The chapter delves into the intricate interaction between polymers and cement, highlighting the significant role polymers play in enhancing the properties of concrete. It explores the mechanisms at the micro and nano-scale, revealing how polymers improve fluidity, water retention, tensile strength, toughness, crack resistance, impermeability, durability, and bonding performance. The review covers various types of polymers, including styrene-butadiene rubber (SB), styrene-acrylic ester (SA), and ethylene-co-vinyl acetate (EVA), and discusses the multi-scale characteristics of cement-based materials. By understanding these mechanisms, professionals can gain insights into developing more resilient and durable concrete structures.
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Abstract
Polymer-modified cement-based materials are commonly used in engineering applications and have achieved good results. The interactions between polymer and cement have received extensive attention. In this paper, the interaction between them is discussed and summarized by reviewing the existing technologies. Traditional experimental methods do not provide a comprehensive picture of the interaction between polymers and cement-based materials, molecular dynamics (MD) simulations were used recently in the study of inorganic-organic phase interactions. People almost reach a consensus on the modification mechanism of polymers on concrete at micro-scale. But at nano-scale, the interaction between polymers and cement is an ongoing work, researches show that it contains several aspects, i.e., chemical bonding, hydrogen bonding, van der Waals forces, etc. Different polymers may have different types of interactions with cement. Understanding these interactions is important to elucidate the relationship between the microstructure and macroscopic properties of polymer-modified cement-based materials. Molecular dynamics simulation has proved to be an effective method to study the interactions between inorganic-organic composites at this stage but has some limitations.
1 Introduction
Cement-based materials are most widely used in construction material in the twentieth century, which is closely related to the daily life of human beings. However, concrete belongs to heterogeneous, brittle composite cementitious material, has a low tensile strength (1/10 ~ 1/20 of compressive strength) [1], that easily cracks under tensile loading [2]. The cracks will weaken the bearing capacity and stability of the structure, and will also accelerate the failure of concrete.
To improve the crack resistance and durability of concrete, one of the approaches is to add modifiers. Polymers are popular as modifiers for concrete. It has been used in many projects and has achieved good results [3‐11]. There are many types of polymers. Among them, the most commonly used polymers are styrene-butadiene rubber (SB), styrene-acrylic ester (SA), and ethylene-co-vinyl acetate (EVA). The polymer can not only improve the fluidity of cement paste [12‐14] and water retention [15, 16] but also improve tensile strength, toughness [17], crack resistance, impermeability [18], durability [19], and bonding performance of concrete [20‐23].
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Until now, some researchers have studied the mechanism of polymer-modified cement-based materials through a range of physical and chemical methods, to reveal the impact of polymer on concrete in macro performance. Cement-based materials have multi-scale characteristics, and the mechanisms of each scale are also different. This article mainly reviews the mechanism at micro-scale (10–3 ~ 10–7 m) and especially nano-scale (<10–7 m).
2 Mechanism at Micro-scale
In recent years, researchers have mainly explored the modification mechanism of polymers on a micro-scale. They believed that the decentralization of polymer particles and forming a polymer film are the main causes of modification. Two famous models, Ohama's and Konietzko's models were proposed based on the characteristics of polymer forming a thin film inside concrete [24, 25]. The Konietzko's model assumed that the polymer film and cement paste penetrated each other to form a mesh structure, while Ohama's model assumed that the cement paste was encapsulated in a polymer film. Fichet et al. observed by SEM that polymer particles were distributed in the middle of cement hydration products and on the surface of unhydrated cement particles, which reduced the hydration rate of cement [26]. With the hydration of cement, polymer emulsion encapsulates the surface of cement particles and hydrated products, filling the pores. The porosity of concrete and the material properties could be improved by this physical effect [27]. The polymer also forms a bridging effect inside cement-based materials [28], consuming the energy generated internally by the external force and increasing the tensile strength of the cementitious material. Su et al. found that polymers also adhere to the inner walls of cement-based material pore channels to form thin films [29], closing the internal pores and improving the durability of cement-based materials. Figure 1 shows the evolution of polymer (SB) morphology and structure in polymer/cement composites cementation materials. Optical microscope and environmental scanning electrical microscope were used to investigate the dispersion and absorption of polymers in mono-dispersed cement system [30]. The mono-dispersed cement system can be achieved at the water to cement ratio of 10:1. Proper amount of SA and SB is beneficial to the dispersion of the cement. SA and SB can be dispersed on the surface of mono-dispersed cement particles as well as the solution in the system proportionally. The absorption density of polymers on the cement particle grows with the increasing polymer to cement ratio (mp/mc). Through sedimentation test, it can be found that the absorption amount of both polymers in the mono-dispersed cement grows with mp/mc but decreases with water to cement ratio (mw/mc). The absorption rate of both polymers in the mono-dispersed cement declines with mp/mc and mw/mc. The polymer coverage on the surface of the mono-dispersed cement particle increases with mp/mc but decreases with mw/mc. Both of the coverage is less than 100%, indicating the absorption of these two polymers on the surface of the cement particle is single layered.
