# Sulfuric Acid Resistance of CNT-Cementitious Composites
## 1 Introduction  
Concrete structures have inevitably suffered from the damage and degradation resulting from aging over a period of years, or external mechanical and environmental impacts during the service period. Therefore, to safely use them, structural health assessments and diagnosis are required by using evaluation equipment and devices. However, as structures have become larger, higher and more complex, this has been difficult because it is impossible to access the damaged area and degradation location in the concrete structures for safety reasons. To address these problems, structural health monitoring (SHM) systems have been developed and widely employed around the world [1; 2; 3; 4]. In addition to SHM systems, self-monitoring or self-sensing composites have been developed and used to evaluate the damage of structures [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. Steel fibers have been employed not only to increase the tensile strength of structures but also to provide self-sensing properties ([PERSON] et al., 2019 [21]; [PERSON] et al., 2018 [22]; [PERSON] et al., 2017 [23]). Recently, there has been a growing interest in using carbon nanotubes (CNTs) as self-sensing materials of concrete due to their excellent electrical conductivity ([PERSON] et al., 2020 [24]; [PERSON] et al., 2020 [25]). These studies mainly focused on structures on the ground, such that there is little research on self-sensing structures located in the ocean and underground. Furthermore, since sewage structures susceptible to chemical erosion constitute about 30% of the national urban infrastructures and are highly likely to suffer corrosion due to sulfuric acid attack, it was essential to provide them with self-sensing capabilities to detect damage. Generally, sulfate ions can easily penetrate into micro-voids and micro-cracks in concrete composites by the transfer mechanism and react with calcium aluminate hydrate and calcium hydroxide to produce expansion hydrationproducts including ettrinigite and gypsum, which results in the swelling of mixtures, bigger size cracks, and the loss of the mechanical properties of concrete [26; 27; 28; 29; 30; 31; 32]. This mechanism is called external sulfate attack [33; 34; 35]. When concrete is exposed to sulfate ions, thaumasite is formed as an alteration of cementitious composites and has very low strength, resulting in the disintegration of the cement paste [36]. CNTs have a significantly low density (1.3-1.4 g/cm\({}^{3}\)) and the amount of CNTs does not exceed up to 2.0% of the weight of the binder ([PERSON] et al., 2020 [37]; [PERSON] et al., 2020 [38]). Moreover, it is very difficult to obtain uniform dispersion of CNTs in the matrix due to van der Waals forces between the CNT particles, and the poor dispersion of CNTs results in low mechanical properties of the cementitious composites. There exist diverse techniques and methods, such as admixtures, silica fume, surface modification of CNT, minimizing the water-to-binder ratio, and ultrasonication to improve homogeneous dispersion in the composites and their mechanical strength [39; 40; 41]. Moreover, uniformly dispersed CNT cementitious composites have excellent self-sensing performance. In spite of numerous studies on the uniform dispersion of CNT cementitious composites, to the authors' knowledge, no one has conducted studies on the durability of CNT cementitious composites subjected to sulfate attack. In this research, to investigate the durability of CNT cementitious mixtures exposed to sulfate attack, powder types of multi-walled or single-walled CNTs were selected and added to the cement mortar with up to 2.0% mass fraction and the specimens were immersed in 5% and 10% sulfuric acid solutions. Several mechanical tests, including weight change ratio and compressive strength change ratio, were carried out before and after degradation induced by sulfate attack.