Quantitative analysis of cement powder by laser induced breakdown spectroscopy

Optics and Lasers in Engineering 49 (2011) 318–323

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Quantitative analysis of cement powder by laser induced breakdown spectroscopy A. Mansoori, B. Roshanzadeh, M. Khalaji, S.H. Tavassoli n Laser Research Institute, Shahid Beheshti University, G.C., 1983963113, Evin, Tehran, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 July 2010 Received in revised form 11 October 2010 Accepted 11 October 2010 Available online 5 November 2010

Determination of elemental composition of cement powder plays an important role in the cement and construction industries. In the present paper, Laser induced breakdown spectroscopy (LIBS) is used for measuring the concentration of cement ingredient. Cement powder samples are pressed into pellets. Laser pulses are focused on the surface of pellets. A microplasma is formed in the front of samples. The plasma emission contains information about the elemental composition of the samples. By assumption of local thermodynamic equilibrium (LTE) and using several standard cement samples, a calibration curve is prepared for each element. The major and minor elements of cement such as Ca, Si, K, Mg, Al, Na, Ti, Mn and Sr are qualitatively and quantitatively determined. For verification of LTE conditions, plasma parameters such as plasma electron temperature and electron density are computed. According to the obtained results, the LIBS technique could be a suitable method for determination of elemental composition in the cement production industries. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Laser induced breakdown spectroscopy Elemental analysis Cement powder Spectroscopy Local thermodynamic equilibrium

1. Introduction Throughout history, cementing materials have played a vital role in people’s life. In terms of volume, different kinds of cement, mainly in the form of concrete are the most commonly used material in the world. Nowadays, more than 1 m3 of concrete is produced per person per year [1]. Permeability, durability and performance are the main factors of concrete structures [2,3]. Worldwide, the damage caused by the corrosion process results in tremendous economic losses totaling in several billion dollars per year [4]. Many studies and efforts have been conducted to improve the quality of concrete and decrease the corrosion [5–9]. One method is to divide the cement powders into various types appropriate for using in specific environmental condition. For this reason, measuring the concentration of major and minor elements is necessary to determine the different kinds of cement and for quality control. Nowadays, analysis of cement is done through expensive and time consuming chemical methods, such as wet chemistry and X-ray fluorescence (XRF). Although XRF is the key technique for characterizing the elemental composition of material in cement factories, it needs expensive apparatus [10,11]. Already, spark induced breakdown spectroscopy (SIBS) has been applied for determination of elemental composition of cement powder [12]. In the present paper, laser induced breakdown spectroscopy (LIBS) is introduced as an accurate method for analysis of cement elements. The LIBS technique has been widely applied to the detection and quantification of trace elements in gaseous [13], solid [14,15] and

liquid [16] samples, with minute or no sample preparation [17]. Several research groups have tried to improve the plasma emission by various techniques [18–21]. LIBS has been used in construction industries for determination of chloride content in different types of cement [22] and in situ analytical assessment of historical buildings [23]. This method offers multi-elemental and fast measurement detection capability under ambient and harsh conditions and is employed for in situ and remote monitoring in industrial processes [24,25]. The main physical process that creates the nature of the LIBS technique is the generation of a high-temperature plasma, induced by a short laser pulse (typically some nanoseconds and ten to hundreds of milli-Joules per pulse) [26,27]. A small volume of the sample is ablated from the surface of a target by the impact of the laser pulses. The ablated mass interacts with a remaining portion of the laser pulse to form a high energetic and hot plasma that contains free electrons, excited atoms and ions. After the laser pulse finishes, the plasma quickly starts to cool down. During this process, the electrons in atoms and ions at the excited electronic states transit into lower states, causing the plasma to emit light with discrete spectral lines [28,29]. The emitted light from the plasma is spectroscopically analyzed and the elements present in the sample are determined through their unique spectral lines [30]. The quantitative elemental composition of the target material is obtained by the preparation of calibration curves using several standard samples [31].

2. The experimental setup n

Corresponding author. Fax: + 98 21 22431775. E-mail address: [email protected] (S.H. Tavassoli).

