Abstract: This study investigates the synergistic effect of graphene oxide (GO) and high-volume mineral admixtures (fly ash, silica fume, slag) on cementitious materials. Compressive strength tests, thermogravimetric–differential thermogravimetric (TG-DTG) analysis, and scanning electron microscopy (SEM) were conducted to reveal multi-scale mechanisms. Results indicate that high mineral admixture replacement delays early strength, while 60% replacement provides the best later strength. With GO incorporation, strength development is accelerated and the upper strength limit is enhanced, enabling 80% replacement systems to achieve optimal performance, with 28-day compressive strength about 30% higher than the control. TG-DTG results show calcium hydroxide (CH) content decreases by 40%–64% with mineral admixtures and by 67%–76% with GO addition, accompanied by a 14%–20% reduction in CH crystal size. SEM observations further confirm that GO induces smaller, regular CH crystals and denser calcium silicate hydrate (C-S-H) gel, significantly reducing pores and cracks. Overall, GO effectively improves mechanical properties and optimizes hydration products and microstructure in high-volume mineral admixture systems, offering a sustainable pathway for low-carbon, high-performance cementitious materials.