The intermediate-range order in peralkaline calcium aluminosilicate glasses is explored by two different models using solid-state 29 Si MAS NMR spectra as the main experimental basis. The two modeling approaches describe the spatial arrangement of the tetrahedral SiO 4 and AlO 4 units, that is, the intermediate-range order (IRO), assuming either a random distribution of structural units or a hierarchy in the IRO. The hierarchy creates a quasi-heterogeneous distribution of the structural units. Two series of calcium aluminosilicate glasses (five glasses for each) are chosen for the present study, compositions of which vary along the join between the anorthite-wollastonite-gehlenite eutectic point and anorthitewollastonite-tridymite point, and parallel to the join. The validity of the two modeling approaches is examined by simulation of the frequency and intensity distributions observed in the 29 Si MAS NMR spectra of the 10 glasses using a sum of resonances predicted for the different types of structural units. The results clearly reveal that the 29 Si MAS NMR spectra support the quasi-heterogeneous IRO for the 10 glasses, since this approach gives a satisfactory fit to the experimental spectra. In contrast, simulations based on the model of random IRO cannot reproduce the 29 Si NMR spectra in a satisfactory manner. The observed hierarchy in the IRO may also explain the compositional dependence of both viscous behavior of the melts and the stability of the 10 glasses.

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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.