<p>Additives, comprising small quantities of organic and inorganic molecules, are crucial for controlling the crystallisation of various (bio)materials; however, their mechanisms remain poorly understood. By integrating in situ high-energy X-ray scattering with potentiometric titrations, we examine the impact of industrially relevant additives on the nucleation pathways of portlandite and gypsum. While both minerals undergo multistep nucleation, portlandite transitions gradually from a disordered to an ordered phase, whereas gypsum exhibits an abrupt transition. These distinct non-classical pathways and their pH conditions correlate with the mineral-specific effects of the additives. Additive effects extend beyond the classical models, such as cation-binding, exerting influence primarily during the prenucleation stage. Moreover, additives demonstrate a dual role by simultaneously delaying and accelerating different stages of nucleation, highlighting their multifaceted impact on the crystallisation process. These findings offer insights for designing tailored additives to optimise industrial crystallisation processes and advance the understanding of biomineralisation mechanisms.</p>