The morphological effect of calcium silicate hydrates (C-S-H) on the elastic behaviour of early-age cement-based materials is studied based on a refined multi-scale elasticity prediction model. The model demonstrates that the growth morphology of low-density (LD) C-S-H has a significant effect on the solid percolation threshold and the elastic behaviour of early-age cement-based materials, especially in the cases of higher water-cement (w/c) ratios or lower hydration degrees. Using the refined model, the growth morphology of LD C-S-H is quantitatively determined and verified by comparing the predicted results with previous experimental data. It is revealed that LD C-S-H grows in a lengthened manner when w/c is above 0.40, and the aspect ratio of LD C-S-H varies according to the availability of free growth space. When the w/c ratio is at the level of 0.50, the realistic growth morphology of LD C-S-H is close to that of a prolate, with an aspect ratio of the order of 10. The elongated growth of LD C-S-H is the key factor enabling cement-based materials with high w/c to set and have a rapid development of elasticity within the first few hours.