## 1 Introduction  
National and international standards have been adapted for manufacturing of 154 kV insulators, specifying the electrical and mechanical performance requirements that the insulators must meet. Size, insulation, electrical and mechanical strength are important for service compatibility in various countries. The design and material of a porcelain insulator varies according to the processing method. American National Standards Institute (ANSI) and the Canadian Standards Association (CSA) worked hard to reconcile these variations, and after a considerable time, the International Electrochemical Commission (IEC) standard achieved consensus and was accepted commercially all over the world [1, 2]. This study aims to investigate the effects of aging on porcelain insulators and provide guidelines for the diagnosis of the extent of damage caused by aging within metal and cement. At present, the lifespan of a porcelain insulator device is predicted to be from 30 to 60 years or more [3]. Once such an insulator is installed and ready for use, it should be able to serve the customer without interruption. It is very difficult to replace all the insulators on a line at once because of safety concerns. Currently, replacement is only carried out if external damage is visible. Since we cannot accurately predict the life expectancy of the porcelain insulator, it is necessary to instead track age-related changes in the porcelain structure. As data from aging studies accumulate, life expectancy may become predictable.  
The lifespan of insulators varies considerably. From an electrical point of view, there should be no arc discharge, and sound insulation properties are required to withstand transient voltages such as lightning. Robust mechanical properties are required due to weather conditions such as wind, ice,snow, and rain [4]. The porcelain insulator is not a single structure of uniform construction, but a complex structure of iron, cement, and ceramics. Deterioration of characteristics after a long period of use require diagnosis of aging [5].  
If the electrical characteristics deteriorate, it leads to partial discharge or flashover, which means the insulator can no longer insulate effectively. If the mechanical characteristics deteriorate, the insulator cannot be used in the transmission tower, posing the risk of a serious accident. This is due to environmental degradation caused by various pollutants including water, and both electrical and mechanical stress [6].  
Investigation has been carried out in order to analyze the degradation of cement by implementing three-dimensional computed tomography (3D-CT), which has a very high resolution and is considered a non-destructive way to analyze the deterioration due to internal defects. When applied to the analysis of porcelain insulators, 3D-CT is particularly useful in detecting degradation of the cement component. Clinker, which is the main component of cement used for porcelain insulators, consists of elite (3 CaOSiO\({}_{2}\)), belite (CaOSiO\({}_{2}\)), aluminate (NaAlO\({}_{2}\)), and ferrite (Fe\({}_{2}\)O\({}_{3}\)). Alite and belite hydrate produce low-crystalline calcium silicate hydrate gel (C-S-H gel) and Ca(OH)\({}_{2}\)[7]. The chemical composition and crystallinity of C-S-H gel varies during the hydration process. Immediately after the hydration reaction, the pH is raised to 10 or more due to the intrusion of H\({}_{2}\)O and the separation of Ca\({}^{2+}\) ions. The low C-S layer is formed on the surface, and as time passes, the Ca\({}^{2+}\) ion concentration increases and the hydrate layer thickens, and then the rehydration reaction occurs and the crystal growth proceeds.  
When the aluminate and ferrite crystal phases react with water, rapid hydration heat occurs and they stabilize to C\({}_{4}\)AH\({}_{19}\) and C\({}_{2}\)AH\({}_{8}\) hydrate. After a period of time elapses, they hydrate to stable C\({}_{3}\)AH\({}_{6}\). Therefore, gypsum is added to generate ettringite hydrate and achieve hydration using C\({}_{3}\)A hydrate [8].