Effects and mechanism of atmospheric multi-acidic gases on cement-based concrete linings of vehicle tunnels

Construction and Building Materials 29 (2012) 438–443

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Effects and mechanism of atmospheric multi-acidic gases on cement-based concrete linings of vehicle tunnels Ming-fang Ba a,c, Chun-xiang Qian a,b,⇑, Yuang Zhuang a,b a

School of Materials Science and Engineering, Southeast University, Nanjing 211189, China Jiangsu Key Lab of Construction Materials, Nanjing 211189, China c Department of Civil Engineering, Ningbo University, Ningbo 315211, China b

a r t i c l e

i n f o

Article history: Received 22 November 2010 Received in revised form 13 September 2011 Accepted 29 September 2011 Available online 29 November 2011 Keywords: Concrete neutralization Vehicle tunnels Multi-acidic gases

a b s t r a c t In order to analyze the effects of atmospheric multi-acidic gases inside vehicle tunnels on concrete, mass gains and neutralizing depths of concrete specimens in accelerated simulation chambers filled with multi-acidic gases or single acidic gas were measured and analyzed. The results showed that the neutralizing rate of specimen in multi-gases chambers was higher by 9.6–12% than that of in corresponding single carbonation chamber. Then concrete specimen exposed to single SO2 attack exhibited high mass gain and low neutralized depth compared with those exposed to single NO2 or CO2 attack. This should be attributed to the facts that gaseous SO2 diffused into concrete nearly all dissolved in pore solution of concrete while larger ratio of gaseous CO2 or NO2 still exit and diffused in un-saturated pores of concrete. Furthermore the products CaSO4xH2O by sulphation in concrete made the further diffusion of SO2 more difficult. Thus the concrete sulphation by SO2 mainly was concentrated on the surface of concrete while the neutralizing process of CO2 or NO2 could promote further inside the concrete. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The air quality is getting worse and worse in urban air and especially in vehicle tunnels. It is well known that motor vehicles are considered as one of the major contributors to the atmospheric pollution [1]. The gaseous multi-pollutants from motor emission mainly consist of CO2, NOX and SO2, which have very direct effects on the deterioration of construction materials. A large number of experts [2–6] reported the maximum concentration of CO2, NO2 and SO2 in vehicle tunnels were several times of that in normal atmosphere levels such as the maximum concentration of CO2 in vehicle tunnel reached 760 ppm, NOx concentration was about 2000 lg/m3 or so and SO2 concentration was 150 or so. Thus it is very necessary to investigate the effects of multi-acidic gases on neutralization of concrete linings inside vehicle tunnels. The above acidic gases are transferred to building material surfaces by two mechanisms [1]: dry deposition, which proceeds by aerodynamic transfer of aerosols to material surfaces, without the aid of hydrometeors; and wet deposition, consisting in the transfer of trace gases and particles occurring in an aqueous form i.e. rain, snow or fog. The effects and mechanism of wetted atmospheric acidic gases on cemented materials have been addressed ⇑ Corresponding author at: School of Materials Science and Engineering, Southeast University, Nanjing 211189, China. Tel./fax: +86 025 52090637. E-mail addresses: [email protected], [email protected] (C.-x. Qian). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.09.016

by a large amount of literatures [7–11] while the effects of dried atmosphere multi-gases on cement-based materials were only in limited attention [12–16] although a few studies have focused on air-hardening and hydraulic mortars [17,18]. However little information is available on the impact of atmospheric multi-gases on cemented materials in vehicle tunnels. Thus the deep deterioration mechanism of concrete by atmosphere multi-gases and corresponding changes of pore structure are still worth studying. In this paper, the neutralizing experiments of concrete specimens with different water-binder ratios by gaseous CO2, NO2 and SO2 were conducted in self-designed neutralizing chambers filled with different acidic gases. Then the neutralized samples were analyzed by means of XRD, DTG and SEM to explain the neutralization mechanism. 2. Experimental 2.1. Experimental materials and specimen preparation The cement used was P.I 52.5 Portland cement according to Chinese standard (GB 175-2007) [19] and its chemical compositions were presented in Table 1. The fine aggregate was quartz sand with 2.7 fineness modulus. The coarse aggregate was basalt with 5–31.5 mm diameter and the high-range water-reducing admixture was used to make mixed concrete with the nearly same fluidity. Specimens (100 mm  100 mm  100 mm) were cast according to the mix proportion in Table 2. After curing at 20 ± 1 °C with 90% relative humidity for 28 days, all the specimens were dried at 60 °C in oven for 3 days and then were sealed by epoxy resin with the exception of the opposite surface. After then they were put into the corresponding simulating chambers for neutralization.

