Polymer-cement Composites Research Articles

Structural and chemical changes from CO2 exposure to self-healing polymer cement composites for geothermal wellbores

Wellbore cement is subjected to a number of mechanical, thermal and chemical stress regimes over its lifetime. Therefore, next-generation wellbore cement formulations need to be evaluated in conditions relevant to these environments. In this work, we investigate the mechanism of the alteration of a novel self-healing polymer-cement composite after exposure to a CO2-rich environment by using synchrotron-based X-ray Fluorescence (XRF), X-ray absorption near edge structure (XANES), and scanning electron microscopy coupled with energy dispersive spectroscopy. Results showed that a chemical alteration of the polymer-cement follows the rim carbonation mechanism, similar to conventional cement, although carbonation takes place to a lesser extent in polymer-cements despite the higher porosity. Along with detailed mechanistic insights on carbonation in polymer-cement composite, the performance of these in CO2-rich environment is further studied using standard compressive strength analysis.

Influence of Lowered Temperature on Efficiency of Concrete Repair with Polymer-Cement Repair Mortars.

This paper presents research dealing with the evaluation of the efficiency of concrete repairs with polymer-cement mortars made at low temperatures with two types of cement and modified by copolymer acrylic-styrene. The low temperature used for the tests, of about 8 °C, is representative for Central Europe, and was established based on the analysis of mean temperatures in Poland during the last 45 years. A comparative analysis of the basic properties of the mortar tested, important from the point of view of repair efficiency, was performed, i.e., flexural and compressive strength, modulus of elasticity, adhesion to the substrate, and porosity for mortars applied and cured at 8 °C and 21°, respectively. The studies were conducted using standard methods and supported with an assessment of microscopic images (1000× magnification). It was shown that when the temperature of polymer-cement composite (PCC) mortar application is lowered to values slightly exceeding the minimum film-forming temperature (MFT) temperature of the polymer modifier, the type of cement determines the effectiveness of the repair. Only for PCC mortar with CEM I sulfate-resistant types of cement was it possible to achieve the same strength parameters as at 21 °C, during 28 days of mortar curing, and at a lowered temperature. Starting from day seven of setting at both above-mentioned temperatures, a relation between the values of the flexural and compressive strength expressed as a quotient of these values, amounting to ca. 0.14–0.19, was found.

Progress and challenges in self-healing cementitious materials

Concrete is a very common material consisting of a mixture of aggregates (sand, gravel, crashed rock) and paste (cement, water and additives). The paste component, made up mainly of cement and water, degrades with time. Despite this shortcoming, research toward the next-generation cement and concrete materials has intensified in the past 10–15 years. Self-healing cementitious materials, in particular, are a research area that has attracted a great deal of attention. A number of novel formulations have demonstrated an increase in mechanical and chemical stability with respect to conventional Portland cement, through the addition of inorganic, organic, and even biological additives. This review reports on the latest developments in cement research related to the synthesis of cement and concrete materials with autogenous healing and/or self-healing capability. These include geopolymers, engineered cementitious materials, bacterial cement composites, microencapsulated self-healing materials, self-healing assisted by shape-memory alloys, and polymer–cement composites. This work describes the performance of each cementitious material and the mechanism responsible for healing, including a section on atomistic simulations and modeling of cementitious materials. A detailed understanding of various cement technologies with autogenous healing and self-healing properties, including their strengths and weaknesses, is critical to determine the areas where new development is needed to enable novel, energy-efficient, and environmentally responsible cement and concrete solutions. To this end, molecular simulations can play a significant role and have already demonstrated promise in achieving an atomic level view of interactions between cementitious materials and other add-on compounds, such as carbon nanotubes or polymers for enhanced reinforcement or autonomous healing properties. Next-generation construction materials are led by self-healing cement and concrete. With a wide range of autonomous healing strategies based on smart polymers and alloys, biotechnology, and molecular modeling, advanced cementitious materials are becoming part of our daily lives. This work represents a comprehensive review on progress, challenges, and opportunities in the development of these materials for infrastructure and beyond.

