Effect of styrene–butadiene rubber latex on the chloride permeability and microstructure of Portland cement mortar

Construction and Building Materials 23 (2009) 2283–2290

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Effect of styrene–butadiene rubber latex on the chloride permeability and microstructure of Portland cement mortar Zhengxian Yang a,b, Xianming Shi a,c,*, Andrew T. Creighton a, Marijean M. Peterson a a

Corrosion and Sustainable Infrastructure Laboratory, Western Transportation Institute, P.O. Box 174250, Montana State University, Bozeman, MT 59717-4250, USA Chemistry Department, Fuyang Normal College, Fuyang, Anhui 236041, People’s Republic of China c Civil Engineering Department, 205 Cobleigh Hall, Montana State University, Bozeman, MT 59717-2220, USA b

a r t i c l e

i n f o

Article history: Received 17 July 2008 Received in revised form 20 October 2008 Accepted 19 November 2008 Available online 30 December 2008 Keywords: Chloride permeability Microstructure Styrene–butadiene rubber latex Polymer-modified mortar

a b s t r a c t We evaluated the chloride permeability and microstructure of Portland cement mortar modified by styrene–butadiene rubber (SBR) latex, using mortar samples with various polymer/cement (P/C) mass ratios. The incorporation of SBR improved the chloride penetration resistance along with the general ionic permeability of the mortar, while increasing its ionic transport resistance and decreasing its electric capacitance. These data suggest that admixing SBR led to denser and more refined microstructure of the cured cement mortar. Field-emission scanning electron microscopy images confirmed such improvements in the pore structure and the formation of an interpenetrating network structure of SBR and cement hydrate phases at relatively higher P/C ratios. Besides slightly reducing Portlandite content and mitigating carbonation with the increasing P/C ratio in mortar, SBR was also found to promote the formation of calcium aluminate trisulfate hydrate phases and facilitated chloride binding. Published by Elsevier Ltd.

1. Introduction The microstructure of mortar and concrete is of considerable importance since it governs their mechanical properties, cement hydration and durability [1–3]. In addition, chloride permeability is recognized as a critical intrinsic property affecting the durability of reinforced concrete [4,5]. The use of polymer as a modifier in new structures seems to be a promising strategy in improving microstructure and enhancing the durability of cement mortar and concrete [6–9]. As one of the popular polymers suitable for admixing into fresh mortar and concrete, styrene–butadiene rubber (SBR) latex has been widely used for a long time [10,11]. The molecular structure of SBR comprises both the flexible butadiene chains and the rigid styrene chains, the marriage of which offers the SBR-modified mortar and concrete many desirable characteristics such as good mechanical properties, water tightness and abrasion resistance especially when an appropriate P/C ratio is used [12–14]. Previous studies indicate that the admixing of SBR latex into fresh mortar and concrete improved the resistance of hardened mortar and concrete to chloride ion penetration [15,16]. Nonethe-

* Corresponding author. Address: Corrosion and Sustainable Infrastructure Laboratory, Western Transportation Institute, P.O. Box 174250, Montana State University, Bozeman, MT 59717-4250, USA. Tel.: +1 406 994 6486; fax: +1 406 994 1697. E-mail address: [email protected] (X. Shi). 0950-0618/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.conbuildmat.2008.11.011

less, the underlying mechanisms responsible for the role of SBR latex in such improvement remain debatable. Polymer films that possibly retard the cement hydration were found to form in the structures of mortar and concrete, and an integrated three-step model has been proposed in which the polymer film formation and the cement hydration processes develop simultaneously [3,5,17]. But interestingly, when tested separately, the polymer films were not effective chloride diffusion barriers on their own [18]. Therefore, what synergistic role SBR plays in reducing the chloride permeability of mortar and concrete is still not fully understood. In addition to polymer film formation, we hypothesize that SBR may alter the microstructure and chemistry of hardened mortar and thus affect the ionic permeability and chloride binding behavior. To test this hypothesis, this work systematically evaluates the chloride permeability and microstructure of SBR-modified Portland cement mortars prepared with various P/C ratios. Electromigration and electrochemical impedance spectroscopy (EIS) measurements were conducted to investigate how the admixing of SBR affects the chloride permeability, the general ionic permeability and the microstructure of hardened mortar. Analytical techniques including field emission scanning electron microscopy/ energy-dispersive X-ray spectroscopy (FESEM/EDX) and Fourier Transform Infrared Spectroscopy (FT-IR) were utilized to shed light on how SBR affects the cement hydration process, microstructural characteristics of the resulting mortar, and possible interactions between the SBR and cement hydrate phases in mortar.

