Interface properties of nano-silica modified waterborne epoxy-cement

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Interface properties of nano-silica modified waterborne epoxy-cement repairing system Bo Pang, Yunsheng Zhang, and Guojian Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04092 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Interface properties of nano-silica modified waterborne epoxy-cement repairing system Bo Panga,b, Yunsheng Zhanga,b,* , Guojian Liua,b

a School of Materials Science and Engineering, Southeast University, Nanjing 211189, China

b Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing 211189, China

ABSTRACT Nowadays, numerous concrete structures are urgently needed to be repaired and strengthened due to the severe safety and durability. In this study, a novel type of silane-based interfacial coupling agent (ICA) is prepared by modifying silane coupling agent (SCA) with hydrothermally treated nano-silica (HTNS). The effect of ICA

on

the

cement

hydration

and

crystalline

form

as

well

as

the

hydrolysis/condensation extent of siloxanes is illustrated. The bonding strength, morphology and propagation of the interface cracks, and the interfacial ductile fracture characterization are investigated. Besides, the coupling mechanism of the ICA in the repaired interface is explored. The results show that HTNS effectively catalyzes SCA hydrolysis and condensation to form Si-O-Si bonding in a neutral environment. The application of ICA on old cementitious matrix not only significantly improves the bonding strength and toughness of the repair interface, but also mitigates the negative effect of dealcoholization of siloxanes on the hydration of 1

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the cement. The repaired interface exhibits simultaneously stiffness, toughness and multi-cracking features in the process of straining. On a micro-level, ICA consumes portlandite of cement hydration and fine-crystallizes to form a layered plug structure in the repaired interface. With the continuous dissolution of portlandite, the nano-silica in ICA forms a fibrous, stable product with ions and enhances the interfacial pore plug effect.

KEYWORDS Cement, repair, silane coupling agent, nano-silica, waterborne, epoxy resin, interface 1 INTRODUCTION As the most widely used construction material around the world, concrete undertakes major infrastructure projects such as bridges, tunnels, dams, railways, and wharves. In recent decades, the amount of concrete starts to mushroom because of the rapid economic development and urbanization, especially in China. As the serviced life of concrete extends, however, numerous inevitable durability issues like cracks, erosion of large area scale or even collapse are frequently seen, which has become a threat to the safety and reliability of construction

1-3

. The life expectancy of about

600,000 bridges in America is 42 years, with an estimated $ 76 billion in needs to deal with the damage problems. From the ASCE’s Failure to Act report on the costs of poor infrastructure, by 2020, deficient bridges and pavement will cost $ 58 billion. If left unattended, those costs will jump to $ 651 billion by 2040. Recently, On March 2


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07, 2018, Senate Democrats have unveiled a proposal to invest $1 trillion in American infrastructure, which includes: $140 billion to ensure Highway Trust Fund solvency over the next decade; plus an additional $140 billion to repair the roads and bridges etc. 4. Similarly, repair and retrofit costs are estimated at $67 billion for Chinese infrastructure in 2016 5. In Europe and some Asian countries, the annual repair cost has exceeded the cost of newly built construction 6. As deterioration of the present concrete constructions become more and more severe with time, repair and retrofit become the most cost-effective solution. Therefore, the concrete repairing industry has been rapidly diversified and developed in recent decades. Various repairing systems spring up, such as cement-based repairing system 7, polymer-based repairing system 8, cement-polymer composite repairing system 9, steel plates fiber repair system

11

10

and carbon

etc. Epoxy mortar is with less shrinkage, superior mechanical

strength and adhesion property, high chemical stability, wide adaptability and good workability. Therefore, it is commonly used in the field of concrete construction repairing 12-15. Since the 20th century, the utilization and production of solvent-based resins have been restricted or even prohibited by strict environmental protection laws and regulations all over the world. Types of waterborne epoxy modified cement-based repair materials (WECM) have been further explored and applied and show excellent mechanical performance and durability

16-18

. However, even if WECM contains a

considerable amount of waterborne epoxy resin (WEP), the organic-inorganic bonding strength and ductility with old concrete is still insufficient or even become 3



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unstable in the presence of water and cement, resulting in a weak and brittle bonding interface between the old concrete and repairing materials. Under strain and continuous fatigue load, the interface becomes the stress concentration area and ends in failure. As the interface issue is the main reason of decrease in the bearing capacity of the repaired structure and the exfoliation of repair material, therefore, the bonding treatment between old and new concrete has become an inevitable challenge in the field of construction repairing 19. The exposed surface of old concrete is mainly covered with CaCO3 (calcite), C-S-H gel, Ca(OH)2 (portlandite) etc. with a loose and porous structures. To form a sound and firm binding with WECM, it is necessary to create an organic-inorganic covalent bond with good wettability. Silane coupling agents (SCA) are additives which can effectively create Si-O-Si covalent bonds between the organic and inorganic interfaces 20. By applying SCA, the interface bonding can be promoted from mechanical binding and electrostatic adsorption into a covalent bond. The application of SCA first appeared in the surface treatment of rubber filler, dental bonding and ceramic modification etc. 21-24. In recent years, the research on the application of SCA in the repairing of concrete structures has also been reported. Qiao et al.

25

improved

the performance of rubber modified cement concrete by using SCA coated rubber particles. The compressive and split tensile strength of the concrete coated rubber improved by 10 to 20% compared with control group. The bond quality of the repaired interfacial transition zone has been studied by Xiong et al. 26 who used SCA aqueous solution to coat the surface of old concrete matrix. They observed a 4


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noticeable increase of bond strength as a results of pull-off bond strength. Wang et al. 27

evaluated the bond strength, micro-hardness, microstructure and surface

morphology of interface between resin and cement paste. The results show that the coupling agent can improve the interface bond strength from 54% to 117% at 7 days to 28 days old. Andrew Stewart et al.