Effect of Different Curing on Mechanical Properties of Polymer - Cement Composites

The polymer-cement composites have special specifications, high strength compared to their low density, satisfactory deformability, deterioration resistance, tailored design. This enables the construction of new elements and the structural recovery of the existing parts made of traditional materials. two sets of mixtures were prepared that consist of mortar and polymer to produce the polymer-mortar composites were cured at room temperature and post-cure at 50 ͦ C. The set includes mortar (cement-sand) without water. The polymer was epoxy which is added to the mortar after mixing the hardener with resin. Set consists of the proportion of polymer (15, 20 and 30%).The effect of the polymer was studied on both sets by computing the density also measuring the hardness, compressive strength and flexural strength of specimens. The addition of polymer showed an improvement in these properties and post-cure have improvement properties .the lowest percent of the polymer showed The highest results were density 1133.3 kg/m3, the hardness of composites was 97.28 shore D. the compressive strength was about 100.816 MPa and the value of flexural strength was about 29.418 MPa.

INFLUENCE OF AGRICULTURAL BIOMASS FLY ASH CEMENT SUBSTITUTION ON THE CARBONATION OF CEMENT AND POLYMER-CEMENT COMPOSITES

Practical use of a new type of combustion waste such as an agricultural biomass fly ash in the building materials requires an assessment of its performance. The paper presents the investigation results on the influence of cement substitution (5% and 30%) by this ash on the cement and polymer-cement composites resistance to carbonation. The composites resistance was assessed on the basis of carbonation process over time (up to 360 days) using the phenolphthalein method. It was found that fly ash from agricultural biomass increases the susceptibility to carbonation of polymer-cement composites to a lesser extent than cement composites compared to composites containing siliceous coal fly ash.

Modern cement-based waterproofings for hydraulic structures protection

Hydrotechnical structures of water management and land reclamation complex from the moment of commissioning are subject to aggressive environmental effects: hydrostatic water pressure, alternating freezing and thawing, moistening and drying, corrosive action of salts dissolved in water, dynamic action of ice. The trouble-free operation of structures with long-term aggressive environmental factors is possible only in the case of providing their protection (reinforcement) with effective insulating, anticorrosive, high-strength, wear-resistant and cavitation-resistant composite materials.
 Waterproofing coatings, which arrange on the surface of reinforced concrete structures, plays an important role in ensuring operational reliability and durability of hydraulic structures. Having rather low cost of waterproofing coatings as compared to the cost of structures the fate of their responsibility in terms of ensuring the durability and operational reliability of the structures is high enough. Waterproofing coatings serve as a barrier to an aggressive environment and thus protect structures from destruction.
 Notwithstanding the importance of waterproofing protection, this issue is not given due attention today. Therefore, filtration of water through the structures of water-reclamation facilities is a common phenomenon. Water filtration through damaged concrete of hydraulic structures causes dissolution and leaching of water with calcium hydroxide (leaching), which further causes the decomposition of other components of the cement stone and leads to the dilution of the concrete structure and to the strengthening of drip filtration. Drip filtration increases over time, then jet filtration develops, which can lead to complete destruction of the structure. Therefore, providing waterproofing protection of structures is an important engineering task in their construction and operation.
 One of the determining factors for the effectiveness of waterproofing is the right selection of material. Traditional waterproofing materials on a bituminous basis have insufficient physical-mechanical properties and durability. However, modern technologies of waterproofing protection of concrete and reinforced concrete structures are based on the use of effective composite materials with high physical, mechanical and protective properties. Depending on the type of binder, waterproofing materials can be based on bitumen, bituminous-mineral, bitumen-polymer, coal, polymer compositions, as well as on the basis of cements and polymer cements.
 The selection of a waterproofing material should be made taking into account the specifics of the operating environment and the conditions of use. The most common in the construction of gluing roll materials on a bituminous basis can only partially satisfy the need for hydraulic engineering, as for the waterproofing of hydraulic structures. There are high requirements with regard to the aggressiveness of the environment and their low repairsability.
 The most promising for use in the technology of arrangement of waterproofing coatings on concrete and reinforced concrete structures of hydraulic engineering facilities of water management and reclamation complex are polymer cement mixtures, binders in which are Portland cement modified with polymer latex dispersion powder (for two-component).
 Creation of effective polymer cement waterproofing compositions is based on optimization of the ratio of interpenetrating meshes of polymers and crystal matrix hydrates. Modification of cement systems by polymers allows to increase adhesion and deformation characteristics, fracture resistance and corrosion resistance of polymer cement composites. Depending on the components included in the mixture, waterproofing coatings may be rigid or elastic. Rigid formulations are a water-mixed, dry mixture that includes a vinyl acetate copolymer and designed for waterproofing concrete and reinforced concrete structures with low water filtration. Elastic waterproofing materials are usually two-component. These materials are used for waterproofing structures that are susceptible to deformation, as well as surfaces with a high degree of water filtration and where cracks up to 1 mm are formed. Elastic two-component formulations are dry mixtures with the addition of redispersible polymer powders. They consist of two components: a dry mixture of modified fine cement and aqueous latex polymer, usually acrylic.
 The content of polymer latex in the mixture has a significant effect on the rheological properties of polymer-cement mixtures and on the physical and mechanical properties of waterproofing coatings made of them. Thus, the introduction of a redispersible polymer powder Axilat L 8262 in a waterproofing mixture in the amount of up to 10% by weight of cement increases the mobility of the mixture from 3,2 to 8,0 cm, increases the adhesive strength of the coating to concrete from 0,45 to 1,95 MPa, increases its bending strength from 6,9 to 7,9 MPa, reduces its water absorption in 24 hours from 7,02% to 0,35% .