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Table 1 Chemical composition of Type I/II low-alkali Portland cement. Item

Spec. limit

Test result

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Loss on ignition (%) Na2O (%) K2O (%) Insoluble residue (%) CO2 (%) Limestone (%) CaCO3 in limestone (%) Tot. alkalies (% as Na2O)

N/A 6.0 max 6.0 max N/A 5.0 max 3.0 max 3.0 max N/A N/A 0.75 max N/A 5.0 max 70 min 0.60 max

20.4 3.7 3.2 63.3 3.2 2.6 2.7 0.1 0.4 0.43 1.7 4.0 98 0.37

Potential compound composition (%) C3S C2S C3A C4AF C4AF + 2(C3A) C3S + 4.75(C3A)

N/A N/A 8.0 max N/A N/A 100 max

56 16 4.5 10 19 78

2. Experimental 2.1. Materials An ASTM specification C150-07 Type I/II low-alkali Portland cement (ASH Grove Cement Company, Clancy, MT.) was used in this study. The chemical composition and physical properties of the cement are listed in Tables 1 and 2, respectively. The fine aggregates used were river sand sifted to allow a maximum aggregate size of 1.18 mm before proportioning and admixing. SBR latex (containing carboxylic acid; dynamic viscosity: 6–46 mPa s; pH value: 8–10; solid content: 51–53%) and deionized water were used in the experiment. 2.2. Sample preparation The mortar samples were prepared with a constant sand-to-cement mass ratio of 2, a constant water-to-cement mass ratio of 0.45, and a polymer (total solids)-tocement ratio (P/C) of 0%, 2%, 6%, 8%, 10%, 12% and 16%, respectively. For both the control (without polymer admixed) and each polymer-modified Portland cement mortar, six samples were prepared to ensure the statistical reliability of chloride ion permeability test results. For the polymer-modified samples, the polymer latex was first mixed into water in a mixer, and then cement was added and stirred thoroughly in a low speed hand mixer for 5 min. Afterwards, sand was added into the mixture and stirred for about 3 min to achieve good workability. After mixing, the fresh mixture was poured into the mold to form a disc of 40 mm diameter and 8 mm thickness, which was carefully compacted to minimize the amount of entrapped air. All the samples were demolded after curing under 20 ± 2 °C and over 80% relative humidity for 24 h. After demolding, the samples without polymer admixed were cured under 20 ± 2 °C and over 95% relative humidity for 27 days. For

the samples admixed with polymer, they were initially cured under 20 ± 2 °C and over 95% relative humidity for 6 days, and then left in the curing chamber with temperature of 20 ± 2 °C and 65% relative humidity for 22 additional days. 2.3. Measurements and characterization 2.3.1. Electromigration measurements The chloride ion permeability of the Portland cement mortars was determined through electromigration experiments, conducted using the glass cell shown in Fig. 1. The glass cell featured a disc-shaped mortar sample that separates the chloride anion source (25 ml 3% NaCl solution) and the chloride anion destination (25 ml 4.3% NaNO3 solution). Each of the two compartments contained one glassy carbon electrode with an exposed surface area of 1 cm2. Once the mortar disc, electrolytes and electrodes were in place, an 8-V DC electric field was maintained across the disc through the two carbon electrodes in the two compartments. During the test, readings of open circuit potential (OCP) of the Ag/AgCl sensor in the destination solution were taken periodically, using a saturated Calomel electrode (SCE) as the reference electrode. In addition, the electric current passing through the disc was monitored using a Gamry Reference 600TM Potentiostat/Galvanostat/ZRA instrument. The Ag/AgCl sensor was used to monitor the evolution of free chloride anion (Cl) concentration in the destination solution, as their OCP data were compared against a standard calibration curve correlating potential readings with known Cl concentrations. 2.3.2. Electrochemical impedance measurements At the completion of the electromigration test, the Gamry Reference 600TM Potentiostat/Galvanostat/ZRA instrument was employed to measure electrochemical impedance spectroscopy (EIS) data in order to characterize the microstructural properties of cement mortar. To this end, a platinum mesh was placed in the left (cathodic) compartment to serve as the counter electrode, whereas the carbon electrode and the SCE in the right (anodic) compartment served as the working electrode and the reference electrode, respectively. The EIS measurements were taken by polarizing the working electrode at ±10 mV around its OCP, using sinusoidal perturbations with a frequency between 300 KHz and 5 MHz (10 points per decade). The Gamry Echem AnalystTM software was used to plot and fit the EIS data. 2.3.3. FT-IR analyses FT-IR analyses were conducted on the samples that had not been tested for electromigration. The samples were ground and mixed with pre-dried FT-IR grade potassium bromide and pressed into pellet with the KBr/sample mass ratio of 100. Transmission infrared spectra of the samples were recorded using Nicolet NEXUS 670 FT-IR spectrometer over the wavenumber range of 4000 to 400 cm1. Each of the samples was scanned 64 times. 2.3.4. FESEM imaging and EDX analyses For surface analyses, the mortar samples were taken out of the glass cell after the electromigration and electrochemical measurements. A strip of cross-sectional sample representing relevant changes in chloride penetration and microstructural properties was cut from the mortar disc along its diameter. After being vacuumdried, the slice samples were then subjected to FESEM/EDX to examine its localized morphology and elemental distributions at the microscopic level, using a Zeiss Supra 55VP PGT/HKL system coupled with the energy dispersive X-ray analyzer. The EDX data were obtained using a micro-analytical unit that featured the ability to detect the small variations of trace element content. We used FESEM/EDX under pressure, typically 102–103 torr, to investigate the effect of SBR latex on the morphology and chemistry of cement hydrates.