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M.-f. Ba et al. / Construction and Building Materials 29 (2012) 438–443 Table 1 Chemical compositions of cement. Label

Oxide compositions (wt.%)

Cement

SiO2 19.94

Al2O3 4.41

Loss (%) Fe2O3 4.01u

CaO 63.79

Table 2 Concrete proportions (kg/m3).

MgO 2.24

SO3 2.48

Na2O 0.10

K2O 0.43

1.78

Table 3 Concentration of multi-gases in neutralization chambers.

No

W/C

Cement

Sand

Gravel

Water

fCu/MPa

No

CNS

C

N

S

NS

C31 C50 C65

0.31 0.50 0.65

450 320 285

760 864 736

1050 1056 694

140 160 185

57.5 42.2 35.6

SO2 (mg/m3) NO2 (mg/m3) CO2 (g/m3)

100 1333 995

– – 995

– 1333 –

1333

1333 1333 –

2.2. Self-designed accelerated neutralization chambers

–

Table 4 Concentration of single acid gases in neutralization chambers.

Hard resin chambers were sealed with glass cement, inside which the acidic gases were in situ produced based on the flowing chemical reactions:

Na2 SO3 ðsÞ þ H2 SO4 ðlÞ ! SO2 ðgÞ þ Na2 SO4 ðSÞ

ð1Þ

CuðsÞ þ HNO3 ðlÞ ! NO2 ðgÞ þ CuðNO3 Þ2 ðSÞ

ð2Þ

Na2 CO3 ðsÞ þ HClðlÞ ! CO2 ðgÞ þ NaClðsÞ

ð3Þ

The relative humidity inside chambers was kept 60–70% by saturated solution of NaNO3. And all experimental chambers were placed in the room at 20 ± 2 °C. The schematic and practical chambers were shown in Fig. 1. 2.3. Determination of multi-acidic gases concentration Based on concentration of acidic gases in vehicle tunnels investigated by several literatures, the concentrations of multi CO2, NO2 and SO2 in accelerated chambers were determined and showed in Table 3, which was proportional to the concentration ratio of multi-acidic gases in vehicle tunnels. 2.4. Determination of single-acidic gases concentration In order to investigate the mechanism of neutralizing deterioration by NO2, SO2 and CO2 respectively, three concentrations for each single acidic gas were designed and Table 4 showed the corresponding concentrations. 2.5. Experimental program 2.5.1. Observation of appearance and mass changes One slice (15 mm  50 mm  50 mm) was cut from specimen C-31 and weighted before being put into simulating chambers. At certain neutralizing age, appearance changes of slices can firstly be observed and the mass changes also can be measured. 2.5.2. XRD, TGA and SEM analysis At certain neutralizing age, specimens C-31 in simulated chambers with the gaseous concentration in Table 3 were taken out and cut into fractions along different distances from exposed surface. Concrete from each fragment was crushed and the aggregate grains were removed. Then the crushed fragments, composed of nearly neat hardened cement paste, were pulverized to the fineness below 0.08 mm and dried only at 35 °C in order not to decompose the eventually present

SO2 NO2 CO2

1

2

3

S1 = 0.1424 N1 = 0.1024 C1 = 0.0979

S2 = 0.2848 N2 = 0.2047 C2 = 0.1958

S3 = 0.5966 N3 = 0.4094 C3 = 0.3916

crystal hydrates. Each pulverized sample was then subjected to X-ray diffraction (XRD) analysis and thermal analysis (TG–DTG–DTA). Furthermore some fragments with aggregate excavated were used for observation with scanning electron microscopy (ESEM).

3. Results and discussions 3.1. The changes of appearance and mass gains After neutralizing for 60 days, the specimens in each neutralizing chamber should have been neutralized to certain extent. It can be seen from Fig. 2 that the appearance for specimen C-31 in chambers with gaseous concentration in Table 3 changed a lot. From Fig. 2 it can be observed that the surface of specimen matrix exposed to single SO2 takes on obvious yellow dots and the one suffering from single NO2 exhibits few changes while ITZ between aggregate and matrix exerts highly corroded boundaries. Regarding the specimens attacked by couple NO2 and SO2, its paste matrix illustrates shallow yellow and its cement-aggregate interface is corroded even more seriously, which indicated the joint effects of NO2 and SO2 on concrete specimen. Moreover, the matrix surface of specimen exposed to ternary CO2, NO2 and SO2 turns into white powder and its cement-aggregate interfaces are also eroded to certain extent, which accounted for joint neutralization effects of NO2, SO2 and CO2. It can be seen from Fig. 3 that the mass changes of specimens C31 varies with neutralizing ages. It was denoted that the slop at each point of the curves represented the corresponding dry

Fig. 1. Schematic and practical pictures for accelerated neutralization experiment.

