Plasma dynamics and chlorine emission characteristics on cement pastes using collinear dual-pulse laser-induced breakdown spectroscopy

The ingress of chloride can lead to the pitting corrosion of rebars, causing severe damage to concrete structures during service. Laser-induced breakdown spectroscopy (LIBS) has been demonstrated as an in-situ and reliable technique that can present cement distribution in concrete profiles, where the chlorine concentration is severely limited in the China standard. Dual-pulse LIBS has frequently been reported to enhance plasma emission. The evolution of the induced plasma generated from cement pastes is studied using an optical diagnostic system coupled with fast photography, shadowgraph, and optical emission spectroscopy, which can simultaneously record time-resolved physical information, including plasma morphology, shockwave propagation, and spatially resolved emission spectra in both single-pulse and dual-pulse configurations. The transition of the action mechanism of the second laser pulse from "initial plasma reheating" to "ablation enhancement" is demonstrated, which is responsible for the change in the plasma morphology from hemispherical to umbrella-shaped. It is observed that the new shockwave formed inside the initial shockwave is produced by the secondary ablation, which propagates faster, and shockwave fusion occurs when it approaches the initial shockwave. The spatially resolved Cl I emission at 837.59 nm is acquired, and the signal-to-background ratio (SBR) closely follows the profile of plasma temperature. The maximum SBR (> 3.0) can be accessed in the region where the plasma temperature reaches its peak (> 9200 K). Under the excitation of dual pulse with an inter-pulse delay of 200 ns, the Cl I emission can be clearly identified in the sample with a chlorine concentration of 0.252 wt% close to the critical value (0.2 wt% in the China standard), and a calibration curve is constructed with limit of detection (LOD) estimated to be 1930 ppm (dual pulse) versus 2550 ppm (single pulse).