Fig. 1.
Evolution of polymer (SB) morphology and structure in polymer/cement composite cementitious materials: (a) Polymer particles dispersed on the surface of cement granules after mixing for 10 min, (b) Polymer particles deformed and fused together after mixing for 3 h, (c) Network polymer film formed at 28 days, (d) Polymer bridges at the interface of cement paste and aggregate at 28 days
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3 Mechanism at Nano-Scale
Although the modification mechanism of polymers has been revealed at the micro-scale, there are still some issues to be solved. How do the polymers interact with the cement-based materials? Researchers are needed to delve further into the nano-scale to explore the intrinsic mechanism. The influence of polymers at the nano-scale consists of two main types, i.e., changing the molecular structure of hydration products and forming interactions with the hydration products, including chemical bonding and intermolecular forces.
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3.1 Changing the Molecular Structure of Hydration Products
Polymers affect the hydration reaction of cement and the molecular structure of hydration products. Some studies have shown that polymers significantly retard early hydration reactions. Zhang et al. found that SA significantly retarded the hydration of cement-based materials [31]. Wang et al. investigated the effect of SB and SA on hydration products, and the results showed that the polymers promoted the formation of calcium aluminate, but reduced the content of calcium hydroxide [32, 33]. The hydration and hardening processes of polymer-modified concrete are mainly influenced by both cement hydration and polymer film formation processes [34]. Polymers also alter the molecular structure of some hydration products (e.g., C-S-H). Wang et al. used NMR to demonstrate the effect of the incorporation of SB on the polymerization of [SiO4]4− tetrahedron in C–S–H gel and the results showed that the degree of polymerization was depressed significantly (Fig. 2) [35]. Peng et al. found that polymer increased the Ca/Si ratio of C-S-H (Fig. 3) [36]. In addition to the degree of polymerization and Ca/Si ratio, polymers also change the layer spacing of C-S-H. Matsuyama first reported that polymers can be intercalated, the used polymers were nonionic poly(vinyl alcohol) (PVA), anionic poly(acrylic acid) (PAA) and cationic poly(diallyl dimethylammonium chloride) (PDC), as indicated by the shift of the (002) basal reflections to smaller diffraction angles (Fig. 4), and later found that the addition of PAA increased the Q2 intensity by 29Si NMR spectra, indicating lengthening of the silicon chain [37]. Subsequently, some scholars also analyzed the shift in the (002) peak position through XRD and found that polyethylene glycol (PEG) caused the expansion of the C-S-H layer spacing [38]. The overall results of the literatures suggested that the success of intercalation depended on the Ca/Si ratio of C-S-H, the nature of the polymer and its molecular weight, and the synthesis process [39].
Fig. 2.
29Si NMR spectra of SB-modified cement pastes hydrated for 28 days [35].
Fig. 3.
Ca/Si ratios of unhydrated cement particles and hydration products for all cement pastes cured for 7 d: (A) Reference, (B) SB-5%, (C)SB-15%, (D) EVA-5%, and (E) EVA-15%. The filled circle represents the data of the unhydrated cement particles and the open square represents these of the hydration products. All data are statistically compared in the panel (F) [36].
Fig. 4.
Changes in (002) basal spacing of C-S-H precipitated with different polymers. Percentages are weight percent of carbon in the complexes [37].
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3.2 Interaction Between Polymer and Cement-Based Materials
3.2.1 Research Using Microstructure Characterization
Researchers have conducted several experiments to investigate the chemical reaction between polymers and cement-based materials. Silva et al. found the presence of calcium acetate (Ca(CH3COO)2·H2O) inside the EVA-modified cement paste by Fourier-transform infrared spectrometer (FTIR) analysis, where the carboxyl group is thought to originate from the hydrolysis of the acetate group in the EVA molecule in a high alkali solution environment [40]. Wang et al. investigated the chemical reaction of SA in cement paste and found that the absorption peak which occurred between 1729 and 1735 cm−1 corresponds to C = O in the COO− group of SA powder modified cement pastes. The same group in pure SA powder appears at 1728 cm−1. Figure 5 shows that the absorption peak of C = O begins to shift to a higher wave number. The possible reason is that the lone pair of electrons belonging to -O- in the COO− group is transferred to the Ca2+ ion in the system, this reduces the electronic cloud density around –O– and also around C = O which is connected to it. The spectrum changes indicate that O → Ca2+ coordination bonds exist [33, 41]. Wang et al. studied the chemical interaction between polyacrylate emulsions and cement paste that the carboxyl groups of polyacrylate chains reacted with Ca2+ from Ca(OH)2 to produce Ca(HCOO)2 (as shown in Fig. 6) [42].