0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.10.005

The experimental setup used for the analysis of cement samples is shown in Fig. 1. A Q-switched Nd:YAG laser operating at fundamental

A. Mansoori et al. / Optics and Lasers in Engineering 49 (2011) 318–323

0.1

Laser beam

XYZ

0.08

Plasma Sample

Si / Ca

Laser P. D. Delay generator

0.6

Si Al

0.5 0.4

0.06

0.3

0.04

0.2

0.02

0.1

0

Power Supply

0

2

4

6

8

10

12

14

16

0

Delay Time (µs)

Echell Spec.

Al / Ca

f =35mm

319

ICCD

Fig. 2. Time evaluation of the signal to background (S/B) ratio, (a) for Al(I) 309.27 nm, (b) for Si(I) 288.16 nm. Laser energy 37 mJ, gate width 20 ms, 20 accumulations, sample type 1886a.

Fig. 1. The schematic diagram of the experimental setup.

wavelength, lL ¼1064 nm is used for plasma generation. Repetition rate, pulse duration, gate width and laser pulse energy is uL ¼ 1 Hz, tL ¼10 ns, tG ¼20 ms and EP ¼37 mJ, respectively. A lens with 3.5 cm focal length focuses the laser beam on the surface of the sample. As the plasma expands and gradually cools, the atomic and ionic emission lines appear. The plasma light emission is collected with a quartz lens and transmitted by an optical fiber to an Echell spectrometer accompanied with an ICCD. A portion of laser light is guided by a beam splitter to a photodiode. The photodiode trigs a delay generator and the delay generator trigs the ICCD at an optimum delay time. The delay time, gate pulse width and ICCD gain are controlled by a computer and the spectral results are saved in the computer. The sample is moved by a computerized XYZ motion stage to have a fresh surface for each laser pulse. In order to quantitatively analyze the elements in cement, nine standard samples from the National Institute of Standards and Technology (NIST) [32] are used. The samples were pressed into pellets using a manual hydraulic press under a pressure of 12 MPa for 5 min. The pellet size is 12 mm in diameter and about 1 mm in thickness.

3. Result Here, the area under the spectral lines is used as the line signal. An imaginary line between two either sides of the spectral line is considered and the area under the line is subtracted as the background. To eliminate some experimental fluctuation, internal standardization method is used in which the spectral signal of each element is normalized to the calcium line as a major element in the cement matrix [29]. The magnitude of each normalized spectral line varies with the experimental delay time. By repeating the experiment at different delay times, the optimum delay in which the spectral signals have their maximum values is determined. Fig. 2 shows typical spectral signals for Al and Si at different delay times. As it can be seen, the maximum signal of Al and Si occurs at 9 and 7 ms, respectively. Table 1 shows the optimum delay times for the elements in this experiment. Fig. 3a shows a typical spectrum of a cement sample at 1 ms delay time at spectral range 240–500 nm. In this delay time, the strongest Ca(II) lines at 393.37 and 396.84 nm and Mg(II) at 279.55 nm are observed. Fig. 3b and c shows a portion of spectrum from 300 to 330 nm and 760 to 775 nm, respectively. This part of spectrum shows narrow and intense atomic emission lines of Al and K elements. These lines are used to evaluate the concentrations of these elements. To evaluate the ability of this technique for quantitative analysis of cement, by the assumption of local thermodynamic equilibrium (LTE) and optically thin plasma, a calibration curve is prepared for Al and Si as major and Mg, K, Na, Mn, Ti and Sr as minor elements. Preparation of calibration curves

Table 1 Detected wavelength of each element, optimum delay time, and corresponding limit of detection (LOD). Laser energy 37 mJ, gate width 20 ms, 20 accumulations, sample type 1886a. Element

Wavelength (nm)

Optimum delay time (ms)

LOD (ppm)

Mg(II) Al(I) Si(I) Na(I) Sr(II) Ti(II) Mn(II) K(I)

279.55 309.27 288.16 588.99 and 589.59 407.77 334.94 257.61 766.49

1 9 7 1 7 9 3 9

24 40.6 14.8 3.7 20.3 34 7.9 40.5

normally requires a series of appropriate Standard Reference Material (SRM). Here, nine cement SRMs from NIST are used for preparation of calibration curves. The certified values of cements SRMs are given in Table 2. The intense spectral lines that have minimal interference from other emission lines are chosen. Table 1 shows the wavelengths selected for each element. The highest correlation coefficient is achieved for sodium when sum of two lines at 588.99 and 589.59 nm are used as signal. The corresponding calibration curves are given in Fig. 4. Each measurement is the accumulation of 20 pulses and each data point represents the mean value of three measurements. The calibration curves are prepared using the standard samples in Table 2. Each calibration curve has been obtained at optimum delay time indicated in Table 1. In Fig. 4 the error bars correspond to the standard deviation of three replicates (n ¼3) and are calculated by "P s¼

