The partial replacement of Portland cement by aluminosilicate-rich supplementary cementitious materials (SCMs) represents a valuable approach to lower the CO₂ footprint of concrete. However, SCMs alter the composition and structure of the principal binding component, calcium-alumino-silicate-hydrate (C-(A)-S-H), a less-crystalline aluminum-substituted calcium silicate hydrate. The uptake and structural impact of alkalis and aluminum are investigated for synthesized C-(A)-S-H phases with different Ca/Si and Al/Si ratios and alkali (Na⁺) contents, using solid-state ²³Na, ²⁷Al, and ²⁹Si NMR as principal analytical tools. ²⁹Si{¹H} CP/MAS NMR data propose that the -87 ppm resonance originates from a second type of Q² pairing sites most likely in the middle of octameric or longer silicate units. This assignment along with constraints induced by the silicate dreierketten structure results in reliable simulations of the ²⁹Si NMR spectra. A quantitative 1:1 relation between the Al/Si ratios from ²⁹Si NMR and the content of fourfold-coordinated Al from ²⁷Al NMR is observed, which reveals that the ²⁹Si Q_p²(1Al) resonance is only associated with Al(IV) in C-(A)-S-H. Combined analysis of the ²⁷Al and ²⁹Si NMR spectra can account for sixfold-coordinated Al being present in the bridging sites of the silicate chains as proposed very recently. The sodium uptake is found to decrease with increasing Ca/Si ratio. For C-(A)-S-H phases with high Ca/Si ratios (Ca/Si ≳ 1.0), sodium promotes the incorporation of Al(IV) in the silicate chains but not at low Ca/Si ratios (Ca/Si ≲ 1.0). The results indicate that the mutual uptake of alkali and Al ions is governed by more than one mechanism and depends on the composition/structure of the C-(A)-S-H phase.