Study of the mechanical and corrosion resistance properties of ferrocement

Ferrocement is a lightweight, homogeneous and versatile structural material which is made as a composite of wire mesh and tightly wound skeletal steel impregnated with cement mortar. Ferrocement excels reinforced concrete system because of the minimal formwork and scaffolding requirement, possibility of casting elements on ground at site, fast erection and decrease in time of construction, and savings in the overall cost. Even though ferrocement has been used for various applications since 1800s, the technology is not properly standardized and unconventional local construction practices continue to prevail. Due to this and the poor workmanship, the durability of ferrocement structures has become an issue. Factors such as the corrosion of mesh and or the skeletal reinforcement, improper mix design etc. lead to deterioration of ferrocement structures. The major corrosion prevention methods used in reinforced concrete systems are application of coating to the steel such as epoxy or cement polymer composite (CPC) coatings and use of corrosion inhibitor in the cementitious matrix. The present study evaluates the possibility of adoption of these corrosion prevention strategies in ferrocement. The objective of the study is to analyse the flexural and corrosion resistance behaviour of four different types of ferrocement systems. The systems studied include ferrocement with normal welded mesh in normal cement mortar(WM); CPC coated weld mesh in normal cement mortar (CPC); normal weld mesh in cement mortar with corrosion inhibitor (WM CI); and CPC coated weld mesh in cement mortar with corrosion inhibitor (CPC CI). The corrosion analysis is done by using half cell potential measurements following ASTM C876-15 and flexural analysis by flexural strength test following IS 516: 1959. The results indicate that usage of both corrosion inhibitor and CPC coating can prominently increase the durability of ferrocement structures.

Cement-based material modified by in-situ polymerization: From experiments to molecular dynamics investigation

A critical obstacle to the design of more sustainable and durable cement-based material is its relatively low flexural strength and susceptibility to brittle fracture. In this paper, a novel cement-polymer composite was developed by in-situ crosslinking polymerization of sodium acrylate monomers and the synchronous hydration of ordinary Portland cement. In-situ polymerization in cement paste can significantly shorten the initial setting time to less than 10 min and the cement hydration process is inhibited during ions dissolution and dynamic balance stage. In-situ polymerization produces the intertwined inorganic-organic network, bridging the defections of cement paste at micro-level. Due to the formation of double-network structure, as compared with OPC, the optimized design can obtain 15% improvement in compressive strength and 200% improvement in flexural strength. The monomer to cement ratio and initiator to monomer ratio are essential factors for improving mechanical properties of composites. Furthermore, molecular dynamics is utilized to investigate the structure, dynamics and mechanical properties of interfacial bonds between sodium polyacrylate molecule and cement hydrate. The carboxyl groups in the polyacrylate provides oxygen sites to accept H-bond from protonated silicate tetrahedron and neighboring water molecules. The calcium and sodium ions play bridging role in connecting oxygen atoms in functional group and oxygen in silicate chain of calcium silicate hydrate. The H-bond, O–Ca–O and O–Na–O salt bridges strengthen the interfacial bonding, which inhibits the cracking development and enhance the ductility of cement-polymer composite. Hopefully, this in-situ polymerization strategy can provide valuable insights on high-performance cement-based composite design.

