Calcium–silicate–hydrate (C–S–H) seeding offers an effective means of accelerating cement hydration. Its performance depends strongly on the stability of the seed particles and the availability of accessible sites for nucleation. This study investigates the effect of chitosan templating on the microstructure, hydration kinetics, rheological behaviour, and early-age strength development of C–S–H seeds. Chitosan-templated C–S–H seeds containing 0, 0.25, 0.50, and 1.00 wt% chitosan (CTS-0, CTS-25, CTS-50, and CTS-100) were synthesized using a biomimetic co-precipitation method and characterized using SEM, BET surface area analysis, TGA–DTG, and isothermal calorimetry. Their performance in cementitious systems was evaluated through rheological measurements and compressive strength testing. CTS-25 exhibited the optimum performance, showing the highest BET surface area, favourable pore characteristics, and a thermally stable C–S–H framework with limited polymer incorporation. These features promoted the greatest hydration acceleration, with a peak heat evolution of 2.09 mW/g and a cumulative heat release of 321 J/g after seven days. Consequently, CTS-25 produced the highest yield stress, viscosity, structural build-up, and early-age compressive strength. It achieved strength improvements of up to 63% over the reference mortar. Excessive chitosan (CTS-50 and CTS-100) causes polymer-rich agglomeration, rapid initial stiffening and suppresses sustained hydration.