28

used X-ray photoelectron spectroscopy and

static contact angle measurements to study the interaction between different SCA and cured cement paste. The results of an increase in bridging silicon and oxygen atoms relative to untreated samples demonstrated successful silane condensation and that a covalent bond was formed between the cement paste and silanes. Currently, the recognized coupling mechanism is that the SCA molecules interact with the remaining hydroxyl groups in the surface of the old concrete to form a covalent bond, which promotes an increase in the bonding strength

29-30

. Besides, the siloxane in the SCA

molecules works only if the Si-O-Si covalent bonds are formed with the remaining hydroxyl groups on the bonding surface under certain conditions

20, 24, 31

. The current

means of using SCA on old cementitious matrix, however, are mainly direct coating or applying without modification, which results in the formation of a large amount of alcohol produced by the hydrolysis. The hydration of cement is severely hindered and delayed along with the alcoholization progress of SCA. Thereby, an unsubstantial layer in the vicinity of the repair interface is then formed to further weaken the bonding effect. In summary, the interfacial modification is the key issue that restricts the large-scale use of WECM as a structural repair material and safeguards the service

life of repaired concrete structures. 5



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The above review features promising developments in SCA that applying SCA on old cement paste could lead to a promising new approach for the organic-inorganic interface bonding. So far, the theoretical researches on the bonding mechanism of WECM mostly focus on the exploration from macroscopic (mechanical properties, adhesion, durability, etc.) to micro-nanoscale (morphology etc.), so it is still an initial stage. On the one hand, avoiding the negative effects of SCA on the chemical reaction of cement is necessary. On the other hand, achieving the coupling effect of SCA and the simultaneous promotion of cement hydration in a moderate chemical environment is also a thorny problem. In the field of cement-based building materials, although the application of polymers has appeared since long ago

32-33

, the in-depth understanding

and research of chemical structures and the chemistry between organic and inorganic surfaces still need to be viewed scientifically and dialectically. The influence of the SCA on the hydration of cement, the morphology of the interface cracks, the characterization of the ductile fracture and the coupling theory of the SCA on the surface of the cement paste are urgent to be studied. More importantly, as a cement-based repair surface, the excessive generation of portlandite and porous products will lead to degradation of the interfacial repair function due to more moisture in the interface

34

. Alcohols produced by directly using SCA will further

promote the degradation of the interfacial bonding. Utilization and certification of the catalytic SCA hydrolysis by HTNS, the filling effects of cracks in the repair interface, and the improvement of the crystal structure and chemical structure of portlandite from the chemical point of view are very important. 6


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In this study, the 3-glycidoxypropyltrimethoxysilane (GPTMS) is applied as SCA. Due to the high catalytic activity of nano-silica on the properties of various types of silica-polymer composites

35-36

as well as the accelerating effects for

beneficial calcium–silicate–hydrate (C-S-H) formation in cement hydration

37-38

. The

hydrothermally treated nano-silica is used to catalytic the hydrolysis and condensation of SCA and to prepare the novel interface coupling agent (ICA) which not only significantly improves the bonding strength and toughness of the repair interface, but also avoids the negative effect of dealcoholization of SCA on the hydration of the cement. To illustrate the effect of ICA on the cement hydration rate, crystalline form, and hydrolysis/condensation extent of siloxanes, conductivity-temperature-salinity simultaneous test, X-ray diffraction (XRD), thermal gravimetric analysis (TGA), nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC) etc. are used. Meanwhile, the adhesive strength between the old cement matrix and WECM has been studied in terms of tensile strength and flexural strength after treating the matrix with ICA, WEP and OPC respectively. Furthermore, Atomic Force Microscope (AFM), scanning electron microscope (SEM) and other means are used to characterize the morphology, viscoelasticity, etc. of the interfacial properties in microdomains. The multiple cracking and crystal evolution rules in the repair interface and characterization coupling mechanism of ICA with cement paste at different scales are further studied. This study provides support for the development of efficient organic-inorganic interface bonding materials in the field of architectural repair. 7



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2. MATERIALS AND METHODS 2.1 Cement Paste Matrix and WECM Samples Preparation Cement paste matrix samples: Step 1, the ordinary portland cement (OPC) and water were mixed with a constant water to cement (W/C) ratio of 0.29 into a paste (stirring speed of 600 rpm, 10 minutes). Step 2, the paste was poured into the molds (2cm*2cm*8cm and tensile mold with a tensile cross-sectional area of 4 cm2 shown in Figure 1 b and c) and vibrated for 10 seconds. Step 3, molded samples were placed into a curing condition of 98% RH at 25 °C. WECM samples: Step 1, the OPC and water were mixed with a constant water to cement (W/C) ratio of 0.29 into a paste. Step 2, the waterborne epoxy resin was mixed with the matched hardener into waterborne epoxy resin mixture (WEP). Step 3, the WEP was mixed with OPC paste with a polymer to cement (P/C) ratio of 0.4 within 20 minutes. Each mixing procedure lasted for 10 minutes at a speed of 600 rpm. 2.2 Interface Coupling Agent (ICA) Preparation Step 1, HTNS: nano-silica was mixed with water with a ratio of 1:19 and stirred by a rotor at 80 °C for 8 hours. Step 2, SCA of 3-glycidyloxypropyltrimethoxy silane (GPTMS) with purity of 8


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97% wt. was mixed with HTNS by the ratio of 1:1 and stirred by a rotor at room temperature for 20 hours. The ICA was then prepared. As a comparison, SCA and non-hydrothermally treated nano-silica were mixed with the same ratio as the control group (CICA). 2.3 Repairing Procedure on Cured OPC Matrix Step 1, after the OPC matrix samples were tested by flexural or tensile tests, the failure-section of the samples was cleaned with a brush. Step 2, the failure-section was evenly sprayed with a layer of ICA with a spray can and been put in the molds. Step 3, after drying for 10 minutes drying at room temperature, the WECM was poured on the failure-section after which the repaired samples were then placed into a curing condition of 98% RH at 25 °C. The ICA preparation process and the repair process are shown in Figure 1 (a).