Table 2 Physical properties of Type I/II low-alkali Portland cement. Item

Test method

Spec. limit

Test result

Air content of mortar % Blaine fineness m2/kg air permeability test

C185 C204

7.9 397

Autoclave expansion % Normal consistency % Compressive strengths, psi (MPa) 1-Day 3-Day 7-Day Setting times min Vicat initial Vicat final Pass 325 mesh % Heat of hydration (cal/g)-7 days False set %

C151 C187 C109

12.0 max 280 min 420 max 0.80 max N/A N/A 1740 (12.0) min 2760 (19.0) min

1998 (13.8) 3472 (23.9) 4600 (31.7)

45 min 375 max 72 min N/A 50 min

111 240 98.7 70.7 85

0.03 26.6

C191

C451

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Fig. 1. Experimental setup for the electro-migration tests.

field) is related to the diffusion coefficient through the Nernst–Einstein equation.

3. Results and discussion 3.1. Effect of SBR latex on the ionic permeability of cement mortar

t¼ Fig. 2 illustrates the typical results obtained from the electromigration test. The breakthrough time t0 is the point after which the Cl concentration in the destination solution (anolyte) increases linearly with time. Under the experimental conditions of this study, t0 was mostly obtained after a chloride concentration of 0.003–0.005 mol/L had been reached in the destination compartment (0.075–0.125 mmol of chloride ions had passed through the mortar sample). The electromigration test for all samples was stopped when an OCP reading reached about 67–70 mV (chloride concentration of 0.020–0.023 mol/L) in the destination compartment. The method used to calculate the apparent diffusion coefficient D of Cl in cement mortars is described as follows. Under an externally imposed electric field with an intensity of E (in V/m), the mobility of ions (the average velocity of the ions per unit of electric

where z, F, R and T are charge number (1 for Cl), Faraday constant (9.64846  104 C/mol), ideal gas constant (8.3143 J/(mol K)) and absolute temperature (298.15 K, or 25 °C) [19], respectively. The chloride ion mobility can be calculated from the time t0 required for the chloride front to penetrate a depth d (in m) of the sample:

t¼

Chloridion Concentration(M)

t0 0

200

400

600

800

1000

Time(minutes) Fig. 2. Evolution of chloride concentration over time in the destination compartment during electromigration test.

d t0 E

The chloride penetration time t0 (in s) was estimated from the monitored Cl concentrations in the anolyte (destination solution). t0 is the point after which the Cl concentration in the anolyte increases linearly with time. Therefore, the diffusion coefficient D of Cl in cement mortars can be estimated by the equation below (in m2/s):

