<p>To synthesize the lightweight calcium silicate powder with excellent physical and chemical properties, the reaction thermodynamics, formation kinetics, and transformation mechanism of tobermorite (5CaO·6SiO₂·5H₂O) using industrial lime and water glass under different conditions were systematically investigated. The thermodynamic calculations indicate that increasing the reaction temperature can promote the synthesis of tobermorite, and the formation of 5CaO·6SiO₂·5H₂O is easier than that of 5CaO·6SiO₂·3H₂O. The effects of various reaction parameters on the physical properties of tobermorite were revealed, and the optimal conditions for the formation of the best-performing tobermorite with a specific surface area of 0.359 m²/g and a bulk density of 0.121&#xa0;g/cm³ were obtained. The morphology of the calcium silicate hydrate evolves from the tinfoil-like agglomerates to well-crystallized flakes and then transits to fibers with a larger aspect ratio with the increasing temperature. The mechanistic correlations between unit cell dimension evolution and preferential crystallographic orientation growth were elucidated, which provides critical guidance for industrial production of high-value-added calcium silicate boards.</p> Graphical Abstract <p></p>