9



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Figure 1 Flow diagram of the ICA preparation and repairing procedure (a) and schematic diagrams of tensile strength test (b) and flexural strength test (c) and slant shear strength test (d). 2.4 Aging and Durability The durability and anti-aging properties of the repaired samples (shown in Figure 1 d) are mainly based on the slant shear strength of the samples before and after the aging tests (immersion aging tests, freezing-thawing aging tests, artificial light aging tests). All the samples are with the cement paste matrix of 1 year old and repaired and cured for 28 days before all the tests. The slant shear strength test method refers to the Standard Test Method for Bond Strength of Epoxy-Resin Systems Used With Concrete, ASTM C882/C882M 39. 10


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Immersion aging tests: All the samples (repaired for 1 month) in the immersion aging tests are immersed in fresh water, alkali solution (5% wt. aqueous sodium hydroxide solution) and acidic solution (10% wt. aqueous hydrochloric acid solution) 40

. Freezing-thawing aging tests: To evaluate the resistance of repaired samples to

freezing and thawing damage, both fresh water and salty water are considered. The artificial prepared substitute ocean water (compound /concentration, g/L; NaCl, 24.53; MgCl2·6H2O, 11.11; Na2SO4, 4.09; CaCl2, 1.16; KCl, 0.695; NaHCO3, 0.201; KBr, 0.101; H3BO3, 0.027; SrCl2·6H2O, 0.042; NaF, 0.003) 41. Artificial light aging tests: The resistance to light aging of all the samples are tested by XENON TEST CHAMBER (Q-SUN XE-3) in both dry and moist condition refer to the standard ASTM D904−99 42. Different interfacial agents containing OPC, ICA, commercial epoxy interface agent (CEA) of SikafloorEpoCem-81 (A component) and type Sikafloor-156 type (B component) produced by Sika Corporation U.S. and WEP of CYDW-100 type (A component) and CYDHD-220 type (B component) produced by Petroleum & Chemical Corporation of China are prepared. Six samples are selected from each experiment and errors were calculated. The main procedure of each experiment is shown in Table 1. Table 1 The main procedure of each ageing experiment. Interfacial

Immersion aging test

Freezing-thawing aging test

Artificial light aging test

11



agent

alkali

acidic

water

substitute ocean water

solution

Page 12 of 53

water

solution

without water

water The samples are frozen in Condition the test samples water or salt solution from 16 for

OPC

The samples are placed

±2°C to -16±2°C within 4

ICA

in airtight boxes containing

hours and are thawed to 16 ±

90 days at a RH of

50 % and 98%, with the wavelength of 340 nm, the CEA

different corrosive solutions

2°C within 2 hours in a irradiance of 0.55 W/m2, the

WEP

for 90 days, 50±3°C.

single cycle, fresh water-300 black standard thermometer cycles,

ocean

water-100 of 63±3°C.

circles.

2.4 Conductivity-Temperature-Salinity Simultaneous Test To study the effect of HTNS and untreated SiO2 on the hydrolysis performance of SCA and the effect of ICA on the hydration of cement, the hydrolysis/hydration exotherm, conductivity and salinity of SCA and cement in solution were investigated by the conductivity-temperature-salinity simultaneous test. SCA and ICA were formulated into a solution of 500 mg/ml, respectively. Then the SCA solution was tested after the addition of distilled water (control), untreated SiO2 (CICA) and HTNS (ICA) immediately. The tests were ended until the conductivity is steady. Furthermore, cement paste with W/C of 9 was tested with adding 1 ml of SCA and nano-silica emulsion mixture (paste-CICA), 1 ml of ICA (paste-ICA) and 1 ml distilled water (paste-control), respectively. The conductivity, temperature, total dissolved solids, and 12


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salinity were tested in an insulated box to prevent the impact of the environment. 2.5 FTIR-TG-XRD Analysis FTIR was tested by a FTIR spectrometer (Nicolet 5700) with wave number range of 400-4000 cm-1 and resolution of 0.1 cm-1. To conduct the FTIR, 1 mg paste powder was mixed with 100 mg KBr to prepare slices. The thermal behavior of the cement paste and nano-silica before and after treated was analyzed using equipment for TG analysis with a Netzsch, TG209 F3 model. All the tested cases were carried out under the following condition: 50 mg samples, heating temperature 10 K/min, heating to 850 oC, platinum crucible, and ambient 100 cm3 of N2. The crystalline phases were analysed by means of XRD, using the Bruker, D8-Discover model equipment. Patterns were collected in the 2 theta scan range 10 to 70o using a step size of 0.02o and a scan speed of 0.25 s/step. 2.6 Meso-Microscopic Morphology Analysis To better observe the crack growth and multi-crack state at the interface layer during fracture, two glass slides were treated with ICA and then bonded together with hardener. 24 hours later, the bonded slides were splitting at different speeds and observed under a digital reflective-polarizing microscope (Axioskop-40POL Zeiss) at the microscopic scale. The morphology and hydration products on the fractured surfaces were observed and analyzed by scanning electron microscope (SEM, FEI 3D) equipped with energy dispersive spectroscopy (EDS). 13



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2.7 NMR-GPC Analysis 29

Si and 1H Nuclear Magnetic Resonance (NMR) with a prepolymer of PSR were

performed using an AVANCE III HD 600 Bruker Spectrometer (Bruker, Switzerland) using deuterated chloroform (ClD3) and deuterated water (D2O) as a solvent. Room temperature GPC was carried out on a PL-GPC220 (Polymer Laboratories) equipped with a PL Mixed-C column using tetrahydrofuran as solvent and eluent. 2.8 Macro Mechanics-AFM Analysis Tapping-mode and force spectrum test-mode with atomic force microscopy (AFM) measurements were performed to evaluate the surface roughness and viscoelasticity of the interface before and after fracture. Both topological and phase images were recorded with a Dimension ICON model device. The data sampling rate was 10 MHz and scan size was 250 µm2 to 1000 µm2. The values of root-mean-square roughness (Rq) and maximum height roughness (Rmax) were calculated over the AFM images.

