**4. Results and Discussions**

*4.1. Morphological Analysis*

Most of the OPC has a large composition of Al2O3, SiO2, and Fe2O3, which are the fundamental elements for creating an amorphous glassy layer at high-temperature conditions [16]. The evaporation of free water molecules and the phase transition of the specimen's composition were caused by the thermal effect of laser irradiation on the surface of cement mortar, resulting in the formation of the glassy layer. Through observing the top, bottom and cross-section surfaces as shown in Figures 2 and 3, respectively. The glassy layer formed on the specimens' processed zone. Moreover, the color of the glassy layer changed from scorched (Figure 2a) to green and even turquoise as seen in Figure 2d,e, respectively. One explanation for this phenomenon could be the presence of metal transition ions in oxidation states, especially ferric ions in the Al3+ and Fe2+ oxidation states. At high temperatures, the Al3+ and Fe2+ exposed green and blue, respectively [17]. Furthermore, Wignarajah et al. [18] reported that the presence of metallic oxides in cement samples contributed to color changes on the processed zone impacted by the laser. Table 4 shows the color of the glassy layer that correspond to the metallic oxides in the material composition.

**Figure 2.** Photographs of the top and bottom surfaces: (**a**) CM0.2; (**b**) CM0.4; (**c**) CM0.6; (**d**) CM0.8; (**e**) CM1.5.

**Figure 3.** Photographs of cross section: (**a**) CM0.2; (**b**) CM0.4; (**c**) CM0.6; (**d**) CM0.8; (**e**) CM1.5.


**Table 4.** Example of colorful glassy layers produced by laser irradiation on the surface of zeolite mortar [18].

During the laser process, a thermal gradient between the melting zone and the substance material was generated, resulting in the development of thermal stress. In addition, the significant temperature gradient between the laser irradiation temperature and the ambient temperature after the scabbling process caused crack formation. Furthermore, the significant reduction of cracks in the top and bottom surfaces with increasing in silica sand proportions of samples was visually observed, as can be seen in the Figure 2.

After the scabbling experiment, the samples were cut in a cross-section to observe the three main material sections: (1) non-processed zone, (2) heat affected zone (HAZ), and (3) processed zone. Furthermore, glassy layer was generated in processed zone. The procedure for obtaining the section view is shown in Figure 4.

**Figure 4.** Illustrating image of three main material sections.

Seo et al. [8] also confirmed that adding silica sand into basic cement-based materials results in decreasing the penetration depth. Meanwhile, Figure 4 reveals a significant decrease in penetration depth while increasing silica sand in proportion from 0.2 to 1.5. In another words, thermal conductivity (қ) describes how quickly heat flows through a material from the hotter side to the colder side under steady-state conditions, and thermal diffusivity (dt) describes how well a material can spread heat [19]. The relationship between thermal conductivity and thermal diffusivity is proportional, as shown by Equation (1). As a result, a decrease in thermal conductivity leads to a decrease in thermal diffusivity.

$$\mathbf{d}\_{\mathbf{t}} = \frac{\kappa}{c\_P \cdot \rho} \tag{1}$$

where, *cp* [J·Kg−1·k−1] is specific heat, қ [W·m<sup>−</sup>1·K−1] is thermal conductivity, dt [m<sup>2</sup>·s<sup>−</sup>1] is thermal diffusivity and *ρ* [Kg·m<sup>−</sup>3] is density. According to Ganeev et al. [20] the Equation (2) expresses the relationship between thermal penetration depth by laser beam energy and the thermophysical properties of the specimen surface as equation below.

$$\mathbf{L\_d} = \sqrt{\mathbf{d\_t} \ast \mathbf{\tau}} \tag{2}$$

where, Ld [m] is thermal penetration depth, and *τ* [s] is pulse duration. Furthermore, higher silica proportion reduced the thermal conductivity of cement mortar [21]. As a result, increasing the silica sand proportions in cement-based materials lowered the scabbling penetration depth. Along with that, the laser beam was absorbed and limited due to the rapid formation of melting layer induced by the heat effect of laser irradiation.

Peach et al. [12] reported that two main mechanisms for laser scabbling of concrete are pore pressure spalling and thermal stress spalling. Pore pressure spalling was caused by the rapid increase in pore pressure caused by the vaporization of free water, while the formation of thermal stress as a result of severe thermal gradients induced by high heat rates and low thermal conductivity of concrete causes thermal stress spalling. However, they were able to achieve those results by using a low-power density laser of 1.768 W/cm2. On the other hand, in present study employed a laser with a high-power density laser of 1.432 × 10<sup>7</sup> W/cm2. the evaporation of material was the major process when using this scabbling method. In addition to the evaporation of the material in the processed zone and the formation of the glassy laser were described above.

The heat affected zone (HAZ) is a zone of the base material that was not melted but impacted by the heat generated during the laser scabbling process. According to Maruyama et al. [22], the temperature for the dehydration reaction of Ca(OH)2 is 400 ◦C or higher. From this study it was reported that microcracks occurred in cement-based materials due to the breaking of chemical and physical bonds caused by temperature. The dehydration of calcium hydroxide is shown in Equation (3) [22]:

$$\text{Ca(OH)}\_{2} + \text{heat} \rightarrow \text{CaO} + \text{H}\_{2}\text{O} \tag{3}$$

As mentioned earlier, increasing the proportion of silica sand in the specimen decreased the thermal conductivity of the samples. As a result, heat transfer from the processed zone was limited. Along with that, the rapid development of the glassy layer absorbed heat energy and reduced the transmission of the heat energy generated by the laser beam. As a result, the heat generated in the samples containing higher silica sand proportion focused on the processed zone, resulting in a larger HAZ. Moreover, the heat of laser process caused the dehydration and decomposition of cement-based materials, thus the color in the HAZ changed and appeared in a whitish grey [23,24]. In all cases, the heat-affected zone can be clearly observed in a grey-white color, as can be seen in Figure 3.
