Construction and Building Materials 23 (2009) 2283–2290

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Construction and Building Materials

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Effect of styrene–butadiene rubber latex on the chloride permeabilityand 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, USAb Chemistry Department, Fuyang Normal College, Fuyang, Anhui 236041, People’s Republic of Chinac Civil Engineering Department, 205 Cobleigh Hall, Montana State University, Bozeman, MT 59717-2220, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 July 2008Received in revised form 20 October 2008Accepted 19 November 2008Available online 30 December 2008

Keywords:Chloride permeabilityMicrostructureStyrene–butadiene rubber latexPolymer-modified mortar

0950-0618/$ - see front matter Published by Elsevierdoi:10.1016/j.conbuildmat.2008.11.011

* Corresponding author. Address: Corrosion andLaboratory, Western Transportation Institute, P.O.University, Bozeman, MT 59717-4250, USA. Tel.: +1 401697.

E-mail address: Xianming_s@coe.montana.edu (X.

We evaluated the chloride permeability and microstructure of Portland cement mortar modified by sty-rene–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 per-meability of the mortar, while increasing its ionic transport resistance and decreasing its electric capac-itance. These data suggest that admixing SBR led to denser and more refined microstructure of the curedcement mortar. Field-emission scanning electron microscopy images confirmed such improvements inthe pore structure and the formation of an interpenetrating network structure of SBR and cement hydratephases at relatively higher P/C ratios. Besides slightly reducing Portlandite content and mitigating car-bonation with the increasing P/C ratio in mortar, SBR was also found to promote the formation of calciumaluminate trisulfate hydrate phases and facilitated chloride binding.

Published by Elsevier Ltd.

1. Introduction

The microstructure of mortar and concrete is of considerableimportance since it governs their mechanical properties, cementhydration and durability [1–3]. In addition, chloride permeabilityis recognized as a critical intrinsic property affecting the durabilityof reinforced concrete [4,5]. The use of polymer as a modifier innew structures seems to be a promising strategy in improvingmicrostructure and enhancing the durability of cement mortarand concrete [6–9]. As one of the popular polymers suitable foradmixing into fresh mortar and concrete, styrene–butadiene rub-ber (SBR) latex has been widely used for a long time [10,11]. Themolecular structure of SBR comprises both the flexible butadienechains and the rigid styrene chains, the marriage of which offersthe SBR-modified mortar and concrete many desirable characteris-tics such as good mechanical properties, water tightness and abra-sion resistance especially when an appropriate P/C ratio is used[12–14].

Previous studies indicate that the admixing of SBR latex intofresh mortar and concrete improved the resistance of hardenedmortar and concrete to chloride ion penetration [15,16]. Nonethe-

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less, the underlying mechanisms responsible for the role of SBR la-tex in such improvement remain debatable. Polymer films thatpossibly retard the cement hydration were found to form in thestructures of mortar and concrete, and an integrated three-stepmodel has been proposed in which the polymer film formationand the cement hydration processes develop simultaneously[3,5,17]. But interestingly, when tested separately, the polymerfilms were not effective chloride diffusion barriers on their own[18]. Therefore, what synergistic role SBR plays in reducing thechloride permeability of mortar and concrete is still not fullyunderstood. In addition to polymer film formation, we hypothesizethat SBR may alter the microstructure and chemistry of hardenedmortar and thus affect the ionic permeability and chloride bindingbehavior.

To test this hypothesis, this work systematically evaluates thechloride permeability and microstructure of SBR-modified Port-land cement mortars prepared with various P/C ratios. Electromi-gration and electrochemical impedance spectroscopy (EIS)measurements were conducted to investigate how the admixingof SBR affects the chloride permeability, the general ionic perme-ability and the microstructure of hardened mortar. Analytical tech-niques including field emission scanning electron microscopy/energy-dispersive X-ray spectroscopy (FESEM/EDX) and FourierTransform Infrared Spectroscopy (FT-IR) were utilized to shed lighton how SBR affects the cement hydration process, microstructuralcharacteristics of the resulting mortar, and possible interactionsbetween the SBR and cement hydrate phases in mortar.