14


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3. RESULTS AND DISCUSSION 3.1 Characterization of SCA Hydrolytic Condensation and Cement Hydration. The hydrolysis and condensation theory of SCA in acid/alkaline-base have been extensively studied and demonstrated 43-44. Okumoto 45 brought up a theoretical study that if hydroxy groups of the substrate are solvated by other water clusters, the activation energies could be very small. Hydrolysis of siloxanes in the neutral environment is much slower than acidic and alkaline environments. However, acidic and alkaline environments, on the other hand, will open epoxy groups and result in a reduced coupling efficiency 44. Therefore, the catalytic effect of nano-silica was used to accelerate the hydrolysis rate of SCA, and alcohol was easier to eliminate before mixing with the cement, thereby alleviating the negative effects. Thus, the conductivity, salinity, total dissolved solids, and temperature of SCA in different condition have been tested to find out the effect of hydrothermal treated/untreated of nano-silica on hydrolysis of SCA and effect of ICA on early cement hydration. As the siloxane is hydrolyzed to silanol, methanol is produced, the conductivity increases with hydrolysis heat releasing. As shown in Fig 2 (a), the period of SCA hydrolysis equilibrium in water is up to 4 days. With an obvious exothermic peak at an early stage (0 to 4 hours), which was related to the process of hydrolysis of the first siloxane group in SCA molecules. When one of the three siloxanes on a silicon atom is hydrolyzed to a silanol group, the chargeability of the silicon atom changes with the rate of hydrolysis slowing down. When the same reaction was triggered by adding 10%wt. CSCA, the conductivity increased sharply, and the rate was 5 times faster 15



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than that of the control one (shown in Figure 2 b). At 1,000 minutes, the exotherm was stabilized, although the reaction did not stop completely. Therefore, it can be speculated that the hydroxyl groups on the surface of nano-silica have an obvious catalytic effect on the hydrolysis of SCA. However, at about 1,100 minutes, the final values of conductivity, salinity and total dissolved solids of CICA sample were 25.21 µS/cm, 0.0063 %, and 12.45 mg/L, respectively, which were less than the control one (39.45 µS/cm, 0.0072 %, and 19.34 mg/L). Therefore, although SCA was catalytically hydrolyzed by nano-silica, the hydrolysis of the first siloxane played the major role in the exothermic peak, which reflected the reaction had not been completely finished. For the case of ICA (Figure 2 c), hydrolysis and condensation of SCA were further promoted. The curves show a strong correlation between the temperature and conductivity, which represents the hydrolysis is dominant. Because of the decrease of electrical conductivity, it is concluded that the hydrolysis of SCA with HTNS is accompanied by the condensation process. Three obvious exothermic peaks are shown in the whole reaction process probably represented the hydrolysis of three siloxanes in SCA molecules. When the reaction was balanced at 1,300 mins, the values of conductivity, salinity and total dissolved solids of ICA sample were 39.80 µS/cm, 0.0072 %, and 19.91 mg/L, respectively. Compared with the control group, the hydrolysis reaction was more complete, and the efficiency increased by 3 times. From the start of the reaction up to about 1500 minutes, the values of the control sample, CICA sample, and ICA sample were 1,895, 1,993 and 2,366, respectively. It demonstrates that the HTNS dramatically accelerates the hydrolysis efficiency of 16


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SCA. The reason for this phenomenon is that the hydrogen bonds of silanols adjacent to the surface are opened after hydrothermal treatment and replaced by water clusters, which accelerate the density of effective hydroxyl groups and water clusters and enhance the catalytic action of nano-silica 46-47. For the case of cement paste with W/C ratio of 9.0 (Figure 2 d), the curves of conductivity and temperature directly reflect the hydration process of clinker. At the first 500 minutes, the temperature rose up sharply to 20.9 oC, which represented the heating from dissolving of cement clinker minerals. Meanwhile, in the range of 0 minutes to 1,500 minutes, conductivity, salinity, and total dissolved solids showed a distinct linear relationship with the temperature changing. In this period (pre-induction, induction of cement hydration) the chemical changes of the solution were dominated by the dissolution of the ions 48-49

. Thereafter, the hydration of cement entered the acceleratory, deceleration and

stabilization periods to form the second exothermic peak. The solution ions gradually from the crystalline state from the saturated state, the temperature rose and the conductivity kept decreasing 49. When the cement was hydrated with adding 10% wt. CICA (Figure 2 e), methanol, which was slowly produced from SCA hydrolysis, had a significant prevention of the hydration of cement. Conductivity rose slowly to about 6,000 µS/cm in 3,000 minutes. It was assumed that SCA, which was not hydrolyzed completely, continued to hydrolysis throughout the cement hydration. At 500 minutes, the hydration exotherm peak value was about 20 oC, followed by an inconspicuous hydration acceleration period. Methanol produced by SCA hydrolysis had a less effect on cement early dissolution, but a greater impediment to the formation of cement 17



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hydrated minerals. Moreover, a large amount of non-hydrolyzed SCA presented in CICA, thus a clear peak in hydrolysis exotherm appeared before 100 minutes. For the case of ICA, compared with the paste-control group, ICA obviously promoted the hydration of cement (shown in Figure 2 f). After a transient induction period, the cement quickly entered the accelerated phase, initial coagulation and curing were completed within 2,000 minutes. The temperature curve integrated value reached upon 6,138 within 2,000 minutes, closes to 7,145 of control cement paste sample in 3,000 minutes. There was only a small exothermic peak within 100 minutes, indicating that very limited hydrolyzed siloxane groups were involved in the ICA, which minimizes hindrance on cement hydration by methanol.

18


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Figure 2. The conductivity, salinity, total dissolved solids and temperature curves of (a) SCA with distilled water (control), (b) SCA with un-hydrothermally treated nano-silica emulsion (CICA), (c) SCA with HTNS emulsion (ICA), (d) cement paste (paste-control), (e) cement paste with untreated nano-silica emulsion (paste-CICA) and (f) cement paste with ICA (ICA).

