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Understanding the fundamental physicochemical processes that occur during the hydration of cementitious materials is essential for developing alternative binders that enable the partial substitution of Portland cement (PC), lowering the carbon footprint associated with the cement industry. Magnesium-silicate-hydrate (M-S-H) stands as a potential alternative binder; however, its inferior mechanical properties attributed to its nanostructure, and the high-water demand for curing, hinder the application of MgO-based cement as PC surrogate. A potential strategy to tackle these major drawbacks is based on controlling M-S-H formation from the early stages, building the properties and nanostructure of this binding phase from its basic building units. The present work provides insights into M-S-H nucleation and early growth gathered by combining titration experiments with electrospray ionization mass spectrometry, analytical ultracentrifugation, and transmission electron microscopy. We evidenced a nonclassical multistep pathway where a highly complex mixture of defined hydrated magnesium (sodium)-silicate oligomeric species exists in solution prior nucleation. Our results suggest that these entities aggregate, yielding an ill-defined M-S-H precursor phase (depleted in Mg compared to the final product) which later transforms into a denser M-S-H interconnected network (Mg:Si ratio ca. 1) with a more defined sheet-like structure that still retains its poorly crystalline character. The identification of oligomeric silicates species prior nucleation is of great importance for developing means to regulate M-S-H (and potentially other hydrous silicates) formation by using a bottom-up approach. This work reveals that, during the hydration process of cementitious materials, the stages prior to the formation of hydrated solids cannot be disregarded, as they could open new avenues for engineering the properties of the final materials.