Hydrated cement paste shrinks mainly because it loses moisture from its really compact pores. As the dampness is shed in these little pores, the surface area pressure of the remaining h2o tends to pull the pores with each other which ends up in a lack of volume over time. Observe: https://www.tjcyindustrialchem.com/news/cement-additives-and-concrete-admixtures-key-differences.html