**1. Introduction**

Scabbling method is a technique for removing thin layers of stone or concrete that can range in thickness from a few millimeters to several centimeters. Scabbling has also been used in many applications, including the decontamination of radioactive surface layers in nuclear power plants removing road markings, many other applications in traffic and construction works. There are several scabbling methods such as electrohydraulic scabbling (EHS), robotic wall scabbler, piston scabbler, and handle scabbler. The EHS method achieves a scabbling depth of up to 1 inch and a scabbling rate of 30 ft2/h, whereas the piston scabbler method achieves a depth of 1/8 inch and a scabbling rate of 130 ft2/h [1,2]. However, there are several drawbacks, including the massive weight of the working system, high current, high voltage, mechanical reaction force, wear down of equipment, and negative environment impact, as well as the operator's health [3].

Laser-aided manufacturing (LAM) has been widely employed to overcome the disadvantages of previous techniques in various fields, including car manufacturing, aerospace, and semiconductor [4–6]. Due to its advantages (e.g., high precision, high speed, noncontact method, remote control capabilities), the applicability of LAM has also been investigated on concrete. Numerous studies on the interaction between laser beam and concrete were carried out. For instance, Seo et al. [7] employed a high-power fiber laser cutting for 50 mm thick cement-based material. The results demonstrated that a line energy of 1.22 × 10<sup>14</sup> J/m<sup>3</sup> was required to fully cut the 50 mm thick cement paste. On the other hand, the effect of material composition on penetration depth, microstructural

**Citation:** Huynh, T.-V.; Seo, Y.; Lee, D. The Effect of Silica Sand Proportion in Laser Scabbling Process on Cement Mortar. *Metals* **2021**, *11*, 1914. https://doi.org/10.3390/ met11121914

Academic Editors: Sergey N. Grigoriev, Marina A. Volosova and Anna A. Okunkova

Received: 6 November 2021 Accepted: 25 November 2021 Published: 27 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

characteristic and chemical changes in the laser cutting of cement-based material were also studied [8,9]. Muto et al. [10] used a 7 kW fiber laser at a scanning speed of 2.5 mm/s, and a spot size of 10 mm. The results pointed out that 100 mm in thickness of concrete slabs was completely cut after 21 passes. In laser drilling concrete field, Kaori et al. [11] obtained drilling diameters of 4 mm to 6 mm and depths of around 50 mm in concrete samples with a compressive strength of 20–100 N/mm<sup>2</sup> by using the appropriate laser settings. Meanwhile, the use of laser for scabbling on cement-based materials has not seemed to be attractive. In addition, to examine the effect of concrete composition, curing regime and aging on the laser scabbling process, Peach et al. [12–15] exploited IPG Photonics YPS-5000 5 kW-Continuous wave laser with nominal beam diameter of 60 mm to scabble on several concrete types, the effect curing regime, aging concrete composition were reported. The authors revealed that two major mechanism of this scabbling method were thermal stress and pore stress spalling of concrete. These results were obtained due to low power density laser. However, the using of high-power density laser in scabbling concrete has not been reported.

Several studies have carried out to investigate the interaction of laser with cementbased materials including laser cutting, laser drilling, laser glazing and laser scabbling. However, the effect of silica sand proportion in laser scabbling on cement-based material has not been studied yet. As a result, it is critical to comprehend the impact of silica sand composition in laser scabbling process. The aim of this study is obtaining a fundamental understanding of the interaction between laser and cement mortar with the differences of silica sand proportions. The formation of the glassy layer and heat-affected zone (HAZ) are the major results in the laser scabbling process on cement mortar. Thus, the study of changes in each region are needed. After laser scabbling process, all samples were evaluated in morphological changes such as color changing and scabbling penetration depth. In addition, the changes in the microstructure of each region of the scabbled samples were also determined by using a scanning electron microscope (SEM). Energy dispersive X-ray (EDX) analysis was also used to analyze changes in the chemical composition and the chemical distribution in each zone. According to the findings of this study, it is believed that the using of laser scabbling on cement based materials is applicable for walls and floor at construction sites, as well as for removing the radioactive layers in nuclear facilities.
