Calcium silicate hydrate is synthesized from steel slag by adding lime and quartz by dynamic hydrothermal method at 185 C. The products mainly consist of disordered calcium silicate hydrate, but more crystallized phase such as 11 Å-tobermorite forms for the presence of inorganic admixtures. The formation of tobermorite is affected by the types of ions according to XRD and FTIR results. Adding 2% crystallized tobermorite is beneficial to form the ordered structure; alkali cations and SO4 2- anions are effective accelerators; Al3+ plays a role of stabilization during the reaction. The products also vary with the initial alkalinities of steel slag.