This paper forms part of a comprehensive study that is being carried out to quantify the influence of a fine glass powder as a supplementary cementing material (SCM) in concrete. The compressive strengths, non-evaporable water content, and time-dependent electrical conductivity of cement pastes modified with fine glass powder and Class F fly ash are compared. The pastes modified with 5% and 10% glass powder show comparable or higher strengths than those of similarly modified fly ash pastes, showing that glass powder can perform in an analogous manner to fly ash at these replacement levels. While 20% glass powder content results in negligible strength enhancement in pastes after 28 days, 20% fly ash modified pastes show a higher rate of strength enhancement in pastes after 28 days, indicating pozzolanic reaction. A factor called strength efficiency is proposed in this paper that can be used as an indicator of efficiency of SCM replacing cement at various ages. Determination of non-evaporable water contents of the plain and modified pastes showed that the glass powder uses less water than fly ash and cement during hydration. Increased cement hydration in the presence of glass powder until about 28 days (partly because of the lower water absorption of glass powder) is evident from the non-evaporable water content values. The glass powder modified pastes show lower effective electrical conductivities than the plain pastes and fly ash modified pastes at 14 days for similar replacement levels.