Moisture content significantly influences strength development in solidified sludge. Ground granulated blast-furnace slag (GGBS) has been found to exhibit cementitious properties in alkaline environments. This paper investigates strength development patterns in portland cement (PC) and GGBS solidified sludge with different curing systems. The effects of initial moisture content (%), PC/GGBS ratio, and cementitious material dosage (kg/m³) on unconfined compressive strength were studied for closed system (CS) and open system (OS). X-ray diffraction (XRD), scanning electron microscopy, and thermogravimetric analysis (TG) tests examined strength and microreaction mechanisms. Results show that strength development and reaction mechanisms in CS differ from those in OS. In contrast to OS, where hydration/carbonation dominates, CS exhibits lower strength due to weakened cement reactions. Here, the PC/GGBS ratio becomes critical as the alkali-activated reaction between GGBS and alkali comes from cement hydration, which serves as the primary source of strength. Clay particles in sludge also participate in alkali-activated reactions in CS. XRD and TG results confirm these mechanistic differences. For in situ curing of high-moisture sludge, a PC/GGBS ratio of 7.5:2.5 and ≥100 kg/m³ cementitious material are recommended for optimal field application.