Self-repairing polymer-modified cements for high temperature geothermal and fossil energy applications

In this work five novel polymer-cement composite formulations were prepared and evaluated as potential cementitious material alternatives to conventional wellbore cement. These cement composites were cured at 200 °C and their mechanical properties, including compressive strength, Young modulus, shear bond strength to steel casing, and self-healing and re-adhering (to steel) capability, all at 20 °C. Thermal stability was also evaluated by curing these cement materials at 200 °C for up to one month followed by determining their mineralogy and chemical composition by X-ray diffraction spectroscopy, 13C NMR, and total organic carbon. Permeability analysis was performed before and after healing a longitudinal fracture on unmodified cements and polymer-cement composites with the later showing lower (2nd/1st) permeability ratios with respect to base cements. Furthermore, a reduction in permeability of up to 80X on average with respect to their unmodified (base) cement, was observed in two polymer-cement formulations suggesting that the introduction of these polymers bring about self-healing capability. Two of the best performing polymer-cement composites were exposed to 30-day curing period at 200 °C showing that their self-healing capability is maintained. The stability of the composite was dependent on the thermal stability of the polymer material which was demonstrated by measuring total organic carbon and NMR spectroscopy. These advanced polymer-cement composites with higher ductility and self-healing capability could be used as alternative wellbore cement materials for geothermal and fossil energy applications.

Effect of Polymer Addition on Performance of Portland Cement Mortar Exposed to Sulphate Attack.

Resistance to degradation contributes greatly to the durability of materials. The chemical resistance of polymer-cement composites is not yet fully recognized. The goal of the research presented in this paper was to assess the performance of polymer-cement mortars under sulphate aggression, as compared to unmodified cement mortar. Mortars with polymer-to-cement ratios from 0 to 0.20 were stored in either a 5% MgSO4 solution or distilled water for 42 months. During this time, changes in elongation, mass, and compressive strength were determined. The results of these investigations, together with the visual and microscopic observations, allowed us to conclude that polymer–cement composites demonstrated better resistance to the attack of sulphate ions than unmodified cement mortar, even when using Portland cement with enhanced sulphate resistance.

Polymer-cement composites with adhesion and re-adhesion (healing) to casing capability for geothermal wellbore applications

Deterioration of cement/casing adhesion in wellbore scenarios can result in unwanted and potentially harmful leakage with the potential of serious repair costs. In this work, the authors explore the use of self-healing polymers added to conventional wellbore cements as a way to bring about self-healing and readhering (to casing) properties to the composite. Self-healing capability was demonstrated by permeability analysis showing that polymer-cement composites reduce flow by 50–70% at cement bulk and at the cement/steel interface. Use of atomistic simulations imply that these polymers have good wetting properties on the steel surfaces. Interactions between steel/polymer and cement/polymer are complementary, resulting in a wider range of bonding patterns. Cracks seem to expose under-coordinated sites that result in more bonding interactions, which agrees well with the permeability measurements showing high degree of healed cracks and cement-steel interfacial gaps together with an overall increased in structural integrity of these advanced polymer-cement composite materials.

Electrokinetic Properties and Aggregative Stability of Polymer–Mineral Dispersions for 3D Printing in Building

An optical microscopy study showed that combination of a polymer with mineral components leads to coagulation of the dispersion. The electrokinetic potential and the modal particle diameter of multicomponent systems based on cement (or chalk) and polyvinyl acetate were found to depend on the polymer to mineral ratio. Modification of these systems with synthesized phloroglucinol-furfural modifier made it possible to obtain aggregation-stable polymer–mineral systems. It was proven that the electrostatic factor is not critical to the aggregative stability of polymer–mineral systems. As the modifier concentration increased from 0 to 0.3% of dry weight of the disperse phase, the absolute value of the ζ potential slightly increased to –30 mV. The peptization of the polymer–cement aggregates occurred almost to the size of primary particles of 6–7 μm. The polymer–mineral dispersions are the basis for polymer–cement composites, which are promising materials for innovative construction technology.

