# Long-Term Effects of External Sulfate Attack on Low-Carbon Cementitious Materials at Early Age
## 1 Introduction
Surprisingly, pre-cured cement pastes exhibited rapid degradation, although sulfate ion ingress did not show significant differences when comparing pre-cured samples to the uncured ones. In contrast, [PERSON] et al. [24] conducted a study suggesting a minimum initial curing period of 14 days for cement-based materials exposed to sulfate. Their findings indicated that a longer initial curing period correlated with improved resistance to sulfate in the cement-based material. Speaking of common practices while moving to the current trend to resolve environmental issues, one of the most effective and widely employed strategies today to jointly mitigate the substantial CO2 emissions from the cement industry and to improve cementitious materials' durability is the partial substitution of clinker with supplementary cementitious materials (SCM) [25]. Due to their pozzolanic effects, these materials not only facilitate the formation of additional C-S-H and reduce the porosity size but also serve as a source of alumina, promoting the development of aluminum-containing phases [26]. The changes in cementitious materials composition significantly influence the mechanisms of ESA. In this context, research indicates that the incorporation of SCM enhances the durability of specimens under sulfate attack by reducing the C3A content [27]. Additionally, numerous studies have demonstrated that the utilization of slag, metakaolin, silica fume, and fly ash improves durability when it comes to ESA [28; 29; 30; 31; 32; 33]. In a recent investigation conducted by [PERSON] et al. [34], specimens incorporating fly ash and slag exhibited enhanced mechanical properties after 90 days. Interestingly, these specimens demonstrated similar properties at 28 days compared to those made with ordinary Portland cement (OPC). Furthermore, these findings corresponded with observations of porosity and capillary water absorption. This suggests that the incorporation of SCM necessitates a 90-day curing period to exhibit their benefits. However, this leaves a gap regarding the mechanisms of ESA on materials cured in sulfate (exposed to sulfate at an early age) since previous studies focus on pre-cured samples when it comes to ESA. Considering all the factors mentioned above, there is a gap in understanding the impact of ESA on samples exposed to sulfate ions at an early age over an extended period, utilizing an adjusted concentration of sulfate. Additionally, there is a need to investigate how ESA affects the resistance of low-carbon cementitious matrices, aligning with the developed low-carbon cement types. For this, this study aims to address two main objectives. Firstly, this study seeks to gain a comprehensive understanding of the long-term effects of ESA on cement paste samples exposed to sulfate ions at an early age. The second objective is to evaluate the resistance of SCM, namely fly ash, blast furnace slag, and metakaolin, under such attack conditions using low-carbon formulations. To achieve this, OPC pastes, binary, ternary, and quaternary blended cement pastes were utilized to address the objectives of the study. The samples underwent exposure to a sodium sulfate solution with a concentration of 15 g/L from an early age, for a duration of three years. This concentration was selected to both create an infinite source of sulfate and to shorten the duration of the test as much as possible while using a relatively low concentration of sulfate. Mass loss, expansion, and physicochemical changes were periodically monitored throughout the experiment, supplemented by visual inspections. The characterization techniques included thermogravimetric analysis, Fourier transform infrared spectroscopy, and the water-accessible porosity test. Furthermore, Raman spectroscopy was employed to monitor the sulfate content in the solutions in which cementitious material samples were immersed.