ðxi MÞ2 1 n

#1=2 ð1Þ

here, M is mean value of different experiments. As it is seen, the calibration curves are linear with regression coefficients between 0.964 and 0.994. Usually, the transitions coupled to the ground state have been affected by self-absorption [33]. In order to decrease the effect of self-absorption, a low pulse energy has been chosen. A decrease in the pulse energy leads to a decrease in the ablated material and accordingly, self-absorption decreases [34,35]. Linearity in the calibration curves shows that the spectral lines have the least possible self-absorption [36]. The limit of detection (LOD) is defined as the minimum detectable   value of net signal or concentration and is evaluated by LOD ¼ 3 s=S where, s and S are the standard deviation of the background and the slope of the calibration curve, respectively [37,38]. Table 1 shows the LOD of each component. The highest and lowest value of LOD is obtained between 3.7 and 40.6 ppm for Na and Al, respectively.

320

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x103

500

Mg

300

0 240

100

Ca

Mg Si

200

290

340 390 Wave length (nm)

Ca Ca

60 Al Al

0 300

Ca

Al

Si

80

20

Ca

Ca Ca

x103

40

Ca

Ca Ca

400

100

Intensity (a.u.)

Ca

Mg

600

Intensity(a.u.)

Intensity (a.u)

700

310 320 Wavelength (nm)

330

30 25 20 15 10 5

440

490

x103 K K

760

765 770 Wavelength (nm)

775

Fig. 3. A typical LIBS spectrum of the cement sample at (a) wavelength between 240 and 500 nm, (b) wavelength between 330 and 330 nm, and (c) wavelength between 760 and 775 nm. Laser energy 37 mJ, delay time 1 ms, gate width 20 ms, 20 accumulations, sample type 1886a.

Table 2 Certified values of nine cement SRMs. The certified values are based on the results of analysis performed at National Institute of Standards and Technology (NIST). Sample

CaO (%)

Al2O3 (%)

MgO (%)

SiO2 (%)

Na2O (%)

SrO (%)

TiO2 (%)

Mn2O3 (%)

K2O (%)

634a 1880b 1881a 1882a 1885a 1886a 1887a 1888a 1889a

65.07 70.26 64.16 70.40 57.58 70.34 39.26 71.22 62.39 70.41 67.87 70.26 60.9 70.34 63.23 70.21 65.34 70.33

5.0157 0.023 5.183 7 0.073 7.067 0.081 39.14 7 0.64 4.0267 0.032 3.875 7 0.035 6.2027 0.046 4.265 7 0.078 3.897 0.12

1.0057 70.0083 1.176 70.020 2.981 70.077 0.51 70.02 4.033 70.033 1.932 70.040 2.835 70.015 2.982 70.067 0.814 70.028

20.493 7 0.068 20.42 7 0.36 22.26 7 0.15 4.0107 0.22 20.9097 0.047 22.38 7 0.27 18.6377 0.038 21.22 7 0.12 20.66 7 0.16

0.0842 70.0029 0.0914 70.0052 0.199 70.007 0.021 70.008 1.068 70.061 0.021 70.003 0.4778 70.0089 0.1066 70.0007 0.195 70.010

0.0735 7 0.0052 0.0272 7 0.0016 0.036 7 0.004 0.024 7 0.002 0.638 7 0.026 0.018 7 0.006 0.322 7 0.013 0.082 7 0.002 0.042 7 0.004

0.2463 7 0.0028 0.2367 0.012 0.3663 7 0.0030 1.786 7 0.005 0.1957 0.014 0.0847 0.009 0.2658 7 0.0035 0.2637 0.006 0.2277 0.010

0.0229 7 0.0011 0.1981 7 0.0020 0.1042 7 0.0016 0.0607 0.001 0.0478 7 0.0015 0.0073 7 0.0004 0.1186 7 0.0013 0.1256 7 0.0036 0.2588 7 0.0073