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

Item Spec. limit Test result

SiO2 (%) N/A 20.4Al2O3 (%) 6.0 max 3.7Fe2O3 (%) 6.0 max 3.2CaO (%) N/A 63.3MgO (%) 5.0 max 3.2SO3 (%) 3.0 max 2.6Loss on ignition (%) 3.0 max 2.7Na2O (%) N/A 0.1K2O (%) N/A 0.4Insoluble residue (%) 0.75 max 0.43CO2 (%) N/A 1.7Limestone (%) 5.0 max 4.0CaCO3 in limestone (%) 70 min 98Tot. alkalies (% as Na2O) 0.60 max 0.37

Potential compound composition (%)C3S N/A 56C2S N/A 16C3A 8.0 max 4.5C4AF N/A 10C4AF + 2(C3A) N/A 19C3S + 4.75(C3A) 100 max 78

2284 Z. Yang et al. / Construction and Building Materials 23 (2009) 2283–2290

2. Experimental

2.1. Materials

An ASTM specification C150-07 Type I/II low-alkali Portland cement (ASH GroveCement Company, Clancy, MT.) was used in this study. The chemical compositionand 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 aggregatesize of 1.18 mm before proportioning and admixing. SBR latex (containing carbox-ylic 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 ratioof 2, a constant water-to-cement mass ratio of 0.45, and a polymer (total solids)-to-cement ratio (P/C) of 0%, 2%, 6%, 8%, 10%, 12% and 16%, respectively. For both thecontrol (without polymer admixed) and each polymer-modified Portland cementmortar, six samples were prepared to ensure the statistical reliability of chlorideion permeability test results. For the polymer-modified samples, the polymer latexwas first mixed into water in a mixer, and then cement was added and stirred thor-oughly in a low speed hand mixer for 5 min. Afterwards, sand was added into themixture 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 diameterand 8 mm thickness, which was carefully compacted to minimize the amount of en-trapped air. All the samples were demolded after curing under 20 ± 2 �C and over80% relative humidity for 24 h. After demolding, the samples without polymer ad-mixed were cured under 20 ± 2 �C and over 95% relative humidity for 27 days. For

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

Item Test method

Air content of mortar % C185Blaine fineness m2/kg air permeability test C204

Autoclave expansion % C151Normal consistency % C187Compressive strengths, psi (MPa) C109

1-Day3-Day7-Day

Setting times min C191Vicat initialVicat finalPass 325 mesh %Heat of hydration (cal/g)-7 daysFalse set % C451

the samples admixed with polymer, they were initially cured under 20 ± 2 �C andover 95% relative humidity for 6 days, and then left in the curing chamber with tem-perature of 20 ± 2 �C and 65% relative humidity for 22 additional days.

2.3. Measurements and characterization

2.3.1. Electromigration measurementsThe chloride ion permeability of the Portland cement mortars was determined

through electromigration experiments, conducted using the glass cell shown inFig. 1. The glass cell featured a disc-shaped mortar sample that separates the chlo-ride 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 glassycarbon electrode with an exposed surface area of 1 cm2. Once the mortar disc, elec-trolytes and electrodes were in place, an 8-V DC electric field was maintained acrossthe disc through the two carbon electrodes in the two compartments. During thetest, readings of open circuit potential (OCP) of the Ag/AgCl sensor in the destina-tion solution were taken periodically, using a saturated Calomel electrode (SCE)as the reference electrode. In addition, the electric current passing through the discwas monitored using a Gamry Reference 600TM Potentiostat/Galvanostat/ZRAinstrument. The Ag/AgCl sensor was used to monitor the evolution of free chlorideanion (Cl�) concentration in the destination solution, as their OCP data were com-pared against a standard calibration curve correlating potential readings withknown Cl� concentrations.

2.3.2. Electrochemical impedance measurementsAt the completion of the electromigration test, the Gamry Reference 600TM

Potentiostat/Galvanostat/ZRA instrument was employed to measure electrochemi-cal impedance spectroscopy (EIS) data in order to characterize the microstructuralproperties 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 elec-trode and the SCE in the right (anodic) compartment served as the working elec-trode and the reference electrode, respectively. The EIS measurements were takenby polarizing the working electrode at ±10 mV around its OCP, using sinusoidal per-turbations 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 analysesFT-IR analyses were conducted on the samples that had not been tested for elec-

tromigration. The samples were ground and mixed with pre-dried FT-IR gradepotassium bromide and pressed into pellet with the KBr/sample mass ratio of100. Transmission infrared spectra of the samples were recorded using NicoletNEXUS 670 FT-IR spectrometer over the wavenumber range of 4000 to 400 cm�1.Each of the samples was scanned 64 times.

