Concrete is one of the most widely used construction materials worldwide due to its versatility and compatibility with various components. However, its production relies on limestone calcination at high temperatures, an energy-intensive process that generates significant CO2 emissions. To mitigate this impact, a promising strategy is to partially replace cement with limestone filler while maintaining the required performance. This research evaluates the effect of replacing 7.5–40% of the total cement mass with limestone filler on chloride-ion permeability and compressive strength. Chloride ingress is a major cause of concrete deterioration, as chloride ions penetrate through diffusion, permeability, or capillary suction, altering the pH and disrupting the passive protective layer on the reinforcement, leading to steel oxidation and expansion. The study investigates six mixtures: two controls (100% GU cement and GUb-8SF cement with 8% silica fume) and four mixtures based on GUb-8SF with 7.5, 15, 25, and 40% limestone filler substitutions. Chloride-ion permeability was assessed using the RCPT (at 91 days) and the Nordic standard method (at 91 days), complemented by compressive strength testing. Results show that increasing limestone filler content generally reduces compressive strength, except at 28 days, where the GU, GUb-8SF, and GUb-8SF-15LF (15% limestone filler) mixes displayed similar performance. Regarding chloride-ion penetration, all mixtures exhibited a very low permeability class (100–1000 coulombs), except for the GU control mix, which showed low permeability (1000–2000 coulombs) according to ASTM C1202 at 91 days.