In an attempt to improve the sustainability of construction and reduce portland cement (PC) consumption, the use of supplementary cementitious materials (SCMs), such as ground granulated blast-furnace slag (GGBS), has become a common practice. On the other hand, to increase the durability of cementitious composites, various internal curing agents, including superabsorbent polymers (SAPs), are often used. Owing to their high capacity to absorb, retain, and release water, SAPs can provide additional water for continuous hydration and lead to more homogeneous microstructures. They are usually neutralized by alkali metals (sodium and potassium) to increase their absorption capacities and keep them stable in PC cementitious matrices. This article discusses the applicability of SAPs in blended systems. It aims to evaluate the effect of GGBS on the water absorption capacity and stability of three partially neutralized SAPs. SAP swelling capacity and kinetics of absorption, pH of binder solutions over time, and the mechanical properties of PC-GGBS matrices are analyzed. The results show that alkali content of up to 4% by weight leads to a GGBS system that is comparable to a stable PC system. Above this limit, SAP degradation starts to take place due to ion exchange with GGBS solution components, resulting in lower compressive strength compared to PC matrices. Thus, the excess of alkalis in SAP networks plays an important role in GGBS aqueous solutions.