2.3.4. FESEM imaging and EDX analysesFor surface analyses, the mortar samples were taken out of the glass cell after

the electromigration and electrochemical measurements. A strip of cross-sectionalsample representing relevant changes in chloride penetration and microstructuralproperties was cut from the mortar disc along its diameter. After being vacuum-dried, the slice samples were then subjected to FESEM/EDX to examine its localizedmorphology and elemental distributions at the microscopic level, using a Zeiss Su-pra 55VP PGT/HKL system coupled with the energy dispersive X-ray analyzer. TheEDX data were obtained using a micro-analytical unit that featured the ability todetect the small variations of trace element content. We used FESEM/EDX underpressure, typically 10�2–10�3 torr, to investigate the effect of SBR latex on the mor-phology and chemistry of cement hydrates.

Spec. limit Test result

12.0 max 7.9280 min 397420 max0.80 max 0.03N/A 26.6

N/A 1998 (13.8)1740 (12.0) min 3472 (23.9)2760 (19.0) min 4600 (31.7)

45 min 111375 max 24072 min 98.7N/A 70.750 min 85

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

Z. Yang et al. / Construction and Building Materials 23 (2009) 2283–2290 2285

3. Results and discussion

3.1. Effect of SBR latex on the ionic permeability of cement mortar

Fig. 2 illustrates the typical results obtained from the electromi-gration test. The breakthrough time t0 is the point after which theCl� concentration in the destination solution (anolyte) increaseslinearly with time. Under the experimental conditions of thisstudy, t0 was mostly obtained after a chloride concentration of0.003–0.005 mol/L had been reached in the destination compart-ment (0.075–0.125 mmol of chloride ions had passed throughthe mortar sample). The electromigration test for all samples wasstopped when an OCP reading reached about 67–70 mV (chlo-ride concentration of 0.020–0.023 mol/L) in the destinationcompartment.

The method used to calculate the apparent diffusion coefficientD of Cl� in cement mortars is described as follows. Under an exter-nally imposed electric field with an intensity of E (in V/m), themobility of ions (the average velocity of the ions per unit of electric

0 200 400 600 800 1000

t0

Chl

orid

ion

Con

cent

ratio

n(M

)

Time(minutes)

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

Fig. 2. Evolution of chloride concentration over time in the destination compart-ment during electromigration test.

field) is related to the diffusion coefficient through the Nernst–Ein-stein equation.

t ¼ zFDRT

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)) andabsolute temperature (298.15 K, or 25 �C) [19], respectively. Thechloride ion mobility can be calculated from the time t0 requiredfor the chloride front to penetrate a depth d (in m) of the sample:

t ¼ dt0E

The chloride penetration time t0 (in s) was estimated from themonitored Cl� concentrations in the anolyte (destination solution).t0 is the point after which the Cl� concentration in the anolyte in-creases linearly with time.

Therefore, the diffusion coefficient D of Cl� in cement mortarscan be estimated by the equation below (in m2/s):

D ¼ dRTt0EzF

Table 3 shows the electromigration test results of the appar-ent diffusion coefficients of Cl� in the cement mortars, with themean calculated from three or four samples. It can be seen fromTable 3 that the incorporation of SBR latex improved the chlo-ride penetration resistance of the mortar, as indicated by the re-duced apparent diffusion coefficients of chloride ions, DCl� . Whenthe P/C ratio was 16%, the value of DCl� decreased by 65% com-pared with conventional Portland cement mortar. As shown inFig. 3a, DCl� decreased linearly with the increase in P/C ratio(i.e., the SBR content admixed in fresh mortar), under the exper-imental conditions of this study.

The evolution of electric current over time was monitored dur-ing the electromigration test. These data were used to calculate theamount of electric charge (Q) passing through the mortar discs byintegrating the current curve over time. Q can be considered as anindicator of electrical conductivity of mortar, derived from both thepore structure characteristics and pore solution chemistry of mor-tar. The results in Table 3 show that the incorporation of SBR latexreduced the general ionic permeability of the mortar, as indicatedby the reduced Q value (integrated over the first 360 min). Interest-ingly, there was a linear correlation between DCl� (characteristic ofchloride permeability) and Q360 min (characteristic of general ionicpermeability), as illustrated in Fig. 3b.