Insights into the physical and chemical properties of a cement-polymer composite developed for geothermal wellbore applications

To isolate injection and production zones from overlying formations and aquifers during geothermal operations, cement is placed in the annulus between well casing and the formation. However, wellbore cement eventually undergoes fractures due to chemical and physical stress with the resulting time and cost intensive production shutdowns and repairs. To address this difficult problem, a polymer-cement (composite) with self-healing properties was recently developed by our group. Short-term thermal stability tests demonstrated the potential of this material for its application in geothermal environments. In this work, the authors unveil some of the physical and chemical properties of the cement composite in an attempt to better understand its performance as compared to standard cement in the absence of the polymer. Among the properties studied include material's elemental distribution, mineral composition, internal microstructure, and tensile elasticity. Polymer-cement composites have relatively larger, though not interconnected, levels of void spaces compared to conventional cement. Most of these void spaces are filled with polymer. The composites also seem to have higher levels of uncured cement grains as the polymer seems to act as a retarder in the curing process. The presence of homogeneously-distributed more flexible polymer in the cement brings about 60–70% higher tensile elasticity to the composite material, as confirmed experimentally and by density-functional calculations. The improved tensile elasticity suggests that the composite materials can outperform conventional cement under mechanical stress. In addition, calculations indicate that the bonding interactions between the cement and polymer remain stable over the range of strain studied. The results suggest that this novel polymer-cement formulation could represent an important alternative to conventional cement for application in high-temperature subsurface settings.

Photoactive polymer-cement composites for tannins removal from wastewaters

The TiO2 nanoparticles exhibit a remarkable photocatalytic activity against wastewater organic pollutants but their separation from the liquid media after the photocatalytic process is a very difficult task. The immobilization of TiO2 on different supports could be a good solution to simplify the cleaning stage after photocatalysis. In this study, two types of photoactive polymer-cement composites named macro-defect free cements (MDF) were obtained both by embedding TiO2 nanoparticles (Degussa P25) in the composite matrix and by photocatalyst deposition on the composites surface through the dip-coating method (in two types of recipe). The composites surface was characterized by SEM and XRD and photocatalytic efficiency was determined against wastewater tannins from woodworking industry. The tannin concentration was monitored by VIS-spectrophotometry, using the Folin-Ciocalteu method. All specimens evidenced photocatalytic activity. The process efficiency is dependent on the light wavelength and on the irradiation duration. The best results were obtained when the MDF containing embedded TiO2 nanoparticles were exposed to 254 nm light (92% after 6 h). Under higher wavelength light irradiation (365 nm), the photocatalytic efficiency decreased to 48.2% for the MDF containing embedded TiO2 and to 39.85% for the MDF coated with a TiO2 film. The obtained results are promising both in terms of environmental protection, considering the persistence of tannins in wastewaters and the difficulties of their removal through the classic methods, and in terms of MDF property improvement, considering its potentially very large range of applications.

Influence of cement substitution by calcareous fly ash on the mechanical properties of polymer-cement composites

The aim of the presented research was to determine the influence of cement substitution with calcareous fly ash on the mechanical properties of polymer-cement composites. Coal combustion products such as calcareous fly ash have been already used in traditional cement composites as a part of cement and considered potential additions for concrete but its introduction into polymer-cement composites is still under preliminary investigation. The morphology of fly ash causes problems with its compatibility with polymer-cement binders but its insertion into those building materials is another way to utilize mineral combustion products that are cumbersome in storage and recycling. The influence of the mineral addition on polymer-cement composites containing 20% of polymer was especially taken into consideration. Mechanical properties of polymercement mortars modified with calcareous fly ash were tested after 28 and 90 days of curing. As a part of preliminary study, activity index of mineral addition was determined. Polymer-cement composites containing calcareous fly ash were characterized by higher flexural and tensile strength comparing to standardized mortar, even for the mortars containing 40% of mineral addition. The negative effect of the polymer-cement composites modification with calcareous fly ash was especially observed on the compressive strength of this composites.

