Cementitious materials show anomalous behaviour during capillary imbibition: a lack of linearity of the mass gain versus the square root of time. This is considered to be caused by the water retention in the calcium silicate (C-S-H) structure (and consequent swelling) during water ingress. However, the strain behaviour of C-S-H during imbibition has never been measured. Additionally, deformations in cementitious materials are not unconstrained “free” deformations, they are influenced by the restriction level provided by the sample. Restriction can be due to the mix composition (relative amount of the deforming phase, C-S-H, versus non-deforming reaction products and aggregates) or due to sample shape. To evaluate the strain behaviour of pure C-S-H during capillary imbibition, strains of compacted C-S-H samples were continuously measured during water ingress. To analyse the effect of sample shape, the imbibition of concrete samples with different geometries (rings and cylinders) was determined. Results show that C-S-H undergoes significant deformation during imbibition, demonstrating its ability to deform when in contact with water. Sample shape influences the matrix restriction and the water flow during primary imbibition (when capillary forces are dominant). However, such differences are small when compared with the larger impact that mix composition has. These results capture the complex interactions between water flow, pore structure, mix composition, and sample shape of cementitious materials during imbibition.