<p>To address the high bulk density, insufficient mechanical strength, and poor thermal stability in tobermorite applications, the crystallization mechanism of tobermorite modified with organic and inorganic additives was systematically investigated. The results indicate that additives enhance the physical properties or crystallization behavior of tobermorite to varying degrees, while organic additives, especially polyethylene glycol, inhibit crystallization of tobermorite owing to the concurrent effects of competitive adsorption and steric hindrance. Inorganic additives exhibit more significant enhancement on physical properties compared to organic additives. Notably, Sr(NO₃)₂ enhances the physical properties and crystallization behavior of tobermorite concurrently compared with other additives. The partial substitution of Ca²⁺ by Sr²⁺ in tobermorite induces lattice expansion, reducing the growth energy barrier of the (001) plane, and promoting ordered interlayer arrangement. At a Sr(NO₃)₂ dosage of 4%, the lightweight spherical porous tobermorite with a relative crystallinity exceeding 90% and an ultralow bulk density of 0.14 g·cm⁻³ was obtained, which holds significant guiding value in the synthesis of lightweight insulation materials and efficient adsorbents.</p>