To study the final hydrolyzate and the extent of hydrolysis of SCA under diverse 19



Page 20 of 53

conditions, qualitative and quantitative analysis was performed by NMR-GPC Analysis. Figure 3 shows the GPC curves of SCA hydrolyzed for 7 days in the blank, distilled water, un-hydrothermally treated nano-silica emulsion, HTNS emulsion at 25oC, respectively. To better distinguish the molecular weight (MW) represented by each peak and the amount of each substance, the curve fitting analysis of the results was applied to differentiate each substance with different MW by different colors and then all the peak areas were calculated. SCA in the blank group (Figure 3 a) was tested with the MW distribution in the range of 200-260 (MW of GPTMS is 236.34). When the SCA was hydrolyzed with distilled water (Figure 3 b), three peaks with MW of 200~260, 370~450, 500~630 and 70~820 were observed in the GPC curve, demonstrating that GPTMS hydrolyzed in water existed mainly as four states, which were monomers, dimers, trimers, and tetramers. The results are essentially consistent with the studied of Gabrielli 44. The proportions of hydrolysates were 40.38%, 7.63%, 28.76% and 23.23%, respectively. Thus, 40.38% of GPTMS is not fully hydrolyzed or unhydrolyzed at all. When untreated nano-silica was present in the GPTMS hydrolysis system (Figure 3 c), the composition of the final hydrolysates did not change. However, the catalytic effect of nano-silica resulted in a decrease of the GPTMS monomer (18.98%), with a significant increase in the proportion of dimers (17.88%) and trimers (42.64%). In contrast, hydrothermally treated nano-silica greatly affected the final products of the GPTMS hydrolysis system that the final SCA hydrolysates was progressing to a state of pentamer (Figure 3 d). The proportions of monomers, dimers, trimers, tetramers, and pentamers after SCA hydrolysis were 20


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3.04%, 3.84%, 3.74%, 6.74% and 82.64% respectively. The incorporation of ICA accelerated not only the degree of SCA hydrolysis but also the monomer condensation rate. Moreover, the final hydrolysates retained the activity as a coupling agent since the hydrolysates eventually exist as an aqueous dispersion of pentameric. Due to the high stability of the final hydrolysates (no sedimentation within 6 months), pentamers are generally assumed to exist as a five-membered ring or other steady state.

Figure 3. The GPC curves of SCA hydrolyzed for 7 days in the blank (a), distilled water (b), un-hydrothermally treated nano-silica emulsion (c), HTNS emulsion (d) at 25oC, respectively. The GPC results have shown further hydrolysis and condensation of SCA and dispersed pentamers in the ICA system. To study the specific detail of hydrolysis and 21



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condensation of the siloxane groups before and after the treatment, NMR was applied for the further analysis. Figure 4 (a) shows the 1H NMR spectra of the specific molecular formula of SCA. To avoid hydrolysis and determine the original molecular formula of SCA, deuterated chloroform was used as the solvent. The NMR peak assignments were in line with reported results

50

. The scenarios of SCA with

hydrothermal treated/untreated nano-silica (SCA+treated SiO2 / SCA+untreated SiO2) and control one (SCA+H2O) were carefully monitored by using

29

Si NMR analysis.

Since SCA was with various hydrolysis states under different conditions after 7 days, the peak intensity and chemical shift of the spectrum showed variations (Figure 4 b, Tn corresponds to a Si atom with a Si-C bond and n bridging oxygen bonds -Si-O-Si-. The results indicated that only portion of the siloxane groups in water was hydrolyzed to silanol moieties or formed -Si-O-Si- structure with unhydrolyzed monomers. The T1 peak was significantly enhanced and the degree of hydrolysis increased with the addition of non-hydrothermally treated nano-silica. Meanwhile, the obvious T2 peak indicated the appearance of a hydrolyzed diradical structure. However, the T0 and T1 peaks almost disappeared when SCA was modified by hydrothermally treated nano-silica. The minor pronounced T2 and T3 peaks were emerged, which indicated that the siloxane groups in SCA were hydrolyzed and partial condensed. Almost all of the siloxanes in the ICA are converted to silanols in the form of methanol, this result was consistent with the experimental research of Gabrielli et al.

44

and the GPC data

of Figure 3. However, in the results of Gabrielli, a shift of the two peaks of H6 '(δ= 2.75 ppm, dd, J = 4.2 and 2.6 Hz) and H6' (δ= 2.94 ppm, t, J = 4.2 Hz) to lower fields 22


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(δ=3.64–3.46 ppm), which caused by hydrolysis of the epoxide ring were observed 44. This phenomenon did not happen in the 1H spectra (Figure 4 c). Therefore, not only most of the siloxane groups of SCA were efficiently converted into methanol in ICA, but the weak acid condition also avoided the consumption of epoxy groups. In the acidic and alkaline condition, although it can promote the hydrolysis, the epoxy group is more unstable. Moreover, GPTMS will quickly form a solid state and lose the coupling effect in strong alkali environment 44, 51.

Figure 4. 1H NMR spectra of SCA (a) in deuterated chloroform (CDCl3),

29

Si

NMR spectra (b) and 1H NMR spectra (c) of the fully hydrolyzed products obtained by reaction of GPTMS in deuterated water (D2O) with treated/untreated SiO2 for 7 23



Page 24 of 53

days.