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

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

DCl� (�10�11m2 s�1) 2.24 1.96 1.60 1.43 1.35 1.14 0.78Q360 min (�104 cm�2) 121.03 100.17 91.81 78.56 72.25 70.64 51.28

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The electromigration test results revealed that with the increas-ing amount of SBR latex admixed into fresh cement mortar, boththe chloride penetration resistance and the ionic transport resis-tance of the hardened mortar tended to be enhanced, althoughsometimes such improvements were small. These findings demon-strate that the use of SBR latex as a cement modifier is a promisingstrategy in improving concrete durability and the beneficial effectsincrease with an increase in the polymer content.

3.2. Effect of SBR latex on the microstructure of cement mortar

3.2.1. Investigation by EISEIS provides information on interfaces and thus was utilized to

shed light on the microstructural properties of the cement mortarsin this study. The complex impedance of the mortar/electrolyteinterface depends on the frequency of the externally imposed ACpolarization signal, allowing the representation of the system withan equivalent circuit typically consisting of resistors and capaci-tors. For this experiment, the equivalent circuit shown in Fig. 4

y = -8.7393x + 2.1742

R2 = 0.9909

0

0.5

1

1.5

2

2.5

0 0.05 0.1 0.15 0.2

P/C

D C

l- (

X10

-11 m

2 s-1)

y = 45.779x + 15.008

R2 = 0.9736

0

20

40

60

80

100

120

140

0 0.5 1 1.5 2 2.5

D Cl- (X10-11 m2s-1)

Q 3

60m

in (

X10

4 Cm

-2)

Fig. 3. (a) Evolution of apparent chloride diffusion coefficient in mortar with theincrease in the P/C ratio; (b) correlation between the chloride permeability (DCl� )and the general ionic permeability Q360 min.

Fig. 4. The equivalent circuit used fo

was used to characterize the interfaces between the counter elec-trode and the working electrode separated by the mortar disc. Con-stant phase elements (Q) instead of capacitances were used in allfittings. Such modification is obligatory when the phase angle ofthe capacitor is different from �90�. The EIS parameters that char-acterize the electrolyte/mortar interface are the ionic transportresistance of the mortar disc (Rmortar) and the capacitance (in thiscase, constant phase element) of the disc (Qmortar). For the elec-trode/electrolyte interfaces, we can assume that each interfacehas a double layer capacitance (in this case, Q1 and Q2, respectively)and a charge transfer resistance (R1 and R2, respectively). In addi-tion, we assign the Warburg impedance (W) to only one interfacecharacterizing the diffusion of species through the interface.

Table 4 presents the fitted equivalent circuit parameters of boththe control and SBR-modified mortar discs, in which nmortar

(0 < nmortar < 1) is the fitting coefficient for Qmortar (with 1 beingthe perfect fit of a capacitor and 0 being the worst).

As shown in Table 4, compared with the conventional Portlandcement mortar disc, the discs modified by SBR latex had higher val-ues of Rmortar and lower values of Qmortar. Rmortar characterizes theporosity of the mortar disc and is a function of the resistance of io-nic transport through the interconnected pores, cracks and poten-tially air voids (containing some carbon dioxide, oxygen and watervapor) in the disc. The increased ionic transport resistance and de-creased electric capacitance of SBR-modified mortar discs suggestthat the incorporation of SBR latex led to denser and more refinedmicrostructure of cement mortar. Such effects were especiallynotable when the P/C ratio in fresh cement mortar exceeded 10%.The reduced porosity and increased density in microstructure havebeen reported to provide cement mortars with higher flexural andcompressive strength, higher microhardness in interfacial zone andlower effective chloride diffusion coefficient in matrix, when poly-acrylic ester emulsion and silica fume were used together [20]. It isalso noted that compared with the conventional Portland cementmortar disc, the SBR-modified mortar discs exhibited slightly

r fitting the impedance spectra.

Table 4Equivalent circuit parameters of the mortar discs.

P/C(%) Rmortar (kX cm2) Qmortar (lS cm�2) nmortar

0 10.57 ± 0.28 213.5 ± 2.61 0.83 ± 0.0082 15.43 ± 0.23 197.8 ± 1.93 0.85 ± 0.0076 159.6 ± 4.01 176.1 ± 0.63 0.88 ± 0.0048 168.5 ± 6.09 143.9 ± 1.21 0.89 ± 0.00410 176.4 ± 4.53 79.80 ± 0.62 0.89 ± 0.00312 243.9 ± 8.72 75.29 ± 0.45 0.89 ± 0.00416 457.8 ± 19.9 73.75 ± 0.66 0.91 ± 0.002

4000 3500 3000 2500 2000 1500 1000 500

g

f

e

d

c

b

aTansmittance(%)

Wavenumber(cm-1)

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%.

