This work investigates the hydration of blended Portland cement containing 30 wt% Na2O–CaO–Al2O3–SiO2 (NCAS) glass particles either as the only supplementary cementitious material (SCM) or in combination with limestone, using 29Si MAS NMR, powder XRD, and thermal analyses. The NCAS glass represents a potential alternative to traditional SCMs, used for reduction of the CO2 emission associated with cement production. It is found that the NCAS glass takes part in the hydration reactions after about 2 weeks of hydration and a degree of reaction of ~50% is observed after 90 days of hydration. The hydrated glass contributes to the formation of the calcium–silicate–hydrate (C–S–H) phase, consuming a part of the Portlandite (Ca(OH)2) formed during hydration of the Portland cement. Furthermore, the presence of the glass and limestone particles, alone or in combination, results in an accelerated hydration for alite (Ca3SiO5), the main constituent of Portland cement. A higher degree of limestone reaction has been observed in the blend containing both limestone and NCAS glass as compared to the limestone–Portland mixture. This reflects that limestone reacts with a part of the alumina released from the hydrating glass, forming the calcium monocarboaluminate hydrate phase. Moreover, it may explain the synergy effect of using a combination of these two SCMs on the late compressive strength which is significantly higher compared to similar Portland cement blends using only NCAS glass or limestone as an SCM.