Fig. 5.
FTIR patterns of cement pastes with and without SAE powder cured for 28 days [33, 41].
3.2.2 Research Using Molecular Dynamics Simulation
While the above studies have demonstrated some chemical interactions between polymers and cement using methods such as FTIR and XPS, the interactions between atoms have not yet been visualized. Furthermore, there are no feasible experimental approaches to explore the van der Waals force between molecules as well as the hydrogen bonding. Molecular dynamics (MD) simulations are based on force fields and can go down to the molecular/atomic scale to characterize the interactions between polymers and cement-based materials. MD was used to study the mechanism of the interaction between polymers and cement hydration products, which makes up for the shortage of existing experimental conditions. The results of molecular dynamics simulations are closely related to the selection of force field types, however, unfortunately, there are no rigorous guidelines for selecting an appropriate force field. There are two major categories of force field types in cement-based materials research，(i) ReaxFF reactive force field, which can simulate the breaking and formation of chemical bonds; (ii) Classical force fields (ClayFF, CSH-FF and COMPASS), which cannot simulate the breaking and formation of chemical bonds.
3.2.3 Interaction at Molecular-Level
A reactive MD simulation based on the ReaxFF force field was employed to study the strengthening mechanisms of C–S–H incorporating polyethylene glycol (PEG), polyvinyl alcohol (PVA) and polyacrylic acid (PAA) polymers. It was found that all three polymers can be inserted into the interlayer of C-S-H and new Si-O-C bonds are formed after breaking the C-C bonds in the PVA, PAA and PEG polymer chains (Fig. 7(a, b, c)) [43], which has been verified by first-nature principle calculations in some similar organic/inorganic system [44, 45]. Additionally, it was reported that the PEG/C–S–H composite displayed a stronger bond due to the breaking of the C–O bond in polymer by the Ca ions presented in the gel and form a new C-Ca bond (as shown in Fig. 7(d)) [46], which has also been proven by the first-nature principle studies, which reduced the system energy and made the system more stable [47]. Although the utilization of ReaxFF force fields can reflect the breaking and formation of chemical bonds, allowing researchers to better understand the interaction between polymers and C-S-H, the parameters in the ReaxFF force field have a huge impact on the results [48].
Fig. 7.
Local snapshots after structural rearrangements: (a) PVA, (b) PAA and (c)(d) PEG (red and yellow sticks indicate silicate tetrahedra, white and red lines indicate water molecules, and the balls of other colors indicate the polymer atoms: grey for carbon, white for hydrogen, red for oxygen, and green for calcium) [43, 46].
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Researchers have also tried to analyze the interaction between polymers and concrete using classical force fields. Hou et al. investigated the interfacial interaction of three polymers, PEG, PVA, and PAA, with C-S-H gels, which exhibited brittle fracture in the z-direction due to the presence of weak hydrogen bond (H-bond) interactions. After the addition of polymers, the stress rebounded at a later stage and the brittle properties were improved. The strongest interaction was found in C-S-H with PAA which was mainly due to two types of strong and stable connections. One was the coordination of double-bonded oxygen atoms from the carboxyl groups around Ca ions from the C–S–H surface (Fig. 8(a)) and the other was the H-bonds formed via high-reactivity nonbridging oxygen atoms from silicate tetrahedra of C–S–H accepting hydrogen atoms from carboxyl groups of polymers (Fig. 8(b)) [49]. Similarly, there were also coordination and H-bond interactions between hydroxyl groups of PVA and C–S–H surface (as shown in Fig. 8(c, d)), and thus PVA was intermediate in terms of affinities, greater than PEG (as shown in Fig. 8(e, f)), in which no obvious connections were found [50]. The interaction strength between the polymer and C-S-H affects the mechanical properties (Young's modulus and tensile strength) of the composite materials, and the higher the interaction strength the better the mechanical properties.