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Neutralization depth /mm

6 5 4

C31-CNS C31-C

3 2 1 0

0

10

20

30

40

50

60

70

80

90

100

Exposure time /d Fig. 4. Neutralization depths of sample C-31 in neutralizing chambers.

3.3. Neutralization depths in single-gas chambers

Fig. 2. Changes of external appearance of corroded concrete by different acid gases.

20.0 C31-S C31-N C31-NS

Mass gain /mg/mm

2

17.5 15.0 12.5 10.0 7.5 5.0 2.5 0.0

0

10

20

30

40

50

60

70

80

90

100

Exposure time /d

The neutralization depth was greatly correlated with the concentration and kind of acidic gases. Fig. 5 illustrates the neutralization depth changes of specimens with different water-binder ratio in single-gas chambers with different concentration shown in Table 4. It could be seen from Fig. 5 that single SO2 gas exhibits the lowest neutralizing depth compared with that of single NO2 or CO2 gases even at the same concentration condition, and also that the neutralizing rate of single NO2 differs little with that of single CO2. It is also notable that specimens in single-NO2 chambers show higher neutralization depths than those in single SO2 chamber, which is contradict with the results of corresponding experimental results of mass gain. This indicated that the high deposition rate of SO2 in concrete did not mean much deeper neutralizing depth. That is to say, SO2 gases mainly deteriorated the surface concrete specimens. It is also shown in Fig. 5 that with increase of waterbinder ratio of specimens the corresponding neutralization depth in single-gas chambers increase obviously, which could be attributed to the higher porosity of specimen with lower water-binder ratio.

Fig. 3. Changes of mass gains of sample C-31 in neutralizing chamber.

3.4. Neutralization mechanism analysis deposition rate of acidic gases. From Fig. 3 it can be also observed that the dry deposition rate decreases with the increase of exposure time in simulating chambers. And it is also obvious that the deposition rate of NO2 in concrete is lower than that of SO2 at the same concentration. Moreover, the co-effects of NO2 and SO2 leads to a small increment of mass gains for each specimen. This again attested that it was very necessary to investigate the joint effect of multi-acidic gases in vehicle tunnels on neutralization of concrete lining.

Neutralization is a combing process of gases diffusion in concrete and chemical reaction between acidic gases and hydrated products of concrete, which reduces the pH values of concrete below the bottom point of stability. With the similarity with carbonation by CO2, SO2 and NO2 gases firstly invaded into concrete by the way of dry deposition, then partially dissolved in pore solutions. Finally the gaseous and dissolved NO2 or SO2 would diffuse into the concrete to further the neutralization depth.

3.2. Neutralization depths in multi-gases chambers

3.4.1. Dissolving and neutralizing process Henry’s constant of gas NO2 is 9.87  108 mol/L Pa at normal temperature and the dissolving and neutralizing process are illustrated as

As neutralization processes, acidic gases dissolve into the pore solution of concrete to form H+ reacting fast with OH in solutions, which leads to the decreased pH values of pore solution in concrete. It is well known that lower pH value of solution makes concrete not to be protected from corrosion any more. Thus the neutralization depth is a crucial parameter to evaluate the neutralization degree of concrete. It is shown from Fig. 4 that the neutralization depths for specimen C-31 increase with different exposure ages in simulated multi-gases and single-CO2 chambers shown in Table 3 and the joint effects of NO2, SO2 and CO2 leads to the increased neutralization depth by 9.6–12% compared with that of single carbonation.

