## 1 Introduction
It is important to note that such surface treatment methods are unlikely to restore or enhance the strength of concrete components, and the research in this specific aspect has been limited. Strontium (Sr), classified as an alkaline earth metal, exhibits chemical characteristics that are similar to calcium (Ca). Calcium is notably a primary and abundant component in hydrated cement composites. However, strontium is characterized by its larger chemical reactivity and ionic size compared to calcium (Sr\({}^{2+}\): 1.13A vs. Ca\({}^{2+}\): 0.99A). Owing to these properties, it is understood that calcium ions can be easily replaced by strontium ions, with the size difference between the two elements leading to a denser microstructure [16; 17]. Based on this principle, the research in the medical field is being conducted on osteoporosis treatments and implant materials using strontium to improve bone density through microstructural modification [18; 19; 20; 21]. Strontium ions are known to react with cement hydrates in aqueous solutions in three different ways: first, by electrostatic adsorption to C-S-H [22; 23]; second, through a Ca\({}^{2+}\)-Sr\({}^{2+}\) ion exchange process [24; 25]; and third, by forming a strong bond with cement hydrates containing aluminum, such as AFt (ettringite) and AFm (monosulfoaluminate) [26]. Based on this understanding, substituting calcium ions in hydrated cement composites with strontium ions can theoretically reduce the microstructure's porosity, leading to a denser structure with improved resistance to water penetration and overall enhancement of durability performance. This hypothesis underpins recent preliminary research into a permeable surface treatment method using strontium cation (Sr\({}^{2+}\)), which has shown significant results in improving surface abrasion resistance and water absorption resistance [27]. Building on this background, this study posits that the use of strontium cation-based permeable surface treatment techniques can alter the strength characteristics of already hardened hydrated cement composites in the near-surface region. While similar research was conducted in the past [28], it had limitations in terms of its experimental scope and result analysis. In this study, the potential of strontium cations as a method for enhancing the durability performance of hydrated cement composites was explored. The cement paste specimens were prepared and treated with strontium cations in solution, and modifications in durability characteristics, such as surface abrasion resistance, water sorptivity, and void content ratio (porosity), were experimentally investigated. Furthermore, special attention was provided to experimentally assessing the potential for improvements in compressive and flexural strength characteristics due to the surface treatment with strontium cations. For the strength tests, the digital image correlation (DIC) technique was utilized to analyze the behavioral characteristics of the specimens. For all test results, \(t\)-test analyses were performed to identify the significance of the difference in mean values between the Sr\({}^{2+}\) treated group and the untreated group. In addition, changes in the microscopic morphology and chemical compositions and structures of the cement hydrates, induced by the strontium ions, were observed through analyses conducted using a scanning electron microscope (SEM), an energy-dispersive spectrometer (EDS), and X-ray diffractometer (XRD).