## 3 Mechanism of ASR and Degradation of Properties of Cement in Porcelain Insulators  
ASR progresses in two distinct steps. Alkali-silica gel mainly reacts with Na\({}^{+}\) and K\({}^{+}\), which are high pH value cations within the cement. These two components absorb water well, causing the  
Figure 2: Cross-sectional composition and three-dimensional computed tomography (3D-CT) analysis photo of new and aged of porcelain insulator. (**a**) Cross sectional analysis; (**b**) 3D-CT of new porcelain insulator; (**c**) 3D-CT of aged porcelain insulators. cement to expand, resulting in internal stresses, and thereby, cement damage. Most of the reaction occurs between the silica contained in cement and the alkali in the cement paste. The silica in solution state contained in the pores reacts with Na\({}^{+}\) and K\({}^{+}\) (having positive ions) to form alkaline silica gelatin (alkali-silica gel) which is unstable in volume. The gel absorbs water and expands, and the hydration reaction is accelerated by rain or melted snow [14,15]. Since the gel is restrained by the surrounding mortar, osmotic pressure is generated by the swelling. Once that pressure is larger than the tensile strength of the cement, cracks occur, leading to additional water migration or absorption and additional gel swelling [16,17]. The generation of gels lowers the rigidity of the cement and affects the electrical/mechanical properties of the porcelain insulator. Amorphous silica in porcelain + alkali of cement + water = alkali-silica gel production,  
\[\text{SiO}_{2(\text{Solid})}+\text{Na}^{+}{}_{(\text{aq.})}+(\text{OH})^{-}{} _{(\text{aq.})}+\text{H}_{2}\text{O}\rightarrow\text{Na}_{2}\text{SiO}_{3} \cdot 2\text{H}_{2}\text{O},\]  
\[\text{SiO}_{2(\text{Solid})}+\text{Na}^{+}{}_{(\text{aq.})}+(\text{OH})^{-}{} _{(\text{aq.})}\rightarrow\text{Na}\text{HSiO}_{3(\text{Solid})},\]  
\[\text{Na}\text{HSiO}_{3(\text{Solid})}+\text{Na}^{+}{}_{(\text{aq.})}+(\text {OH})^{-}{}_{(\text{aq.})}\rightarrow\text{Na}_{2}\text{SiO}_{3}\cdot 2\text{H}_{2} \text{O}.\]  
The silica contained in the cement and the alkali react with water, generating the alkali-silica gel. Alkali-silica gel is combined with a solid form of SiO\({}_{2}\) and 2 NaOH in the form of a solution in the cement. When this comes into contact with water, Na\({}_{2}\)SiO\({}_{3}\cdot 2\text{H}_{2}\text{O}\) is formed. Among these SiO\({}_{2}\), cationic Na\({}^{+}\) with high pH and OH\({}^{-}\) react to form NaHSiO\({}_{3}\) in a solid state. NaHSiO\({}_{3}\), Na\({}^{+}\), and OH\({}^{-}\) are combined to form Na\({}_{2}\)SiO\({}_{3}\cdot 2\text{H}_{2}\text{O}\), resulting in cement erosion and etching. Cracking of the cement also occurs due to water expansion. This reaction is the starting point for the deterioration of the mechanical properties of the porcelain insulator [18,19]. Figure 3 shows the mechanism of the process by which ASR occurs in the porcelain insulator. Stage I is the stage where internal pores are generated by deterioration during the cement curing process, a long service period, or mechanical/electrical stress. These also include the micro-pores created mainly in the initial stage of the manufacturing process and then enlarged by water permeation. In Stage II, alkali-silica gel is formed by reaction with water. The SiO\({}_{2}\) component within the cement reacts with high pH Na\({}^{+}\) and K\({}^{+}\) ions and generates Na\({}_{2}\)SiO\({}_{3}\cdot 2\text{H}_{2}\text{O}\). Stage III is a process in which alkali-silica gel erodes silica and alkali components in the cement causing the expansion of the cracks. Solid state SiO\({}_{2}\) and aqueoussolutions of Na\({}^{+}\) and OH\({}^{-}\) form NaHSiO\({}_{3}\). Then, volume expansion occurs. Finally, Stage IV is crack propagation or growth due to water permeation. The high stresses inside the cement expand the volume of the cement and lead to cracks. In general, it has been reported that amorphous silica is highly susceptible to ASR.