D¼

P/C=0% P/C=2% P/C=6% P/C=8% P/C=10% P/C=12% P/C=16%

zFD RT

dRT t 0 EzF

Table 3 shows the electromigration test results of the apparent diffusion coefficients of Cl in the cement mortars, with the mean calculated from three or four samples. It can be seen from Table 3 that the incorporation of SBR latex improved the chloride penetration resistance of the mortar, as indicated by the reduced apparent diffusion coefficients of chloride ions, DCl . When the P/C ratio was 16%, the value of DCl decreased by 65% compared with conventional Portland cement mortar. As shown in Fig. 3a, DCl decreased linearly with the increase in P/C ratio (i.e., the SBR content admixed in fresh mortar), under the experimental conditions of this study. The evolution of electric current over time was monitored during the electromigration test. These data were used to calculate the amount of electric charge (Q) passing through the mortar discs by integrating the current curve over time. Q can be considered as an indicator of electrical conductivity of mortar, derived from both the pore structure characteristics and pore solution chemistry of mortar. The results in Table 3 show that the incorporation of SBR latex reduced the general ionic permeability of the mortar, as indicated by the reduced Q value (integrated over the first 360 min). Interestingly, there was a linear correlation between DCl (characteristic of chloride permeability) and Q360 min (characteristic of general ionic permeability), as illustrated in Fig. 3b.

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Table 3 Apparent diffusion coefficients of chloride ions and electric charge passing through the mortar samples. P/C 11

2

1

DCl (10 m s ) Q360 min (104 cm2) 

0%

2%

6%

8%

10%

12%

16%

2.24 121.03

1.96 100.17

1.60 91.81

1.43 78.56

1.35 72.25

1.14 70.64

0.78 51.28

The electromigration test results revealed that with the increasing amount of SBR latex admixed into fresh cement mortar, both the chloride penetration resistance and the ionic transport resistance of the hardened mortar tended to be enhanced, although sometimes such improvements were small. These findings demonstrate that the use of SBR latex as a cement modifier is a promising strategy in improving concrete durability and the beneficial effects increase with an increase in the polymer content. 3.2. Effect of SBR latex on the microstructure of cement mortar 3.2.1. Investigation by EIS EIS provides information on interfaces and thus was utilized to shed light on the microstructural properties of the cement mortars in this study. The complex impedance of the mortar/electrolyte interface depends on the frequency of the externally imposed AC polarization signal, allowing the representation of the system with an equivalent circuit typically consisting of resistors and capacitors. For this experiment, the equivalent circuit shown in Fig. 4

D Cl- (X10-11 m 2s -1)

2.5 y = -8.7393x + 2.1742 2 R = 0.9909

2 1.5 1 0.5 0 0

0.05

0.1

0.15

0.2

Q 360min (X104 Cm -2)

P/C 140 120 100 80 60 40 20 0

y = 45.779x + 15.008 2 R = 0.9736

was used to characterize the interfaces between the counter electrode and the working electrode separated by the mortar disc. Constant phase elements (Q) instead of capacitances were used in all fittings. Such modification is obligatory when the phase angle of the capacitor is different from 90°. The EIS parameters that characterize the electrolyte/mortar interface are the ionic transport resistance of the mortar disc (Rmortar) and the capacitance (in this case, constant phase element) of the disc (Qmortar). For the electrode/electrolyte interfaces, we can assume that each interface has a double layer capacitance (in this case, Q1 and Q2, respectively) and a charge transfer resistance (R1 and R2, respectively). In addition, we assign the Warburg impedance (W) to only one interface characterizing the diffusion of species through the interface. Table 4 presents the fitted equivalent circuit parameters of both the control and SBR-modified mortar discs, in which nmortar (0 < nmortar < 1) is the fitting coefficient for Qmortar (with 1 being the perfect fit of a capacitor and 0 being the worst). As shown in Table 4, compared with the conventional Portland cement mortar disc, the discs modified by SBR latex had higher values of Rmortar and lower values of Qmortar. Rmortar characterizes the porosity of the mortar disc and is a function of the resistance of ionic transport through the interconnected pores, cracks and potentially air voids (containing some carbon dioxide, oxygen and water vapor) in the disc. The increased ionic transport resistance and decreased electric capacitance of SBR-modified mortar discs suggest that the incorporation of SBR latex led to denser and more refined microstructure of cement mortar. Such effects were especially notable when the P/C ratio in fresh cement mortar exceeded 10%. The reduced porosity and increased density in microstructure have been reported to provide cement mortars with higher flexural and compressive strength, higher microhardness in interfacial zone and lower effective chloride diffusion coefficient in matrix, when polyacrylic ester emulsion and silica fume were used together [20]. It is also noted that compared with the conventional Portland cement mortar disc, the SBR-modified mortar discs exhibited slightly

Table 4 Equivalent circuit parameters of the mortar discs.