3.2 Properties of Interfaces Between the Old Cement Paste and WECM The final purpose of the repair interface modification is to improve the prerequisite interface properties of strength and toughness as well as the durability. Therefore, tensile and flexural strength were tested to compare the repair effects by modifying the old cement paste matrix with fresh ordinary cement paste (OPC modified), waterborne epoxy resin (WEP modified) and ICA (ICA modified). The viscoelasticity and roughness before (modified surface) and after fracture (modified interface) were compared by applying ICA and WEP. Since the experimental cement paste base was mixed with type 52.5 OPC at low W/C of 0.29, therefore, the initial flexural strength, tensile strength (black line) reached up to 13MPa and 3MPa (Figure 5 a and b). The repaired sample treated with the same type OPC paste as the interfacial component (gray line) was with the same elasticity modulus but was with a lower strength (7.3 Mpa and 0.6 MPa), while a significantly decrease of fracture ductility after the repair was observed. WEP treated could improve the resistance to ductility, but the lower modulus of elasticity and strength made the sample more vulnerable to tensile stress (red line). ICA-modified interface prominently improved the flexural and tensile ductility of the repaired sample, and the failure process is divided into two stages. In the first stage, the interface showed a similar stiffness to the cement matrix, which was related to the incorporation of cementitious materials in WECM. In the second stage, the elastic modulus of the sample decreased before 24


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fracture, which showed a higher ductility and better mechanical properties. The ultimate fracture strength was same with cement paste matrix, but the flexural and tensile ductility increased by 147 % and 156 %, respectively. The two-stage mechanical curve proves the coupling effect between cement hydration and epoxy resin, which is consistent with the previous research 52. From the results of the AFM force-distance curves (Figure 5 c), the WEP and the fracture interface basically showed the same micromechanical properties. The stiffness exhibited by the WEP is related to the higher brittleness of epoxy resin itself. When the interface was modified by ICA, the cement-based surface brought out obvious characteristics of both stiffness and toughness. The stiffness phase of the viscous force curves had a broader distribution. The smaller stiffness related to the nano-silica and cement hydrates while the larger one was due to the SCA coupling effect. The fractured ICA treated interface showed a very strong viscoelasticity, which indicated that the fracture process will consume a large amount of fracture energy. The dissipation phase was directly related to the ductile fracture and multiple cracking of the interface. The Rmax, Rq, and Ra roughness values were calculated by the interface parameters (Figure 5 d). The roughness of the ICA interface increased significantly after the fracture, while the roughness of the WEP did not change much. The viscoelastic and roughness data demonstrated that the interface of ICA treatment could form fractures at different dimensions and scales during deformation to consume more fracture energy. Therefore, a higher tolerance, toughness, and strength during impact and sustained loading on the interface was achieved. The tolerance of the interface bond to strain 25



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and stress is also reflected in the durability of the repaired cement samples (shown in Figure 5 e). Compared with the control group, the shear bond strength after different aging tests reflect that repair interface bonding is much more vulnerable to damage by freezing and thawing cycles, especially in the cases with salt solutions 53. During the experiment, five of six WEP samples broke through the interface in less than 100 cycles. This is related to the limited bonding area created by the numerous air voids (Figure 6 a and b) in WEP. In the case of ECA, due to its strong sealability and impermeability, its capacity for vapour pressure and interface frost damage of cement matrix has been greatly reduced. This is also mentioned in its product specification. Although the adhesion performance of ICA is weaker for the initial adhesion than ECA, its durability and comprehensive anti-aging performance are even better. Besides, the ICA is competent for repairing the wet interface, and the application of the ECA is greatly limited due to its insolubility in water. Since all four repairs cases were using WEMC as surface material, where the cement component protects the internal organic material from light ageing damage, the comprehensive performance meets relevant standards for light aging.

26


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Figure 5. Flexural strength (a) and tensile strength (b) of the repaired samples by ordinary cement repair (OPC modified), waterborne epoxy repair (WEP modified) and ICA treated WECM repair (ICA modified). The interfacial viscoelasticity (c) and roughness (d) before and after fracture of ICA treatment and WEP treated cement paste surface. The slant shear strength (e) of samples after ageing test.

The topography of the AFM images (Figure 6) validated the conjecture of the 27



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causes of viscoelasticity shown in Figure 5. If the cement matrix is directly repaired by WEP, due to the low solid content of WEP, a lot of air voids with a diameter of about 1 µm in the interface layer were formed (Figure 6 a). As the result, the effective bonding area was reduced by about 20%. In addition, the interface structure did not change significantly when the WEP repaired interface was fractured, only with sporadic visible bulges observed (height difference of 30 to 40 nm shown in Figure 6 b). This phenomenon showed the lack of toughness cracking during the progress of the interface failure. AFM images of WEP verify the viscoelasticity results shown in Figure 5 that no significant change after WEP interface fracture. The obvious nano-silica was observed in the rough interface treated by ICA which was assumed to improve the stiffness of the repaired interface (Figure 6 c). When the interfacial failure was triggered, the appearance of the irregular rugged terrain with the range of 20nm to 120nm was then observed (Figure 6 d). This is directly related to the high viscoelasticity results of the ICA treated interface in the process of failure shown in Figure 5. When the nano-indenter of was tested into the bump area, the high viscoelasticity curve was obtained. The bumpy interfaces also verified the ductile fracture behavior and the consumption of high fracture energies.

28


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Figure 6. AFM images and section tortuosity of surface and fracture interface of waterborne epoxy repair (a and b) and ICA treated WECM repair (c and d)

The macroscopic mechanic experimental results have demonstrated that interfacial treatment of ICA improved the fracture toughness and bonding strength. To investigate the specific process and extent of crack propagation, two pieces of 29



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ICA-treated glass slides were observed with a digital reflective-polarizing microscope at different splitting times. When the interface was split within 2 seconds (Figure 7 a), numerous cracks were observed with the interval of 20 µm to 100 µm at the fracture surface. Additionally, the crack propagation progress was blocked and transformed into short cracks of 100 to 200 µm in length. After zooming in, the crack stopped extending when contacted with fine cracks in different directions at the interface. By drying the sample in Figure 7 (a) to make the interface coupling component shrink and observe the change of the base glass. Numerous cracks were found (shown in Figure 7 b) in the base glass that consistent with the direction of interface crack propagation. Since the untreated base glass showed almost brittle fracture during the deformation without any multiple cracking, thus this proved that the ICA induced multi-crack cracking and brought significant toughening effect onto the interface as well as the substrate. For the case of the fracture interface of splitting time within 10 seconds (Figure 7 c), the phenomenon of cracks blocking was not observed, but the cracks still showed a multidirectional development and presented in various forms. To study the crack growth process in situ, the sample was observed with a slow splitting loading rate and found that the cracks were developing by very short leaps and bounds. Each crack grew in different directions and this phenomenon was associated with the blocking effect of internal nano-silica in ICA and the toughness brought by condensation of coupling agent on the interface.

