# Long-Term Effects of External Sulfate Attack on Low-Carbon Cementitious Materials at Early Age
## 1 Introduction  
In general, two types of sulfate attacks are distinguished. The first is known as internal sulfate attack (ISA, or ISR for internal sulfate reaction, or DEF for delayed ettringe formation), where sulfate originates from raw materials [1, 2]. The second is external sulfate attack (ESA), and the source of sulfate, in this case, is the surrounding environment (groundwater, soil, seawater, etc.) [3, 4]. When it comes to external sources, sulfate, a pervasive agent contributing to the deterioration of concrete structures, is inevitable due to its presence in various external sources where constructing RC structures is a necessity. Upon contact with RC structures, sulfate ions penetrate the cementitious matrix through the porous network, initiating chemical reactions with hydrated cementitious materials to produce ettringe and/or gypsum [5, 6, 7, 8, 9]. This process induces internal stresses, leading to expansion and cracking in the structure. Consequently, the structural integrity weakens,permeability increases, and a feedback loop ensues, facilitating further migration of sulfate ions. This phenomenon involves complex interactions among physical, chemical, and mechanical factors [10; 11; 12; 13; 14]. Despite ESA having historical roots dating back to 1818 [15], it remains a complex issue, influenced by many factors, such as cation type (calcium, sodium, or magnesium sulfate), concentration, pH of the surrounding environment, the type of cement used, and many other [16]. A study by [PERSON] et al. [17] compared the degradation effects of blended cement under sodium sulfate and magnesium sulfate attack. Their findings revealed distinct mechanisms of attack, with magnesium sulfate causing significantly more damage compared to sodium sulfate. Over the years, considerable efforts have been invested in developing methods for testing ESA resistance of all types of binder and concrete. However, each method comes with its strengths and limits, and many are criticized for their dissimilarity with respect to real-world field exposure conditions. Notably, various testing standards, such as ASTM C 1012, the Asian method, the Swiss method, and CSA A 3004-C8, employ high concentrations of sodium sulfate (greater than 30 g/L) [18] to generate an infinite source of sulfate and accelerate the test. A study conducted by [PERSON] [19] revealed that the preferential formation of ettringite occurs when a low concentration of sulfate is selected, while higher sulfate concentrations lead to an increased production of gypsum. These elevated concentrations, coupled with other testing conditions like extreme temperatures and harsh dry and wet cycles, are intended to accelerate the attack for concrete durability testing against sulfate. However, it is crucial to acknowledge that such conditions may alter the characteristics of the ESA, raising questions about the representativeness of these methods. In addition, if not prefabricated, on-site cast concrete is directly exposed to the surrounding environment from an early age [20; 21]. A recent investigation compared the resistance of prefabricated and cast-in-site concrete against ESA, revealing that prefabricated concrete demonstrates greater resistance to ESA [22]. Curing is recognized for enhancing the properties of structural concrete, including microstructure, mechanical strength, and durability, by promoting the hydration of cementitious materials, reducing creep and shrinkage, and providing protection against environmental aggressions [23]. However, real practices consist of exposing concrete to sulfate ions in certain environments at an early age. Most studies and standards focus on cured samples, typically after 28 or 90 days of curing before attacking the samples, leaving unanswered questions about the impact of ESA on RC structures during their early stages. A recent study [20] investigated the degradation mechanisms induced by ESA on both cured and uncured cement pastes.