0

0.5

1

1.5

2

2.5

D Cl- (X10-11 m 2s -1) Fig. 3. (a) Evolution of apparent chloride diffusion coefficient in mortar with the increase in the P/C ratio; (b) correlation between the chloride permeability (DCl ) and the general ionic permeability Q360 min.

P/C(%)

Rmortar (kX cm2)

Qmortar (lS cm2)

nmortar

0 2 6 8 10 12 16

10.57 ± 0.28 15.43 ± 0.23 159.6 ± 4.01 168.5 ± 6.09 176.4 ± 4.53 243.9 ± 8.72 457.8 ± 19.9

213.5 ± 2.61 197.8 ± 1.93 176.1 ± 0.63 143.9 ± 1.21 79.80 ± 0.62 75.29 ± 0.45 73.75 ± 0.66

0.83 ± 0.008 0.85 ± 0.007 0.88 ± 0.004 0.89 ± 0.004 0.89 ± 0.003 0.89 ± 0.004 0.91 ± 0.002

Fig. 4. The equivalent circuit used for fitting the impedance spectra.

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SBR films. When a high content of SBR latex is used, both cement hydration and SBR film formation proceed well to generate a comatrix phase with an interpenetrating network structure of SBR and cement hydrate phases that binds the sand particles strongly. The reduced porosity and other alternations at the microstructure level leads to macroscopic changes in the properties of the SBRmodified mortar, such as reduced water permeability, higher ionic transport resistance, and lower electric capacitance.

a b

Tansmittance(%)

c d e

3.2.2. Investigation by FT-IR FT-IR is one of the versatile tools widely used for molecular characterization and has been found to be very useful in predicting the degree of hydration and monitoring the dynamics of changes during the hydration reaction in Portland cement [21]. In this study, FT-IR was utilized to evaluate the effects of polymer modification on the hydration processes and possible interactions between the SBR and cement hydrate phases. The typical FT-IR spectra of SBR-modified cement mortars with various P/C ratios are shown in Fig. 5. It can be observed from the spectra that few changes occurred when the P/C ratio remained below 10%. However, when P/C ratio exceeded 10%, the absorption peak at 3642 cm1 corresponding to the –OH groups in Portlandite had a slight decrease, although the decreasing trend was not obvious at higher P/C ratios. The peaks at 1077 cm1 and 997 cm1 corresponding to silicate phases (SiO4) underwent changes among these samples; the former increased slightly while the later decreased. The changes indicate that the cement hydration was influenced by the admixing of SBR latex into fresh mortar. Furthermore, it is apparent that the absorption peaks at 874 cm1 and 1442 cm1 corresponding to carbonate phases (CO3) mostly due to the carbonation showed a gradual decrease with the increase in the SBR content. In contrast to plain mortar, the reduced carbonate content in SBR-modified mortars suggests an improvement in carbonation resistance and such improvement became remarkable when the P/C ratio exceeded 10%. In addition, the absorption peaks at 1640–1645 cm1 and 3440–3446 cm1 derived from molecular water exhibited a slight decrease when the P/C ratio exceeded 10%, implying a reduction in water absorption by the mortar.

f g

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber(cm ) Fig. 5. Typical FT-IR spectra of SBR-modified cement mortars with a P/C ratio: (a) 0%, (b) 2%, (c) 6%, (d) 8%, (e) 10%, (f) 12% and (g) 16%.

higher values of nmortar, likely due to lower porosity and higher microstructural regularity of the discs. Combing the data in Tables 3 and 4, a strong correlation can be seen between DCl and both Rmortar and Qmortar. As the P/C ratio increased, DCl decreased linearly, Rmortar increased nonlinearly and Qmortar decreased nonlinearly. It is concluded that these three parameters were all indicators of the microstructure and chemical composition of the saturated mortar matrix, in which the ionic species transport mainly via natural diffusion or electromigration. We can attribute the findings from the electromigration and EIS data to the following mechanisms. For the SBR-modified mortars, the large pores can be filled with SBR or sealed with continuous

SBR CHAIN

SBR CHAIN

O

SBR CHAIN

O

C

C O

C

C

O

C O

C O

O O

O O

Ca

Ca

Interaction with Ca2+ ions

Binding to cement grains

Anhydrous cement grains or cement hydrates

O O SBR CHAIN

O Ca O

C

C O

O

O

O

O

C

SBR CHAIN

C

SBR CHAIN

Ca O

O Ca O

C

C O

O

O

O

O

C

C

Ca O

SBR CHAIN

Fig. 6. Possible interactions between SBR particles with cement grains or cement hydrates.

