## 1 Introduction  
When it comes to the disposal of radioactive waste, various concerns must be addressed. One of the most critical issue is ensuring the long-term safety of the disposal site once it has been released from regulatory oversight (after 300 years) [1]. Numerous radionuclides have long half-lives, indicating that they will continue to be hazardous for longer than 300 years. Therefore, it is essential to adopt a disposal method that will minimize the danger of radiation exposure to current and future generations. Near-surface disposal is a sought-after disposal method that involves holding waste in a facility designed to isolate it from the environment by utilizing both natural and engineered barriers. This disposal approach is suitable for low- and intermediate-level radioactive waste, the majority of which consists of radionuclides with short half-lives and a few with long half-lives that decay over thousands of years beyond the operational lifetime of the disposal site [2]. As the second phase of the Gyeongju LILW radioactive waste repository, the first near-surface waste disposal facility in South Korea was developed [3]. The first stage of the LILW facility is a 100-m-deep underground silo with a capacity of 35,200 cubic meters (130,000 drums) [4]. The near-surface repository was constructed with a capacity of 32,875 cubic meters (125,000 drums) fewer than 30 m below the surface [4]. This disposal includes a subterranean gallery, three mobile cranes, and twenty concrete vaults. These man-made barriers were incorporated to supplement natural barriers in order to meet the regulatory design goals of isolating radionuclides from the environment. Important criteria to consider when choosing barrier structures and materials are their long-term durability, compatibility with site characteristics, associated media and availability [5]. Concrete structures at the Gyeongju LILW site are made of Ordinary Portland cement, as is the case with many near-surface disposal sites [6]. Ordinary Portland cement and composite cements (with supplementary cementitious materials in the form of iron blast furnace slag, silica fume, pozzolans, and fly ash) are the commonly used types of cement due to the well-understood technology transferred from civil engineering research and many years of experience with its performance in various environments [78]. In addition to Portland cement, other materials are being evaluated to determine their suitability for use in the construction of engineered barriers to radioactive waste repositories [9]. For instance, geo-polymers produced by mixing a source of silicates and aluminates (such as fly ash or metakaolin) with an alkali activator have been proposed for use, since they were allegedly utilized in the construction of ancient pyramids [9]. Despite this claim, there is still uncertainty surrounding their long-term performance; and their use of caustic Na/K(OH) as an activator presents handling challenges [9]. When evaluating the long-term performance of a near-surface waste disposal facility, it is often assumed that concrete barriers will maintain their initial chemical buffering function for thousands of years [10]. Consequently, many studies on the long-term performance of concrete engineered barriers focus on the evolution of hydraulic behavior over long timescales, with the assumption that the concrete barriers will retain their initial designed hydraulic properties for a certain period of time before disintegrating into porous sand with poor hydraulic properties (rapid water infiltration) [11]. This does not account for the progressive degradation of concrete barriers resulting from geochemical changes. Concrete is a mixture of water, hydrated cement, and fine aggregates, such as sand and gravels [12]. Portland-based concrete is made of several anhydrous minerals, mainly calcium silicates (alite, belite, aluminates, and ferrites), and traces of gypsum [13].