NO2 ðgÞ þ H2 O $ NO2  H2 O HNO2 ¼

½NO2  H2 O PNO2

2NO2 þ H2 O $ NO2 þ 2NO3 þ 2Hþ As little produced

NO 2,

NO2  H2 O $ NO3 þ 2Hþ Hþ þ OH $ H2 O

ð4Þ ð5Þ

Eq. (5) can be simplified as

KN ¼

½NO3   ½Hþ 2 KHN  PNO2

ð6Þ ð7Þ

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M.-f. Ba et al. / Construction and Building Materials 29 (2012) 438–443

10

Neutralization depth (mm)

Neutralization depth (mm)

4

3

2

SO2-0.31 SO2-0.50

1

SO2-0.65 0 0.0

0.1

0.2

0.3

0.4

0.5

8 6 4

NO2-0.31 NO2-0.50

2

NO2-0.65 0 0.0

0.6

0.1

0.2

0.3

0.4

0.5

0.6

Concentration of acid gases (g/L)

Concentration of acid gases (g/L)

(b) NO2

(a) SO2 Neutralization depth (mm)

10 CO2-0.31 CO2-0.50

8

CO2-0.65

6 4 2 0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Concentration of acid gases (g/L)

(c) CO2 Fig. 5. Neutralization depth of specimens in single-gas chambers.

The Henry’s constant of gas CO2 at the same condition is 3.36  107 mol/L Pa and its dissolving and ionizing processes are expressed as

CO2 þ H2 O $ CO2  H2 O HCO2

H2 CO3 $ Hþ þ HCO3

HCO3 $ Hþ þ CO2 3

KC 1 ¼

KC 2

h i COl2 ½CO2  H2 O ¼ ¼ PCO2 PCO2

½Hþ ½HCO3  ½H2 CO3 

h i ½Hþ  CO2 3  ¼  HCO3

ð8Þ

ð9Þ

ð10Þ

Furthermore Henry’s constant of gas SO2 is 1.22  105 mol/ L Pa at the same condition and its dissolution and neutralizing process are shown as follows:

SO2 ðgÞ þ H2 O $ SO2  H2 O KHS ¼

SO2  H2 O $ Hþ þ HSO3

HSO3 2SO2 3

$ Hþ þ

SO2 3

KS1 ¼

½SO2  H2 O PSO2

½Hþ   ½HSO3  ½SO2  H2 O

½H   ½SO2 3  ½HSO2 

ch

ð12Þ S

ð13Þ

2SO2 4

ð14Þ

Ca2þ þ SO2 4 þ xH2 O $ CaSO4  xH2 O

ð15Þ

þ O2 $

3.4.2. Analysis of formulation products 3.4.2.1. Results of XRD analysis. It is well known that CO2 converts calcium hydroxide and eventually also hydrated calcium silicates into calcium carbonate. Thus XRD, TG and SEM analysis were con-

ð11Þ

þ

Ks2 ¼

CO2 was 0.76 while that of SO2 is nearly zero at the same condition. This indicates that nearly all SO2 dissolves in pore solution of concrete while about 30% gaseous NO2 or CO2 still diffuse in pore solution of concrete after diffusing into concrete. As the diffusion coefficient of gaseous phase is higher by 3–4 magnitudes order than that of liquid phase, thus the neutralization rate of NO2 and CO2 in concrete are much higher than that of SO2. Thus during the neutralization process by multi-acidic gases, the neutralizing rate of NO2 is the highest compared to that of CO2 and SO2.This contradicts with Marinonia’s report [1] that gas NO2 served as catalyst for concrete sulphation of concrete lining in vehicle tunnel.

q

ch

ch S x

SO2

S ch Sx S

Sx: CaSO4⋅ xH2O A: Ca(OH)2NO3(H2O) A S: CaSO4⋅ H2O ch: Ca(OH)2 C: CaCO 3 q: SO2 ch NO2

SxC

A

ch

SO2+NO2

AC

SO2+NO2+CO2

In term of ideal gas state equation and Henry’s law, the ratio of gaseous phase to dissolved phase (D/G) can be calculated. The D/G of NO2 in pore solution of concrete at 20 ± 2 °C was 0.70 and that of

10

20

30

40

50

60

70

80

2-Theta- Scale Fig. 6. XRD patterns of sample C-31 suffering different acidic gases.

