Waste materials such as fly ashes, slags, silica fume and recycled glass are often used as supplementary cementitious materials (SCMs) in blended cements, where they can partly replace Portland clinker and contribute to a reduction in the environmental footprint of concrete. The SCMs are generally less crystalline or amorphous compounds rich in silica and/or alumina. Their pozzolanic reactivity is partly ascribed to the amorphous structure, which complicates their analysis and quantification in hydrated cement blends using a range of conventional analytical tools. In this context, solid-state NMR represents an important analytical tool, as it allows quantification of crystalline and amorphous components in an equal manner. This chapter illustrates the potential of solid-state NMR in studies of pozzolanic waste products in blended Portland cements including silica fume, fly ashes, slags and glasses. A general description of the quantitative solid-state NMR approach is given and the advantages and limitations of the method in studies of waste materials and blended cements are discussed. It is shown that solid-state NMR can provide accurate measures for the degree of SCM reaction for the different SCMs along with the reaction degrees of the main phases in Portland cement. Moreover, it is illustrated that valuable information about the principal hydration product, the calcium-alumino-silicate-hydrate (C-(A)-S-H) phase, and the impact of SCMs on its composition and structure can be derived from 27Al and 29Si NMR spectra. Finally, it is shown that similar solid-state NMR approaches can be used to follow the carbonation process in end-of-life concrete exposed to enforced carbonation.