0.3572 7 0.0039 0.6467 0.014 1.228 7 0.029 0.0517 0.014 0.2067 0.011 0.0937 0.004 1.1007 0.024 0.5267 0.010 0.6057 0.015

4. Plasma parameters 4.1. Plasma temperature For drawing calibration curve it was assumed that the plasma is in the local thermodynamic equilibrium (LTE) condition. In order to show the validity of the above assumption, it is necessary to compute the plasma temperature and electron density. Several methods for measuring of the plasma temperature based on the absolute or relative line intensity are available. Here, Boltzmann plot method is used for measuring of the plasma temperature. Assuming that the level population obeys a Boltzmann distribution, the intensity of a spectral line is obtained by Iij ¼ ns Aij ¼

Aij gi s Ei =kT ne U s ðTÞ

ð2Þ

where, Aij and gi are the transition probability and the statistical weight for the upper level, respectively. ns indicates the population density of species S. After linearization of expression (2), the familiar form of the Boltzmann plot equation is obtained  s  Iij n E  i ð3Þ ¼ ln ln gi Aij U s ðTÞ kT When the left-hand side of equation (3) is plotted vs. Ei, the plasma temperature is obtained from the slope of plot, which is

equal to 1/kT [27]. The choice of the emission lines is based on their highly relative strengths and their wavelengths being spread over a wide range of energy levels. Here, four spectral lines of calcium, 370.6, 393.37, 396.85 and 445.48 nm are selected for preparation of the Boltzmann plot in several delay times and their relevant spectroscopic constants are obtained from the NIST database. Fig. 5a shows a typical Boltzmann plot at delay time 7 ms. The plasma temperature as a function of delay time is illustrated in Fig. 5b. Error bars in this figure correspond to the standard deviation of different experiments. As it is shown, heat exchange between the plasma and surrounding environment causes the plasma cooling. The plasma temperature decreases from 12,000 to 7300 K when the delay time increases from 1 to 9 ms. 4.2. Plasma electron density The broadening of a spectral line contains some information about the emitter and surrounding environment. A spectral line is broadened due to different mechanisms, but under typical LIBS condition, Stark broadening predominantly determines the line profile. Stark broadening is a consequence of the interactions of the electric fields near the radiator and thus, it is proportional to the electron number density   ne DlStark ¼ 2 w ð4Þ 1016

Al l 309.27nm/Ca ll 396.85

0.07

0.05

R2 = 0.9936

0.03 0.04

0.15 Intensity Ratio Kl/ Call

Intensity Ratio Ti ll/Ca ll

0.09

0.05

0.06 0.08 0.07 Weight Ratio (Al/Ca)

0.09

0.1

Intensity Ratio Si l/Ca ll

Intensity Ratio Al l/Ca ll

A. Mansoori et al. / Optics and Lasers in Engineering 49 (2011) 318–323

K l 766.49nm/Ca ll 396.85

0.12

0.09

0.06

R2 = 0.9644

0

0.005

0.01

0.015

0.02

0.025

0.05

Ti ll 334.94nm/Ca ll 396.85nm

0.04 0.03

R2 = 0.9695

0.02 0.01

0.5

0.001

0

Intensity Ratio Mg ll/Ca ll

0.09 R2 = 0.9896

0.03

0.17

0

0.003

0.006 0.009 Weight Ratio (Sr/Ca)

0.012

0.015

Intensity Ratio Mn ll/Ca ll

Intensity Ratio Sr ll/Ca ll Intensity Ratio Na l/Ca ll

0.12

0

Na l (588.99+589.59) nm/Ca ll 393.37

0.14 0.11 0.08

R2 = 0.9932

0.05 0.02

0

0.002

0.004

0.006

0.005

0.006

0.4

0.3

R2 = 0.978

0.2 0.2

0.21

0.22

0.23

0.24

0.25

0.26

Weight Ratio (Si/Ca)

Sr ll 407.77nm/Call 306.85nm

0.06

0.002 0.003 0.004 Weight Ratio (Ti/Ca)

Si l 288.16nm/Ca ll 396.85

Weight Ratio (K/Ca) 0.15

321

0.008

0.01

Weight Ratio (Na/Ca)