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M.-f. Ba et al. / Construction and Building Materials 29 (2012) 438–443

phase in neutralized concrete. It seemed that the possible reaction was carried out as

0.10 NO2

-1 DTG /%⋅°C

0.05 0.00

SO2

-0.05

CO2

SO2+NO2+CO2

-0.10

SO2+NO2

-0.15 -0.20 -0.25

0

200

400

600

800

1000

1200

T /°C Fig. 7. DTG patterns of sample C-31 suffering from different acidic gases.

ducted to analyze the formation products during concrete neutralization by multi-acidic gases. At the age of 60 days for neutralization, samples C-31 in simulated chambers with gaseous concentration in Table 3, taken at the depth of 3 mm from exposed surface, was analyzed by XRD patterns in Fig. 6. It could be seen that CaSO4.H20 and CaSO4xH2O have been identified here as new formulation phase from samples suffered to SO2 attack. This indicated SO2 reacted with calcium hydrates of concrete by Eq. (15) to form CaSO4.H20 and CaSO4xH2O. One point needing to be cleared here was that the peak of formed CaSO4xH2O is nearly overlapped with that of quarts. From the XRD patterns of samples exposed to single NO2, it is observed that Ca(OH)2NO3(H2O) was testified as typical nitrate

NO3 þ Ca2þ þ 2OH þ 2Hþ ! CaðOHÞ2  NO2  H2 O

It is well known that formed CaSO4xH2O produces a considerable expansion in concrete while the resultant Ca2(OH)2(NO3)2 does not. Thus further diffusion of SO2 into deeper concrete gets more difficult compared to that exposed to single NO2 attack. This again can be used to explain that the neutralized sample suffering from single SO2 has higher mass gain and lower neutralization depth compared to the sample exposed to single NO2 as seen in Fig. 5. From Fig. 6 it is also deduced that the sample exposed to three acidic gases showed the severe neutralization degree, which can be determined from the disappeared peak of hydroxide in samples. Furthermore, the diffraction peak of formed CaCO3 is also very obvious in the sample exposed to three acidic gases. 3.4.2.2. Results of DTG analysis. The corresponding results of thermal analyses of samples (Fig. 7) were consistent with above results of XRD analysis. From Fig. 7 it could seen that the sample by SO2 attack showed obvious peaks at about 111–141 °C corresponding to the loss of hygroscopic water from CaSO4xH2O, compared with samples attacked by only NO2 or CO2. It is also present in Fig. 7 that the sample exposed to multi-gases attack shows much lower peaks at about 420–450 °C denoting the decomposition of calcium hydroxide compared with the sample exposed to single CO2 attack in Table 3. This again indicated the joint effects of multi-acidic gases lead to much severe neutralization degree of concrete. 3.4.2.3. Results of ESEM analysis. The ESEM observations of samples show that the neutralized concrete has highly porous structure and

(a) NO2

(b) SO2

(c) SO2+NO2

ð16Þ

(d) SO2 +NO2+CO2

Fig. 8. ESEM images of samples C-31.

M.-f. Ba et al. / Construction and Building Materials 29 (2012) 438–443

contain crystal of products of chemical degradation. From Fig. 8b–d it is illustrated that the crystals of CaSO4xH2O are visible in samples attacked by SO2, However certain crystals of Ca3(OH)2NO3 are found in samples of Fig. 8a, c and d attacked by NO2. The effect of multi-gases gives rise to the most porous structure of concrete. Thus the results of SEM observation correspond well with the findings obtained by XRD and DTG analysis. 4. Conclusions (1) The experimental results of concrete neutralization in simulated atmosphere condition in vehicle tunnels showed that gaseous SO2 and NO2 contributed a lot to the neutralization degree compared with that of only carbonation. The effect of multi-gases with the same proportion with the composite acidic gases in vehicle tunnels led to the much severe neutralization degree i.e. the neutralization depth by multigases was higher by 9.5–12% than that of single carbonation. (2) As the highest solubility of SO2 compare to CO2 and NO2, SO2 diffused into concrete nearly all dissolved while larger ratio of gaseous CO2 and NO2 still existed and diffused in un-saturated pores of concrete, which could be used to explain the fact that sulphation by SO2 was only concentrated at the surface of concrete and while the corresponding neutralizing process of NO2 and CO2 could promote further inside concrete. Furthermore during whole neutralization process, the neutralizing rate of NO2 was the highest compared to that of CO2 and SO2. (3) Sulphation of concrete formed CaSO42H2O and CaSO4xH2O, whose expansive effect made the further diffusion slower. This was why concrete sample attacked by single SO2 had higher mass gain but lower neutralized depth compared with that exposed to single NO2. The reaction of NO2 with calcium oxides formed Ca2(OH)2(NO3)2H2O, which took on tiny lamina by observation of SEM, which could also be indicated by the disappeared peak of calcium hydroxides in XRD patterns.

Acknowledgment The authors are highly appreciated the financial support from the National Basic Research Program of China (Grant No. 2009CB623203) and Nanjing Key Construction Project of Nanjing Yangtze River Tunnel (7612005822).

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