In addition to the study of polymer-C-S-H interface interactions, MD simulations have been applied to study the effect of polymers on cement hydration. The adsorption of PCE on the cement particles, represented as C2S, was investigated. The results revealed that the presence of PCE perturbed the dense water layer above the C2S surface and lowered the water density, which affected the hydration reaction of cement [51]. Chaudhari et al. studied the effect of carboxylic and hydroxycarboxylic acids on cement hydration. It suggested that the chelate complex of the hydroxycarboxylic acid retards cement hydration by adsorbing onto the reactant cement mineral phases particularly hydroxylated C3S [52].
3.2.4 Effect of Service Environment on the Interaction
It is known that the interaction between different materials plays a crucial role in the structural integrity and durability of concrete under extreme environmental conditions. In fact, it is difficult to probe the effect of the service environment on polymer-cementitious material interactions at the molecular level by existing technical means. MD can bridge this gap very well.
Many macro-scale experimental studies have explored the adverse effects of external environmental exposures on the epoxy/concrete interface. However, there is a lack of sufficient intrinsic understanding of how the bond evolution and stress transfer at the epoxy/concrete interface are disrupted or altered by aggressive environments. Hou and Wang et al. used MD to reveal the mechanism of debonding between epoxy and C-S-H under sulfate environment conditions, they found that water molecules and sulfate ions weakened the interaction between C-S-H and epoxy resin by breaking the Ca-O and H-bonds [53]. Yu et al. also demonstrated that water molecules disrupt the bonding interaction between the epoxy resin and C-S-H, leading to a reduction in bond strength [54]. Zhang conducted a study on the effect of temperature on the bonding properties between tannic acid and C-S-H and showed that the bonding properties were improved after high temperatures due to the increase in contact area caused by the larger radius of gyration of tannic acid [55].
Fig. 8.
Overall view of calcium silicate hydrates with (a) (b) PEG (c) (d) PVA (e) (f) PAA intercalated and enlarged snapshots of the Os-Hp connection. (Red and yellow sticks represents the silicate tetrahedra, green and purple balls correspond to the intralayer calcium atoms and the interlayer calcium atoms respectively, white and red lines represent water molecules and hydroxy, balls of other colors are for the polymer atoms: grey for carbon, white for hydrogen, red for oxygen, and blue dotted line represents H-bonds) [49].
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4 Summary
The application of polymers can improve the mechanical properties and durability of cement-based materials. The modification mechanism of polymers at micro-scale and nano-scale have been studied. It is recognized that at the micro-scale the film-forming action of polymers inside cement-based materials can bridge the microcracks to improve the mechanical properties of the materials, and also close the pores to improve the durability. However, it is difficult to reveal clearly the basic mechanism of the action of polymers with cement-based materials at the micro-scale.
The action of polymers on cement-based materials at the nano-scale consists of two parts. Firstly, it changes the number and molecular structure of the hydration products, such as the Ca/Si ratio, the polymerization of [SiO4]4− tetrahedron and the layer spacing of C-S-H gel. Secondly, it is the molecular interaction between polymers and cement-based materials. Currently, the chemical interaction between polymers and cementitious materials have been demonstrated by XPS and FTIR techniques, and these chemical interactions are generated mainly between the polymer with polar functional groups and cement-based materials. However, some commonly used polymers such as SB that does not contain polar functional groups but still shows good performance. The interaction between polymers and cement-based materials is still controversial.
The application of the first-nature principle and molecular dynamics can also solve the problems of interaction between polymers and cement-based materials, and some progress has been made in this area. However, there are still some shortcomings in molecular dynamics simulation studies that need further investigation: (1) Limited by the bulk volume and time length of the molecular dynamics simulation itself, its simulation results may differ from the actual experimental results, and it is an important research task to relate the simulation results to the experimentally observed macroscopic behavior; (2) Most of the current studies represent cement-based materials with individual calcium silicate hydrates, without considering the effect of other compositions and porosity within the material on its performance; (3) Although force fields such as ClayFF and CVFF have been used to describe the interaction between polymers and cement-based materials and are considered reasonable, such classical force fields cannot describe the chemical reactions that exist between the polymer and the cementitious material, and therefore more suitable force fields need to be developed for polymer-cement systems.
Understanding of the nature of materials may bring new solutions for materials. Elucidation of the modification mechanism of polymers on cement-based materials at nano-scale is helpful to understanding the nature of the material, which may open the gate to create a new generation of polymer-cement based composite.
Acknowledgement
The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 51872203 and 51572196) and the Top Discipline Plan of Shanghai Universities-Class I (2022-3-YB-17).
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