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S/Al(based on EDX data)

Fig. 7. Representative FESEM images of SBR-modified cement mortars with a P/C ratio: (a) 0%, (b) 2%, (c) 6%, (d) 8%, (e) 10%, (f) 12% and (g) 16%, all taken at magnification approximately 7600 times.

0.2 0.15 0.1 0.05 0 0%

2%

6%

8%

10%

12%

16%

P/C Fig. 8. Average S/Al elemental ratio in the SBR-modified mortars as a function of P/C ratio.

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We propose the following mechanisms to account for the reduced Portlandite content and improved carbonation resistance of the SBR-modified cement mortar. Usually, a small amount of carboxylic acid is chemically bound onto the polymer particle surface. These groups, ionized in the highly alkaline environment of fresh mortar, tend to interact with calcium ions from the cement hydrates, which generally results in improved stability of the polymer latex and adhesion of the polymer-modified mortar to existing substrates [22–24]. Fig. 6 illustrates the possible interactions between SBR particles with cement grains or cement hydrates, which is consistent with the conclusion obtained by other researchers in that polymer modifier absorbs calcium on the formed polymer film and thus reduces the formation of Portlandite with the increase of polymer content in mortar [25]. On the other hand, at higher P/C ratios, the SBR latex particles distributed more homogeneously in the mortar and the microstructure of the mortar became denser and more refined. As a result, the porosity and pore size of the SBR-modified mortar were eventually reduced, which led to reduced availability of airborne CO2 (as well as oxygen and water) in mortar and thus enhanced carbonation resistance of the hardened mortar.

though the effect of SBR on the microstructure of the cement hydrates remained unclear. In the case of a P/C ratio of 10% (e), more continuous polymer bridges are observed and the bridges could be seen situating at the surface of the sand particles as well as in the bulk binder matrix. When the P/C ratio was increased to 12% (f), the polymer bridges became wider and a coherent polymer film formed in the modified mortar. The interpenetrating network structure of SBR and cement hydrate phases was formed to bind the sand particles together. The interpenetrating network structure fully developed when the higher P/C ratio of 16% (g) was employed where the formed polymer film became thicker and continuous. When compared with the control sample (a), the SBR-modified samples seemed to have a second matrix integrated throughout the mortar surface. The porosity and pore size of this second matrix became smaller with the increasing P/C ratio, indicating that continuity of the SBR film gradually increased and became notable in the case of a high SBR content. These microscopic observations agreed well with the experimental data and mechanisms discussed earlier. 3.2.4. Investigation by EDX Using an appropriate accelerating voltage (20 kV) with a scan time of 60 s per sampling point, the EDX data provided the chemical composition at a large volume, as in the depth of approximately 1 lm from the surface to the mortar bulk matrix. The EDX data were taken from six different sites on the surface of each mortar sample subsequent to the electromigration and electrochemical measurements. Each site corresponded to a selected area of approximately 40 lm  40 lm. Fig. 8 shows the S/Al elemental ratio of the surface on mortar samples with various contents of SBR latex admixed, with each data point averaged from the multiple EDX measurements. It can be seen that average S/Al ratio on mortar surface increased with the rising P/C ratio, which suggests that SBR latex facilitated the formation and stability of sulfate-containing AFt phases (e.g., ettringite). Tricalcium aluminate (known as C3A phase) is an important phase in cement, most of which is readily dissolvable by water. Hydration of C3A in the presence of gypsum initially produces ettringite, which is usually unstable and gradually reacts with C3A to form more stable monosulfate (known as AFm phase). The related reactions are shown as follows.

3.2.3. Investigation by FESEM Recent technological advances in FESEM enable the observation to be performed under a weak vacuum and thus allow better retention of moisture in the samples. As such, cement hydration and microstructure of the cement mortars can be studied without suffering from the micro-shrinkage or crystallization due to moisture evaporation [26]. Fig. 7a–g show the representative FESEM images of mortar surfaces with various P/C mass ratios subsequent to the electromigration and electrochemical measurements, all of which were taken at magnification of approximately 7600 times. In contrast to the control sample (a) that exhibited a loosely connected microstructure, the structure of the SBR-modified mortar was compact even when only a small amount (e.g., P/C of 2%) of SBR was admixed (b). The polymer binders were found on the mortar surface although the connections between the fissures were very thin at a P/C ratio of 6% (c). At a P/C ratio of 8% (d), some scattered polymer bridges (confirmed by relatively higher carbon content at these sites based on EDX data) were clearly visible on the mortar surface even 3.0

0.24

DCl-

0.20

2.0

0.16

DCl- ( x 10

-11

2

-1

Cl%

1.5

0.12

1.0

0.08

0.5

0.04

0.0

Cl ( wt%, based on EDX data )

m .s )

2.5

0.00 0%

2%

6%

8%

10%

12%

16%

P/C Fig. 9. Apparent chloride diffusion coefficient (left) and average Cl content (right) as a function of P/C ratio.

