The focus of this study is to elucidate the role of particle size distribution (PSD) of metakaolin (MK) on hydration kinetics of tricalcium silicate (C3S-T1) pastes. Investigations were carried out utilizing both physical experiments and phase boundary nucleation and growth (pBNG) simulations. [C3S + MK] pastes, prepared using 8%mass or 30%mass MK, were investigated. Three different PSDs of MK were used: fine MK, with particulate sizes < 20 µm; intermediate MK, with particulate sizes between 20 and 32 µm; and coarse MK, with particulate sizes > 32 µm. Results show that the correlation between specific surface area (SSA) of MK's particulates and the consequent alteration in hydration behavior of C3S in first 72 hours is nonlinear and nonmonotonic. At low replacement of C3S (ie, at 8% mass ), fine MK, and, to some extent, coarse MK act as fillers, and facilitate additional nucleation and growth of calcium silicate hydrate (C—S—H). When C3S replacement increases to 30%mass, the filler effects of both fine and coarse MK are reversed, leading to suppression of C—S—H nucleation and growth. Such reversal of filler effect is also observed in the case of intermediate MK; but unlike the other PSDs, the intermediate MK shows reversal at both low and high replacement levels. This is due to the ability of intermediate MK to dissolve rapidly–with faster kinetics compared to both coarse and fine MK–which results in faster release of aluminate [Al(OH)4-] ions in the solution. The aluminate ions adsorb onto C3S and MK particulates and suppress C3S hydration by blocking C3S dissolution sites and C—S—H nucleation sites on the substrates' surfaces and suppressing the post-nucleation growth of C—S—H. Overall, the results suggest that grinding-based enhancement in SSA of MK particulates does not necessarily enhance early-age hydration of C3S.