Densifying hydration products of alite by a bio-inspired admixture

•Tannic acid (TA) can be used to mediate the hydration of tricalcium silicate (alite) to achieve better microstructure.•TA can form complex with calcium ion through chelating, thus retarding the hydration of the alite.•TA can promote the growth of calcium hydroxide along x-axis.•TA can modify the atomic structure of calcium silicate hydrate.•TA has a unique ability to radically reduce inter-hydrate pores of cement paste.