30


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Figure 7. The polarized transmitted light microscopy images of the fresh fracture 31



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interfaces with a splitting time of 2 seconds (a) and 10 seconds (c). The image (b) of dried sample (a) at 100 oC for 24 hours. The in-situ images (d) of the crack expansion process at the interface with splitting time of 1 minute.

3.3 Characterization of Interfacial Chemistry Reaction and Morphology To

quantitatively

and

quantitatively

characterize

the

SCA

hydrolytic-condensation as well as the catalytic action of nano-silica before and after hydrothermal treatment, various samples were tested by using thermogravimetric analysis (TGA) and FTIR. Furthermore, the grafting efficiency of ICA and SCA on hydrated cement paste was further studied. The absorption band (Figure 8 a) at the scope of 3000 to 2800 cm-1 resulted from the asymmetric stretching of the -CH bond from the organic moieties of SCA 54. The strong and broad -OH stretching-vibrational absorption bands at 3300 cm-1 to 3500 cm-1 are derived from water molecules. Since all the samples were evaporated to dryness at room temperature, the peak intensity here may be related to adsorbed water from the surface of the nano-silica. It can be seen from the results that the -OH peak intensity of the hydrothermally treated nano-silica was relatively higher, and the intensity of -OH peak decreased after reaction with SCA, while the peak intensity of -CH- absorption band and the Si-O-Si absorption band (1000 to 1250 cm-1) increased significantly. Therefore, it can be proved that the HTNS has higher surface activity and higher catalytic efficiency. The reason may be related to that after hydrothermal treatment, the hydrogen bonds between the vicinal silanol and germinal silanol on the silica surface were broken and 32


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the conjugated hydrogen bonds then are formed with the surrounding water clusters 55. The increase of the relative density of active silanol and water clusters enhanced the catalytic effect on hydrolysis and condensation of SCA. The TGA results show that after hydrothermal treatment, adsorbed water on the surface of the silica has been increased by about 10% wt. while grafted SCA increases from 0.9% wt. to 4.4% wt. (Figure 8 b). When the cement paste samples of 28 days old were treated by SCA and ICA respectively, the peak intensity of -CH with the band range of 2800 cm-1 to 3000 cm-1 and Si-O-Si band increased to some extent (Figure 8 c). By comparing the absorption intensity, the effect of grafting treatment of ICA was much more obvious than mixing SCA directly. Moreover, after repeated experiments, it was found that the sharp -OH peak (~3600 cm-1, related to calcium hydroxide) of cement paste samples after treatment was reduced apparently or even disappeared

52

. Therefore, it was

speculated that the calcium hydroxide in cement paste had a direct correlation with the grafting effect of ICA and SCA. To verify the above conjecture, cement paste samples with a water to cement ratio of 5: 1 were hydrated in the closed test tubes at 90 oC for 7 days to obtain a fully hydrated hydration product. Then the fully hydrated cement samples were treated by ICA with the same procedure. Then, the mineralogy of all the treated samples was analyzed by XRD. The results of the TGA curves (Figure 8 d) demonstrated that ICA did indeed contribute to the consumption of calcium hydroxide in cement paste, in which the calcium hydroxide in 28 days old cement paste decreased from 2.2 wt.% to 1.9 wt.% and the one in the fully hydrated cement paste decreased from 15.1 wt.% to 2.9 wt.% after ICA treatment. The XRD 33



Page 34 of 53

results of mineralogy also certify the depletion of portlandite by ICA. Evident portlandite diffraction peaks appeared in the XRD pattern of the 28 days old cement paste, whereas disappeared completely after ICA treatment. The peak intensities of portlandite in XRD pattern of the fully hydrated cement paste increased significantly with the disappearance of the C2S, C3S peaks (Figure 8 f). Therefore, the significant decrease of the portlandite diffraction peaks further corroborated the consumption of calcium hydroxide by ICA. Of course, calcium hydroxide should be converted into other chemical forms such as C-S-H, or into the fine crystalline form that was hardly detected. In summary, the consumption of portlandite is advantageous relative to the improvement of interfacial properties, and the formation of low-strength portlandite is more likely to happen with a high water-cement ratio at the interface and in air voids.

34


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Figure 8. FTIR spectra (a) and TGA curves (b) of dried (room temperature/ 24 hours) samples of hydrothermal treated/untreated nano-silica and the ones treated with SCA. FTIR spectra (c) and TGA curves (d) of dried (room temperature/ 24 hours) cement paste (hydrated for 28 days) and the one treated with ICA and SCA. XRD patterns of the ICA-treated/untreated cement paste hydrated for 28 days (e) and untreated/SCA/ICA-treated fully hydrated cement paste (f).

The microstructures and coupling products at the micrometer-nanometer scale 35



Page 36 of 53

were further studied by SEM and EDS analysis on the cement surface and fracture bonding interface. By comparing the treated and untreated bonding interface transition zone (BITZ) at the longitudinal sections with the same cement matrix. A visible crack throughout the untreated interface was observed, while no evident crack was observed at the ICA treated BITZ. After zooming in situ, no obvious bonding and crack arresting behavior was found in the untreated interface with a crack width of 2 µm to 5 µm observed. For the ICA treated side, numerous of fibrous products were found in the interface, which may be related to the crack arresting behavior during the deformation. Compared with the smooth portlandite crystals on the surface of the untreated cement paste (Figure 9 b), abundant coupling products (Figure 9 c) were attached to the surface of the portlandite after ICA treatment. When the repaired interface was fractured 24 hours and 7 days after repairing, respectively, a complex of coral-like combination structure of cement hydration products and ICA-coupling components was observed at the interface. To better observe the cracks filling effect of ICA on the cement cracks and pores at BITZ, the fracture surface of fully dried cement paste was treated with fresh ICA and the repair surface at longitudinal section was cut (Figure 9d). A clear crack blockage pattern (shown in dotted lines) can be observed, and most of the interconnected cracks with a width of 1 to 3 µm at the interface were plugged into ones less than 100 nm. After zooming in, the cracks were filled with numerous of nano-silica with a particle size of 50 to 200 nm, which provided reaction sites for the formation of pore plugs by subsequent chemical reactions of ICA and hydration products. After repairing for 24 hours (Figure 9 e), this 36