Z. Yang et al. / Construction and Building Materials 23 (2009) 2283–2290 2287

higher values of nmortar, likely due to lower porosity and highermicrostructural regularity of the discs.

Combing the data in Tables 3 and 4, a strong correlation can beseen between DCl� and both Rmortar and Qmortar. As the P/C ratio in-creased, DCl� decreased linearly, Rmortar increased nonlinearly andQmortar decreased nonlinearly. It is concluded that these threeparameters were all indicators of the microstructure and chemicalcomposition of the saturated mortar matrix, in which the ionic spe-cies transport mainly via natural diffusion or electromigration.

We can attribute the findings from the electromigration and EISdata to the following mechanisms. For the SBR-modified mortars,the large pores can be filled with SBR or sealed with continuous

CO C O

O O

CO C O

O O

Ca

SBR CHAIN

Interaction with Ca2+ ions

Bindiceme

CO C O

O O

C OCO

OOCa Ca

SBR CHAIN

SBR CHAIN

SBR CHAIN

Fig. 6. Possible interactions between SBR partic

SBR films. When a high content of SBR latex is used, both cementhydration and SBR film formation proceed well to generate a co-matrix phase with an interpenetrating network structure of SBRand cement hydrate phases that binds the sand particles strongly.The reduced porosity and other alternations at the microstructurelevel leads to macroscopic changes in the properties of the SBR-modified mortar, such as reduced water permeability, higher ionictransport resistance, and lower electric capacitance.

3.2.2. Investigation by FT-IRFT-IR is one of the versatile tools widely used for molecular

characterization and has been found to be very useful in predictingthe degree of hydration and monitoring the dynamics of changesduring the hydration reaction in Portland cement [21]. In thisstudy, FT-IR was utilized to evaluate the effects of polymer modi-fication on the hydration processes and possible interactions be-tween the SBR and cement hydrate phases. The typical FT-IRspectra of SBR-modified cement mortars with various P/C ratiosare shown in Fig. 5.

It can be observed from the spectra that few changes occurredwhen the P/C ratio remained below 10%. However, when P/C ratioexceeded 10%, the absorption peak at 3642 cm�1 corresponding tothe –OH groups in Portlandite had a slight decrease, although thedecreasing trend was not obvious at higher P/C ratios. The peaksat 1077 cm�1 and 997 cm�1 corresponding to silicate phases(�SiO4) underwent changes among these samples; the former in-creased slightly while the later decreased. The changes indicatethat the cement hydration was influenced by the admixing ofSBR latex into fresh mortar. Furthermore, it is apparent that theabsorption peaks at 874 cm�1 and 1442 cm�1 corresponding tocarbonate phases (�CO3) mostly due to the carbonation showeda gradual decrease with the increase in the SBR content. In contrastto plain mortar, the reduced carbonate content in SBR-modifiedmortars suggests an improvement in carbonation resistance andsuch improvement became remarkable when the P/C ratio ex-ceeded 10%. In addition, the absorption peaks at 1640–1645 cm�1

and 3440–3446 cm�1 derived from molecular water exhibited aslight decrease when the P/C ratio exceeded 10%, implying a reduc-tion in water absorption by the mortar.

CO C O

O O

Ca

Anhydrous cement grains or cement hydrates

ng tont grains

SBR CHAIN

CO C O

O O

C OCO

OOCa Ca

SBR CHAIN

SBR CHAIN

les with cement grains or cement hydrates.

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 magnificationapproximately 7600 times.

