Portland cement, the ‘glue’ in concrete, is the most widely used construction material in the World with an annual production of more than 4 billion tons. This large amount results in a significant CO2 footprint, approx. 7% of the anthropogenic CO2 emissions, implying that current research of the cement scientific community and industry focusses on reducing this impact. A reduction by 20 – 40% in CO2 emissions can be achieved by substitution of the Portland clinkers by supplementary cementitious materials (SCMs), where calcined clays and carbonated recycled concrete fines attract most research attention at the moment. The latter type of SCM can be obtained by enforced carbonation of cement fines from end-of-life concrete, producing a material mainly composed of calcium carbonate and an alumina silica-gel [1]. A common feature for hydrated Portland cement, calcined clays, and carbonated recycled concrete fines is that the principal components are present as amorphous phases. Thus, solid-state NMR, in particular 27Al and 29Si NMR, has been an invaluable tool for characterization of the structure, composition, and reactivity of these phases in Portland cement – SCM binders, which will be illustrated in the present contribution by three examples.
(i) For calcined clay - limestone Portland cements, 27Al and 29Si NMR has been used to obtain information about the degree of reaction of the calcined clay (metakaolin) and the evolution of the Al-bearing phases during hydration. The principal binding phase is a less-ordered calcium-(alumino)-silicate-hydrate (C-(A)-S-H) phase, where the combi-nation of 27Al and 29Si NMR has provided quantitative information on Al in this phase, giving a more detailed picture on the capacity of the C-(A)-S-H phase to incorporate Al under different compositional conditions and at different hydration stages.
(ii) 27Al and 29Si NMR investigations of aqueous carbonation of Portland cement pastes, incorporating different types of SCMs, reveal that nearly all calcium-bearing phases can be carbonated and thereby, that the initial calcium content governs the CO2 uptake capacity. A model for deconvolution of the 29Si NMR spectra of the carbonated cement pastes has been established, which has led to a structural model for the alumina-silica gel that accounts for the Al/Si ratio and the degree of polymerization of the gel [2]. 
(iii) 1H, 13C, and 27Al NMR experiments at very high magnetic field (22.3 T) has been utilized to characterize the structural environments for these elements in the cement hydrate phase, monocarbonate (Ca4Al2(OH)12(CO3)·5H2O). Here, the combination of the very high magnetic field and ultra-fast MAS (νR = 100 kHz) has allowed resolution of the 22 distinct 1H resonances in a 1H – 1H SQ-DQ (BABA) MAS experiment. The 1H resonances are assigned to distinct sites in the hydrogen network from the correlations in the 2D spectrum combined with DFT calculations of 1H chemical shifts.      

[1] M. Zajac, I. Maruyama, A. Iizuka, J. Skibsted, Cem. Concr. Res. 174, 107285 (2023)
[2] F. M. Neto, R. Snellings, J. Skibsted, Cem. Concr. Res. 177, 107428 (2024).