Effect of the degree of corrosion on bond performance of Cement Polymer Composite (CPC) Coated steel rebars

Currently, large infrastructures (bridges, highways, etc.) are designed for more than 100 years. To achieve long service life, coated rebars (mostly, cement polymer composite (CPC) coated rebars) are being used to enhance the corrosion resistance. However, inadequately coated rebars can lead to premature corrosion. This can also affect the bond between the rebar and the concrete. To assess the effect of CPC coating on bond strength, pull-out specimens of (150×150×100) mm with 12 mm diameter rebar with 100 mm embedded length were cast and tested. For this, three replica specimens with two types of reinforcement namely, i) Uncoated steel ii) CPC coated steel were cast. To induce corrosion, additional five specimens with CPC coated steel rebars were cast with premixed chloride and cured for 28 days. During the curing period, continuous monitoring of corrosion potential and rate was done and degree of corrosion was assessed. The effect of degree of corrosion on bond of steel-concrete-coating interface was quantified. The CPC coated rebars without corrosion exhibited 10% bond reduction. CPC coated rebars with corrosion exhibited 30-70% reduction in bond strength. Also, the corrosion is found to adversely influence the stiffness of the bond.

The influence of the mineral additives on the carbonation of cement composites

In practice, it is usually assumed that the durability of basic structural elements should not be less than the expected durability of the building. Reinforced concrete constructions, designed and made with quality requirements, are durable. However, each reinforced concrete structure, from the moment it is made, is gradually degraded, and one of the reasons for its destruction is often the corrosion of the concrete and hence the reinforcement. For the long-term use of reinforced concrete constructions, the most important thing is that a thin protective layer on the steel surface protects the reinforcing steel from rusting. One of the most common causes of corrosion in the reinforcement is the damage of this layer as a result of the carbonation of the concrete cover. The rate of carbonation depends on many factors, such as concrete compactness, concentration of CO2, concrete moisture, content of free calcium dioxide, content of mineral additives, etc. When the carbonated layer reaches the steel reinforcement layer, protection of steel from corrosion ceases to exist. The aim of the presented research was to determine the influence of mineral additives used as a compound of concrete on the carbonation of concrete and polymer-cement composites.

Polymercement Composites in Environment with Increased Concentration of Sulfate Ions and Extreme Temperatures

Polymer cement matrix based materials were designed within the frame of the research presented in this paper. Resistance of these materials to combination of several adverse factors was observed. Considerable proportion of input materials used were components from alternative material resources. Primary binder was replaced with fly ash and blast furnace slag. Dominant proportion of filling mass was taken up by agloporite – porous aggregate produced by self-baking from fly ash. The focus of the research was assessment of degradation of materials after long-term exposition (up to 90 days) to increased concentration of sulphate ions and high temperatures. Degree of degradation was evaluated on the basis of physico-mechanical and physico-chemical tests combined with microstructural analyses.

Photoactive glazed polymer-cement composite

Macro defect free cements (MDF), a kind of polymer-cement composites, are characterized by remarkably high mechanical properties. Their flexural strengths are 20–30 times higher than those of conventional cement pastes, nearly equal to that of an ordinary steel. The main drawback of MDF cements is their sensitivity to water. This paper presents a method to both diminish the negative impact of water on MDF cements mechanical properties and to enlarge their application by conferring photoactivity. These tasks were solved by glazing MDF cement with an ecological glaze containing nano-particles of TiO2. Efficiency of photocatalytic activity of this material was tested against methylene blue aqueous solution (4.4mg/L). Influence of the photocatalyst concentration in the glaze paste and of the contact time on the photocatalysis process (efficiency and kinetic) was studied. The best obtained photocatalysis yield was of 97.35%, after 8h of exposure to 254nm UV radiation when used an MDF glazed with 10% TiO2 in the enamel paste. Surface of glazed material was characterized by optic microscopy, scratch test, SEM, XRD, and EDS. All these properties were correlated with the aesthetic aspect of the glazed surface aiming to propose using of this material for sustainable construction development.

Polymer-Cement Composites with Self-Healing Ability for Geothermal and Fossil Energy Applications

Sealing of wellbores in geothermal and tight oil/gas reservoirs by filling the annulus with cement is a well-established practice. Failure of the cement as a result of physical and/or chemical stress is a common problem with serious environmental and financial consequences. Numerous alternative cement blends have been proposed for the oil and gas industry. Most of these possess poor mechanical properties, or are not designed to work in high temperature environments. This work reports on a novel polymer-cement composite with remarkable self-healing ability that maintains the required properties of typical wellbore cements and may be stable at most geothermal temperatures. We combine for the first time experimental analysis of physical and chemical properties with density functional theory simulations to evaluate cement performance. The thermal stability and mechanical strength are attributed to the formation of a number of chemical interactions between the polymer and cement matrix including covalent bonds...