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3CaO  Al2 O3 þ 3CaSO4  2H2 O þ32H2 O ðC3 AÞ

ðGypsumÞ

¼ 3CaO  Al2 O3  3CaSO4  32H2 O ðEttringite; AFt phaseÞ

3CaO  Al2 O3 þ 3CaO  Al2 O3  3CaSO4  32H2 O ðC3 AÞ

ðEttringite; AFt phaseÞ

¼ 2ð3Cao  Al2 O3  CaSO4  12H2 OÞ þ CaSO4 þ 8H2 O ðMonosulfate; AFm phaseÞ

When SBR latex was added, however, some of the C3A was contained inside cement grains with no initial access to water and no further access to ettringite (as shown in Fig. 6). As such, SBR promoted the reaction of tricalcium aluminate with gypsum and thus facilitated the formation and stability of ettringite. Fig. 9 shows that as the P/C ratio increased, the hardened mortar consistently exhibited higher Cl content (based on EDX data) and lower apparent diffusion coefficient of chloride ions (DCl ). These trends along with the increase of S/Al with the P/C ratio can be explained by the following mechanism. As the content of admixed SBR increased, more AFt phases formed in the hardened mortar matrix, during the electromigration test some of which converted to chloroaluminate via the partial substitution of the sulfate anion by the chloride anion in the calcium sulfoaluminate hydrates. One possible reaction of such chloride binding is shown as follows: 

3CaO  Al2 O3  3CaSO4  32H2 O þ 6Cl

¼ 3CaO  Al2 O3  3CaCl2  32H2 O þ 3SO2 4 4. Conclusions This experimental study evaluated the chloride permeability and microstructure of SBR-modified Portland cement mortars, which were prepared with various polymer/cement (P/C) mass ratios, a constant water/cement ratio of 0.45 and a constant sand/cement ratio of 2. Electromigration tests demonstrate that the incorporation of SBR latex improved the chloride penetration resistance of the mortar, as indicated by the reduced apparent diffusion coefficients of chloride anion (DCl ). The SBR latex also reduced the general ionic permeability of the mortar, as indicated by the reduced electric charge (Q) passing through the samples. Subsequent to the electromigration test, each mortar sample was subjected to the EIS measurements with a large frequency range of 5 mHZ  300 KHZ. The incorporation of SBR latex in cement mortar increased its ionic transport resistance and decreased its electric capacitance, which are governed by the pore structure characteristics and pore solution chemistry of the mortar. The FESEM images suggest that the admixing of SBR latex in fresh mortar altered the morphology and microstructure of the hardened mortar. Through the cement hydrate matrix, a continuous polymer film became more and more visible with the increasing P/C ratio in the mortar. At a P/C ratio higher than 10%, the interpenetrating network structure of SBR and cement hydrates was found to bind the sand particles together. The FT-IR spectra indicate that the incorporation of SBR latex in cement mortar slightly reduced Portlandite content and mitigated carbonation. The EDX data indicate that the admixing of SBR latex promoted the formation of AFt phases and facilitated chloride binding via the partial substitution of the sulfate anion by the chloride anion in the calcium sulfoaluminate hydrates during the electromigration test. This work brought new insights into the interaction mechanisms between cement hydration and SBR latex modifier. It pro-