0

0.05

0.1

0.15

0.2

0% 2% 6% 8% 10% 12% 16%

P/C

S/A

l(bas

ed o

n E

DX

dat

a)

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

2288 Z. Yang et al. / Construction and Building Materials 23 (2009) 2283–2290

Z. Yang et al. / Construction and Building Materials 23 (2009) 2283–2290 2289

We propose the following mechanisms to account for the re-duced Portlandite content and improved carbonation resistanceof the SBR-modified cement mortar. Usually, a small amount ofcarboxylic acid is chemically bound onto the polymer particle sur-face. These groups, ionized in the highly alkaline environment offresh mortar, tend to interact with calcium ions from the cementhydrates, which generally results in improved stability of the poly-mer latex and adhesion of the polymer-modified mortar to existingsubstrates [22–24]. Fig. 6 illustrates the possible interactions be-tween SBR particles with cement grains or cement hydrates, whichis consistent with the conclusion obtained by other researchers inthat polymer modifier absorbs calcium on the formed polymer filmand thus reduces the formation of Portlandite with the increase ofpolymer content in mortar [25]. On the other hand, at higher P/Cratios, the SBR latex particles distributed more homogeneously inthe mortar and the microstructure of the mortar became denserand more refined. As a result, the porosity and pore size of theSBR-modified mortar were eventually reduced, which led to re-duced availability of airborne CO2 (as well as oxygen and water)in mortar and thus enhanced carbonation resistance of the hard-ened mortar.

3.2.3. Investigation by FESEMRecent technological advances in FESEM enable the observation

to be performed under a weak vacuum and thus allow better reten-tion of moisture in the samples. As such, cement hydration andmicrostructure of the cement mortars can be studied without suf-fering from the micro-shrinkage or crystallization due to moistureevaporation [26].

Fig. 7a–g show the representative FESEM images of mortar sur-faces with various P/C mass ratios subsequent to the electromigra-tion and electrochemical measurements, all of which were taken atmagnification of approximately 7600 times. In contrast to the con-trol sample (a) that exhibited a loosely connected microstructure,the structure of the SBR-modified mortar was compact even whenonly a small amount (e.g., P/C of 2%) of SBR was admixed (b). Thepolymer binders were found on the mortar surface although theconnections between the fissures were very thin at a P/C ratio of6% (c). At a P/C ratio of 8% (d), some scattered polymer bridges(confirmed by relatively higher carbon content at these sites basedon EDX data) were clearly visible on the mortar surface even

0% 2% 6% 8%

0.0

0.5

1.0

1.5

2.0

2.5

3.0

DCl-

C

P/C

DC

l- ( x

10

-11m

2 .s-1 )

Fig. 9. Apparent chloride diffusion coefficient (left) and

though the effect of SBR on the microstructure of the cement hy-drates remained unclear. In the case of a P/C ratio of 10% (e), morecontinuous polymer bridges are observed and the bridges could beseen situating at the surface of the sand particles as well as in thebulk binder matrix. When the P/C ratio was increased to 12% (f),the polymer bridges became wider and a coherent polymer filmformed in the modified mortar. The interpenetrating networkstructure of SBR and cement hydrate phases was formed to bindthe sand particles together. The interpenetrating network structurefully developed when the higher P/C ratio of 16% (g) was employedwhere the formed polymer film became thicker and continuous.When compared with the control sample (a), the SBR-modifiedsamples seemed to have a second matrix integrated throughoutthe mortar surface. The porosity and pore size of this second matrixbecame smaller with the increasing P/C ratio, indicating that con-tinuity of the SBR film gradually increased and became notable inthe case of a high SBR content. These microscopic observationsagreed well with the experimental data and mechanisms discussedearlier.

3.2.4. Investigation by EDXUsing an appropriate accelerating voltage (20 kV) with a scan

time of 60 s per sampling point, the EDX data provided the chem-ical composition at a large volume, as in the depth of approxi-mately 1 lm from the surface to the mortar bulk matrix. TheEDX data were taken from six different sites on the surface of eachmortar sample subsequent to the electromigration and electro-chemical measurements. Each site corresponded to a selected areaof approximately 40 lm � 40 lm.

Fig. 8 shows the S/Al elemental ratio of the surface on mortarsamples with various contents of SBR latex admixed, with eachdata point averaged from the multiple EDX measurements. It canbe seen that average S/Al ratio on mortar surface increased withthe rising P/C ratio, which suggests that SBR latex facilitated theformation and stability of sulfate-containing AFt phases (e.g.,ettringite). Tricalcium aluminate (known as C3A phase) is animportant phase in cement, most of which is readily dissolvableby water. Hydration of C3A in the presence of gypsum initially pro-duces ettringite, which is usually unstable and gradually reactswith C3A to form more stable monosulfate (known as AFm phase).The related reactions are shown as follows.

10% 12% 16%

l%

0.00

0.04

0.08

0.12

0.16

0.20

0.24

Cl ( w

t%, based on E

DX

data )

average Cl content (right) as a function of P/C ratio.

