IEEE Photonics Journal Stability Enhanced Online Powdery Cement
situation to some extent. However, it requires use of radioactive sources, so that the equipment has a
lot of inconvenience in operation, management, and maintenance. In addition, the measurement sta-
bility of PGNAA can be seriously affected by effects such as neutron absorption and scattering, γ-ray
self-absorption, self-shielding, etc. [1], [2]. For the purpose of product quality control, there is urgent
demand for developing new technologies suitable for on-line quantitative analysis of cement raw
materials.
The laser-induced breakdown spectroscopy (LIBS) technique, which fundamentally utilizes line
emission radiated from the micro-plasma that goes through vaporization, dissociation, and exci-
tation or ionization of the sample surface, is able to perform simultaneous multi-element analysis
of powdery samples without any special instrumentation alterations. Therefore, it is an attractive
tool for application to cement plants. LIBS has been greatly developed in recent decades and
widely used in many fields, such as metallurgy [3], [ 4], geology [5], [6], environment [7], [8], etc.
For concrete analysis, A. Mansoori et al. employed LIBS to analyze the pressed cement ingredient
pellets, the major and minor elements of cement such as Ca, Si, K, Mg, Al, Na, Ti, Mn and Sr were
qualitatively and quantitatively determined [9]. Gondal et al. employed a dual pulsed LIBS system
for detection of weak spectral line of sulfur in concrete using the S(II) peak at 545.38 nm as a marker
for quantifying sulfur content in concrete, the limit of detection (LOD) achieved was approximately
38 mg/g [10]. Weritz et al. employed LIBS for investigation of the sulfur content in concrete by using
the S(I) spectral line at 921.3 nm, both depth profiles and spatial resolved sulfur distributions were
presented measured on concrete cores originating from a highly sulfate contaminated clarifier [11].
Gehlen et al. used LIBS to analyze drill cores by scanning the hydrated cement sample surface with
laser pulses, and a LOD according to DIN32645 of 0.1 mass% was achieved for chlorine in hydrated
cement using the Cl(I) 134.72 nm emission line [12]. We designed a laboratory LIBS apparatus
that mainly comprising a sealed optical module and an analysis chamber for application in cement
plants for on-site quality analysis of cement, and good agreement was found between the laboratory
measurement results from the LIBS method and those from the traditional method, with the absolute
measurement errors of less than 0.5% [13]. It is notable that all the above studies were performed
in the laborator y conditions. However, in the case of industrial application, the on-line LIBS mea-
surement stability would be difficult to be guaranteed due to the complexity of the environmental
conditions and the lack of sample preparation process. To date, there are many studies focusing
on stability improvement in LIBS measurements. Scaffidi et al. combined the nanosecond and fem-
tosecond laser in an orthogonal preablation spark dual-pulse LIBS configuration, even without full
optimization of interpulse alignment, ablation focus, large signal, signal-to-noise ratio, and signal-to-
background ratio enhancements were observed for both copper and aluminum targets [14]. Popov
et al. developed a small chamber in brass to enhance the sensitivity of single-pulse LIBS for element
deter mination in air, an increase of a factor of 3 and 10 for As and Fe signals was detected in the violet
range (230-240 nm) [15]. George et al. found that there existed an optimum pressure range that op-
timized the LIBS signal strength and quality, which provided quantitative detection with linearity over
at least two orders of magnitude [16]. Hou et al. proposed an algorithm based on the characteristics
of atomic and ionic lines under various plasma conditions to reduce the signal fluctuation that caused
by the varying plasma temperature and electron density, the relative standard deviation (RSD) was
reduced from 2.93% (Cu(I) 406.264 nm) and 2.13% (Cu(II) 217.941 nm) to 1.68% (combined
intensity) [17].
In order to apply the LIBS technology to industrial cement production line, a novel experi-
mental LIBS device with a pneumatic sampling module has been developed for possibly on-
line monitor i ng of the powdery cement raw materials quality. Field test illustrated that the mea-
surement instability mainly depended on: 1) the change in the powder concentration of the
ejected gas-powder stream; 2) the fluctuation of the pulsed laser output power; 3) the change
in the fluorescence collection efficiency. Accordingly, the gas-powder stream stabilization technol-
ogy, pulsed laser power stabilization technology, and optical excitation and collection efficiency
enhancement were proposed to improve the measurement stability of the experimental LIBS
device.
Vol. 9, No. 5, October 2017 6804010