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combination structure is a continuous sheet laminated structure. By EDS analysis, the Ca content was as high as 53.84% while Si content was 1.34%. Then it was speculated that it might be a self-assembled composite structure of ICA coupling components and finely crystalline calcium hydroxide. After 7 days of hydration (Figure 9 f), layered calcium hydroxide disappeared, forming numerous fibrous coral structure. The increase of Si content from 1.9% to above 7.2% in the EDS results revealed that such a structure transforms into a C-S-H-like structure or a more stable and un-soluble calcium-silicon composite structure.

37



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38


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Figure 9. The SEM images of ICA treated/ untreated longitudinal section (a) and (a)1 and (a)2 of cement paste bonding interface transition zone (BITZ) after failure. The SEM images of cement paste surface before (b) and after (c) ICA treatment at 24 hours. The SEM images of the cracks filling effects of ICA at longitudinal section of the repair interface (d) and (d)1 and (d)2. The SEM images and EDX spot scan of fracture section before (e) and after (f) ICA treatment repaired for 24 hours and 7 days.

Based on the above results and microscopic morphology, the hydrolysis and condensation process of SCA by HTNS and possible coupling mechanisms of ICA are inferred as shown in Figure 8. The surface properties of nano-silica have been studied extensively, and are proved that silanols are hydrogen bonded to each other and/or adsorbed water molecules

56-57

. Ortho silanol is the smallest unit of silicone polymer,

there are four silanol groups on the surface without any intramolecular hydrogen bond (IMHB)(shown by blue dotted line). All hydrogen bonds can interact with the surrounding monomers to further polymerize, so this is also an important reason for their instability. When the nano-silica branch number increases, most of the surface silanol groups form IMHB with each other and reduce its catalytic effect (Figure 10 a). IMHB is difficult to open by mechanical means such as stirring and ultrasonic treatment, but hydrothermal treatment is very easy to cross the barrier. Moreover, the surrounding clusters of water molecules readily form hydrogen bonds with the silanol immediately and result in the stable clusters of water molecules on the surface of the 39



Page 40 of 53

nano-silica. The catalytic effect of water clusters with different molecules number on hydrolysis and condensation of alkoxysilanes has already been theoretically demonstrated by Okumoto et al. 45. Even under neutral conditions, water clusters and hydrogen bonds with siloxanes can significantly reduce the energy barrier of both hydrolysis and condensation, with a silicon front-side nucleophilic attack and concomitant bond interchanges. According to the results portlandite content, crystal structure, and SEM images, it is speculated that ICA can transform well-crystallized hexagonal plate to fine-grained near-square plate at the interface. Meanwhile, during the polymerization of ICA and WEP and the hydration of cement, lots of layered structure of a polymer-portlandite-polymer is formed. This agrees well with the study of Galmarini Sandra et al.58, who simulates and synthesizes the calcium hydroxide structure formed in aqueous solution. The structure of the near-quartet is related to its lower surface energy. During the recrystallization of calcium hydroxide, silanols are hydrogen bonded to the surface hydroxyl groups and dehydrated to attach ionically to the (001) surface of calcium hydroxide. This may be the reason of sharp decline in the diffraction peak at (001) surface in the XRD results. During the continuous dissolution of portlandite, the nano-silica in ICA forms a fibrous, stable product with ions and enhances the interfacial pore plug effect (as shown in Figure 9 f). In contrast to WEP and OPC treated repairing interfaces（Figure 10 c）, ICA-treated samples were able to form stable fracture interfaces and multi-dimensional cracking states upon fracture.

40


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Figure 10. Mechanism of the catalytic effect of hydrothermally treated of nano-silica on SCA hydrolysis and condensation (a), fine-grained effect and 41



Page 42 of 53

interfacial coupling effect of ICA on portlandite (b) and damage schematic (c) of interface treated by different means. 4. CONCLUSION In this study, we use HTNS as catalytic to promote the hydrolysis of SCA. A novel and the stable cement paste interface coupling agent is prepared. The effect of SCA on OPC hydration and the catalytic effect of nano-silica on SCA have been studied for the first time. Secondly, the macroscopic and microscopic mechanical properties and fracture properties of the repaired interface on OPC surface are further analyzed and studied. The results prove that ICA significantly improves the hydrolytic condensation efficiency of SCA and avoid the negative impact of methanol generated during SCA condensation on cement hydration. The repaired interface adhesion, stiffness, tenacity, and viscoelasticity are further improved while the interfacial cracks are filled with ICA to a certain extent. Additionally, the effect of the ICA on the portlandite content and the crystal structure at the interface have been first observed and the coupling mechanism is speculated based on the morphology of the layered structure and the atomic arrangement of the crystal and the coupling agent. AUTHOR INFORMATION Corresponding Author *Yunsheng Zhang. E-mail: [email protected] Note The authors declare no competing financial interest. 42


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ACKNOWLEDGMENT

Authors gratefully acknowledge the financial support from 973 National Basic Research Program of China (No. 2015CB655102), and National Natural Science Foundation of China (51678143).

ABBREVIATIONS

SCA, silane coupling agent; ICA, interface coupling agent; ECA, commercial epoxy interface agent; WEP, waterborne epoxy resin; WECM, waterborne epoxy modified

cement-based

repair

materials;

GPTMS,

3-glycidoxypropyltrimethoxysilane; HTNS, hydrothermally treated nano-silica; OPC, ordinary portland cement.

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Interface properties of nano-silica modified waterborne epoxy-cement

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