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Calciumsilicate hydrates (C–S–H) are the most abundanthydration products in ordinary Portland cement paste. Yet, despitethe critical role they play in determining mechanical and transportproperties, there is still a debate about their density and exactcomposition. Here, the site-specific mass density and compositionof C–S–H in hydrated cement paste are determined withnanoscale resolution in a nondestructive approach. We used ptychographicX-ray computed tomography in order to determine spatially resolvedmass density and water content of the C–S–H within themicrostructure of the cement paste. Our findings indicate that theC–S–H at the border of hydrated alite particles possiblyhave a higher density than the apparent inner-product C–S–H,which is contrary to the common expectations from previous works onhydrated cement paste. [ABSTRACT FROM AUTHOR]