2290 Z. Yang et al. / Construction and Building Materials 23 (2009) 2283–2290

3CaO � Al2O3ðC3AÞ

þ3CaSO4 � 2H2OðGypsumÞ

þ32H2O

¼ 3CaO � Al2O3 � 3CaSO4 � 32H2OðEttringite; AFt phaseÞ

3CaO � Al2O3ðC3AÞ

þ3CaO � Al2O3 � 3CaSO4 � 32H2OðEttringite; AFt phaseÞ

¼ 2ð3Cao � Al2O3 � CaSO4 � 12H2OÞ þ CaSO4 þ 8H2OðMonosulfate; AFm phaseÞ

When SBR latex was added, however, some of the C3A was con-tained inside cement grains with no initial access to water and nofurther access to ettringite (as shown in Fig. 6). As such, SBR pro-moted the reaction of tricalcium aluminate with gypsum and thusfacilitated the formation and stability of ettringite. Fig. 9 showsthat as the P/C ratio increased, the hardened mortar consistentlyexhibited higher Cl content (based on EDX data) and lower appar-ent diffusion coefficient of chloride ions (DCl� ). These trends alongwith the increase of S/Al with the P/C ratio can be explained by thefollowing mechanism. As the content of admixed SBR increased,more AFt phases formed in the hardened mortar matrix, duringthe electromigration test some of which converted to chloroalumi-nate via the partial substitution of the sulfate anion by the chlorideanion in the calcium sulfoaluminate hydrates. One possible reac-tion of such chloride binding is shown as follows:

3CaO � Al2O3 � 3CaSO4 � 32H2Oþ 6Cl�

¼ 3CaO � Al2O3 � 3CaCl2 � 32H2Oþ 3SO2�4

4. Conclusions

This experimental study evaluated the chloride permeabilityand microstructure of SBR-modified Portland cement mortars,which were prepared with various polymer/cement (P/C) mass ra-tios, a constant water/cement ratio of 0.45 and a constant sand/ce-ment ratio of 2.

Electromigration tests demonstrate that the incorporation ofSBR latex improved the chloride penetration resistance of the mor-tar, as indicated by the reduced apparent diffusion coefficients ofchloride anion (DCl� ). The SBR latex also reduced the general ionicpermeability of the mortar, as indicated by the reduced electriccharge (Q) passing through the samples.

Subsequent to the electromigration test, each mortar samplewas subjected to the EIS measurements with a large frequencyrange of 5 mHZ � 300 KHZ. The incorporation of SBR latex in ce-ment mortar increased its ionic transport resistance and de-creased its electric capacitance, which are governed by thepore structure characteristics and pore solution chemistry ofthe mortar.

The FESEM images suggest that the admixing of SBR latex infresh mortar altered the morphology and microstructure of thehardened mortar. Through the cement hydrate matrix, a continu-ous polymer film became more and more visible with the increas-ing P/C ratio in the mortar. At a P/C ratio higher than 10%, theinterpenetrating network structure of SBR and cement hydrateswas found to bind the sand particles together.

The FT-IR spectra indicate that the incorporation of SBR latex incement mortar slightly reduced Portlandite content and mitigatedcarbonation. The EDX data indicate that the admixing of SBR latexpromoted the formation of AFt phases and facilitated chloridebinding via the partial substitution of the sulfate anion by the chlo-ride anion in the calcium sulfoaluminate hydrates during the elec-tromigration test.

This work brought new insights into the interaction mecha-nisms between cement hydration and SBR latex modifier. It pro-

vided improved understanding of the effect of admixed SBR latexon the microstructure, chemistry, ionic permeability and chloridebinding behavior of Portland cement mortar. Such knowledge isexpected to contribute to the effort of searching for effective mea-sures to improve the durability of cement mortar and concrete in achloride-laden environment.

Acknowledgements

This work was supported by the Research and Innovative Tech-nology Administration under the US Department of Transportationthrough the University Transportation Center research grant. Theauthors would like to extend appreciation to Dr. Recep Avci ofthe Imaging and Chemical Analysis Laboratory at Montana StateUniversity for the use of FESEM/EDX instrumentation and Dr. Tre-vor Douglas of the Department of Chemistry and Biochemistry atMontana State University for the use of FT-IR. We also greatlyappreciate the donation of SBR latex from BASF that was used inthis work.

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