2.6

Mg ll 279.55nm/Ca ll 396.85nm

2.2 R2 = 0.973

1.8 1.4 0.01

0.04

0.02

0.03 0.04 Weight Ratio (Mg/Ca)

0.05

0.06

Mn ll 257.61nm/Ca ll 396.85

0.03 0.02 R2 = 0.9799

0.01 0

0

0.001

0.002

0.003

0.004

0.005

Weight Ratio (Mn/Ca)

Fig. 4. Calibration curves for (a) Al/Ca, (b) Ti/Ca, (c) K/Ca, (d) Si/Ca, (e) Sr/Ca, (f) Mg/Ca, (g) Na/Ca, and (h) Mn/Ca, The calibration curves are prepared using the standard samples in Table 2. Each calibration curve has been obtained at optimum delay time indicated in Table 1.

here, DlStark is spectral line broadening (nm), w the electron impact parameter (or half-width) and ne is the electron number density (cm  3) [27,39]. For determination of electron density by Eq. (4), a line with less possible self-absorption should be selected. Self-absorption is not small for the lines which have the ground state as the lower level. Here, the line Ca 442.5 nm is selected, in which the lower energy level is not ground state. The Stark broadenings extracted by fitting a Gaussian profile to the spectral line. Fig. 6 shows electron density of plasma as a

function of delay time. As it is illustrated, a decrease in the electron number density is observed with time. This is due to the recombination of free electrons with ions. 4.3. Local thermodynamic equilibrium (LTE) The necessary condition for LTE that gives the corresponding lower limit of electron number density, ne (cm  1) is given by

322

A. Mansoori et al. / Optics and Lasers in Engineering 49 (2011) 318–323

34

ln (Iij/giAij)

32

y = -1.329x + 36.271 R2 = 0.9916

30 28 26

2

3

4

x103

1 µs 3 µs 5 µs 7 µs 9 µs

12 Temperature (k)

5 Ei (eV)

10.5

6

7

(11912 ± 58.7) k (9816 ± 328.7) k (9406 ± 241.2) k (8512 ± 638.8) k (7327 ± 243.6) k

References

9 7.5 6

0

1

2

3

4 5 6 Delay time (µs)

7

8

9

10

Fig. 5. (a) Boltzmann plot of the Ca lines emitted (td ¼ 7 ms), (b) temporal behavior of plasma temperature. Sample type 1886a.

Electron Density (cm-3)

2

x1016 1 µs 3 µs 5 µs 7 µs 9 µs

1.9 1.8 1.7

(1.93 ± 0.0086) x 1016 cm-3 (1.67 ± 0.0188) x 1016 cm-3 (1.61 ± 0.0343) x 1016 cm-3 (1.534 ± 0.0199) x 1016 cm-3 (1.529 ± 0.0393) x 1016 cm-3

1.6 1.5 1.4

0

2

4 6 Delay time (µs)

8

10

Fig. 6. Temporal behavior of electron density. Sample type 1886a.

McWhirter criterion [29,30] pffiffiffi ne Z 1:6  1012  T ðDEÞ2

identified by this method. For quantitative analysis, an internal standard method is used, in which the emission intensities of each element are normalized to the emission intensity of a proper Ca atomic line. Using the area under the peak instead of peak values, together with the background subtraction improves the correlation coefficient in the calibration curves. All calibration curves are linear with regression coefficients more than 0.96. The values of plasma temperature and electron density are evaluated and it is demonstrated that plasma is at the LTE condition in the time range used in the experiment. The LOD for each element is evaluated. Results show that the concentration of cement elements can be determined with a limit of detection smaller than 40 ppm. This proves that LIBS has an appropriate accuracy for multi-elemental analysis of cement powder.

ð5Þ

where, DE (ev) is the highest energy transition for which the condition holds. In our experiments, DE¼2.8 ev for Ca and plasma temperature varies from 12,000 to 7300 K. The lower limit of electron number density is at the order of 1015 cm  3 while, the minimum experimentally measured electron number density is at the order of 1016 cm  3 (see Table 1). Therefore, the assumption of LTE is completely fulfilled.

5. Conclusion The results presented in this paper illustrate the use of LIBS as a diagnostic and elemental analysis technique for cement powder pellets. Major and minor elements are qualitatively and quantitatively

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