, as well as the exploration of low-loss, high-efficiency dispersion processes, are urgent issues that need to be addressed.
• Mechanisms governing nanomaterials’ effects on microstructure and properties of PMC require further refinement. (1) Advanced characterization techniques should enable qualitative and quantitative analysis of nanomaterial effects on PMC microstructural evolution and hydration mechanisms; (2) Given the substantial hydration influence of both components, NPMC-specific hydration models can be established to investigate dynamic characteristics and reaction kinetics; (3) Current studies focus predominantly on single-phase interactions with cement-based systems, overlooking mutual polymer-nanomaterial interactions within NPMC. Pronounced interfacial interactions between specific nanomaterial surfaces and organic polymers yield hybrid-phase properties distinct from individual components, consequently altering system responses. Comprehensive understanding and utilization of these interactions will enhance synergistic optimization efficiency, enabling significant performance enhancements in cement-based materials.