vided improved understanding of the effect of admixed SBR latex on the microstructure, chemistry, ionic permeability and chloride binding behavior of Portland cement mortar. Such knowledge is expected to contribute to the effort of searching for effective measures to improve the durability of cement mortar and concrete in a chloride-laden environment. Acknowledgements This work was supported by the Research and Innovative Technology Administration under the US Department of Transportation through the University Transportation Center research grant. The authors would like to extend appreciation to Dr. Recep Avci of the Imaging and Chemical Analysis Laboratory at Montana State University for the use of FESEM/EDX instrumentation and Dr. Trevor Douglas of the Department of Chemistry and Biochemistry at Montana State University for the use of FT-IR. We also greatly appreciate the donation of SBR latex from BASF that was used in this work. References [1] Ollitrault-Fichet R, Gauthier C, Clamen G, Boch P. Microstructural aspect in a polymer-modified cement. Cem Concr Res 1998;28(12):1687–93. [2] Koleva DA, Hu J, Fraaij ALA, van Breugel K, de Wit JHW. Microstructural analysis of plain and reinforced mortars under chloride-induced deterioration. Cem Concr Res 2007;37(4):604–17. [3] Beeldens A, Gemert D, Schorn H, Ohama Y, Czamecki L. From microstructure to macrostructure: an integrated model of structure formation in polymermodified concrete. Mater Struct 2005;38(6):601–7. [4] Basheer L, Kropp J, Cleland DJ. Assessment of the durability of concrete from its permeation properties: a review. Constr Build Mater 2001;15(2):93–103. [5] Ohama Y. Polymer-based admixtures. Cem Concr Compos 1998;20(2): 189–212. [6] Van Gemert D, Czarnecki L, Maultzsch M, Schorn H, Beeldens A, Łukowski P, et al. Cement concrete and concrete-polymer composites: two merging worlds. A report from 11th ICPIC congress in Berlin, 2004. Cem Concr Compos 2005;27(9):926–33. [7] Fowler DW. Polymers in concrete: a vision for the 21st century. Cem Concr Compos 1999;21(5):449–52. [8] Kardon JB. Polymer-modified concrete: review. J Mater Civil Eng 1997;9(2):85–92. [9] Ohama Y. Principle of latex modification and some typical properties of latexmodified mortars and concretes. ACI Mater J 1987;84(6):511–8. [10] Lewis WJ, Lewis G. The influence of polymer latex modifiers on the properties of concrete. Composites 1990;21(6):487–94. [11] Ohama Y. Recent progress in concrete-polymer composites. Adv Cem Based Mater 1997;5(2):31–40. [12] Shaker FA, El-Dieb AS, Reda MM. Durability of styrene–butadiene latex modified concrete. Cem Concr Res 1997;27(5):711–20. [13] Barluenga G, Hernández-Olivares F. SBR latex modified mortar rheology and mechanical behaviour. Cem Concr Res 2004;34(3):527–35. [14] Wang R, Wang PM, Li XG. Physical and mechanical properties of styrene– butadiene rubber emulsion modified cement mortars. Cem Concr Res 2005;35(5):900–6. [15] Rossignolo JA, Agnesini MVC. Durability of polymer-modified lightweight aggregate concrete. Cem Concr Res 2004;26(4):375–80. [16] Ohama Y. Polymer-based materials for repair and improved durability: Japanese experience. Constr Build Mater 1996;10(1):77–82. [17] Isenburg JE, Vanderhoff JW. Hypothesis for reinforcement of portland cement by polymer Latexes. J Am Ceram Soc 1974;57(6):242–5. [18] Zeng S. Polymer modified cement: hydration, microstructure and diffusion properties. PhD thesis, University of Aston; 1996. [19] Atkins PW. Physical chemistry. 5th ed. Oxford: Oxford University Press; 1994. [20] Gao JM, Qian CX, Wang B, Morino K. Experimental study on properties of polymer-modified cement mortars with silica fume. Cem Concr Res 2002;32(1):41–5. [21] Mollaha MYA, Yu W, Schennach R, Cocke DL. A Fourier transform infrared spectroscopic investigation of the early hydration of Portland cement and the influence of sodium lignosulfonate. Cem Concr Res 2000;30(2):267–73. [22] Dennis R. Latex in the construction industry. Chem Ind 1985;15(5):505–11. [23] Chandra S, Flodin P. Interactions of polymers and organic admixtures on portland cement hydration. Cem Concr Res 1987;17(6):875–90. [24] Page CL, Page MM. Durability of concrete and cement composites. Washington: CRC Press LLC; 2007. p. 365–88. [25] Afrid MUK, Ohama Y, Iqbal MZ, Demura K. Behavior of Ca(OH)2, in polymer modified mortars. Int J of Cem Compos Lightweight Concr 1989;11(4):235–44. [26] Sarkar SL, Xu A. Preliminary study of very early hydration of superplasticized C3A + gypsum by environmental SEM. Cem Concr Res 1992;22(4):605–8.