## 1 Introduction  
Superabsorbent polymers (SAPs) have emerged as a valuable solution for enhancing the durability of concrete structures, particularly in the context of internal curing, which counteracts the detrimental effects of shrinkage and leads to improved cracking behavior. These polymers, consisting of a natural and/or synthetic water-insoluble 3D network of polymeric chains crosslinked by chemical or physical bonding, possess the remarkable ability to absorb large amounts of liquids from the environment (in amounts up to 1500 times their own weight) and transform into insoluble gels [1, 2]. The utilization of SAPs in cementitious materials has expanded the frontiers of construction in the past two decades, enabling the development of smart, innovative, and sustainable structures [3, 4]. Previous research has demonstrated the multiple benefits of SAPs in enhancing the performance of cementitious materials beyond internal curing, as SAPs have been found to also promote crack sealing, enhance autogenous healing, control rheological properties, and increase resistance against salt-scaling under freeze-thaw conditions [5, 6, 7, 8, 9]. Internal curing has proven to be an effective strategy in reducing chloride ingress and carbonation in cementitious materials, thus enhancing the durability of concrete structures. The use of lightweight aggregates (LWAs) as internal curing agents, for example, has demonstrated significant improvements in limiting the penetration of chloride ions. Studies conducted by [PERSON] [10] and [PERSON] [11] have shown that incorporating pre-wetted fine lightweight aggregates in concrete mixtures results in reduced chloride ingress compared to reference mortar mixtures without internal curing. The authors found that the interfacial transition zone surrounding the lightweight aggregate particles exhibits lower porosity, leading to improved quality and densification of the cement paste. Similarly, [PERSON] et al. [12] tested concrete mixtures internally cured with saturated LWAs and found that the migration coefficient of the internally cured concrete was slightly lower than the migration coefficient of the plain concrete (15-30% lower), which the authors attribute to increased cement hydration and reduced porosity at the interfacial zone. When it comes to SAPs and their effects on chloride ingress, a few studies have been published on the topic. [PERSON] and [PERSON] [13] attributed the reduction in chloride ingress caused by the internal curing of SAPs to the general densification of the paste phases as expressed by their increased gel space ratio. The authors also explained that this is different from what occurs with LWAs, where the effect of internal curing that reduces chloride transport is attributed to the improved quality of the interfacial transition zones (which does not necessarily occur with SAPs). Beyond that, they also found that adding SAPs without additional water causes an increase in the resistance of concrete to the penetration of chloride ions. In the case of SAPs added to the mixtures with additional water, the penetration of chloride ions could only be reduced if the hydration degree could be sufficiently increased. With similar results, [PERSON] et al. [14] concluded that the decrease in the chloride ion penetration in cementitious materials with SAPs was due to improvements in the interface transition zone between the SAP void and hardened binder matrix. Reduced chloride ingress in concrete mixtures studied by [PERSON] et al. [15], [PERSON] et al. [16], and [PERSON] et al. [17] was also attributed to a refinement in the pore structure and consequently limited diffusion of chloride ions caused by internal curing promoted with SAPs. In a more recent study, [PERSON] et al. [18] showed the effect of sealing and healing promoted by SAPs on the chloride ingress of cracked mortars. In the study, the authors found that the addition of SAPs improved the performance compared to a reference mixture with the same total water-to-cement ratio.