Despite a wide range of applications, naturally occurring minerals suffer from some limitations for industrial use. Consequently, many research efforts have been conducted to develop a fast, optimized, and sustainable methodology to produce synthetic minerals. In the case of calcium silicate hydrates (CSH), the hydrothermal flow approach allows to mimic the environment at high temperature and pressure of the natural geological processes for the synthesis of xonotlite under sub- and supercritical conditions in only few seconds. The ultra-fast, flexible, and effective production of xonotlite particles reported in this work expands its use towards industrial requirements, especially for applications to cement-based materials. In this context, CSH nanominerals can be used to impart new functionality or to accelerate the hydration process of cement for developing green cement materials. This study sheds light on the acceleration effect of crystalline xonotlite seed, measured using isothermal calorimetry and synchrotron radiation based X-ray microtomography, as a means of lowering the cement content without compromising the performance of the paste.