<p>Ye'elimite (C₄A₃$) is the principal mineral phase in calcium sulfoaluminate (CSA) cement, a promising low-carbon binder. However, its excessively rapid hydration reaction poses significant challenges for practical applications, necessitating effective retardation strategies. Conventional chemical retarders can delay CSA hydration but often suffer from unstable performance and narrow dosage windows, limiting their engineering feasibility. In this study, a novel retardation mechanism based on ionic solid-solution doping with B³⁺ was proposed and elucidated for the first time. Experimental results reveal that a portion of B³⁺ ions substitutes for Al³⁺ ions and enters the C₄A₃$ lattice, while the remaining B³⁺ ions accumulate on the particle surface, forming a low-reactivity B-containing amorphous shell layer. This core–shell structure significantly impeded the contact between water and the reactive C₄A₃$ core, thereby slowing down the hydration process. Additionally, the continuous dissolution of B₂O₄²⁻ into the pore solution further contributed to the delay of hydration. This dual-retardation mechanism offers a stable and controllable approach to regulate CSA hydration kinetics. The findings provide new insights into microstructural engineering of CSA and suggest a promising pathway toward expanding its applicability in low-carbon construction materials.</p>