Increasing early age reactivity of cement replacements is a barrier to reducing the embodied carbon of blended Portland cements. Mechano-chemical activation is an emerging alternative to conventional thermal activation for clays, which can accelerate early age reactivity. Knowledge gaps on the structure and reactivity of mechano-chemically activated kaolinitic clays include the influence of Fe-bearing phases and the mineralogical characteristics of kaolinites from different sources. This study evaluated the effectiveness of mechano-chemical vs. thermal activation for an Fe-rich clay containing disordered kaolinite and 24 wt% goethite, and a low-Fe clay containing highly ordered kaolinite. In the Fe-rich clay, mechano-chemical activation simultaneously caused dehydroxylation of kaolinite to form meta-kaolinite, and dehydration of goethite to form hematite. Agglomerates of intermixed meta-kaolinite and goethite/hematite nanoparticles were shown to have similar Al and Si environments after thermal or mechano-chemical activation (as determined by STEM-EDX, ²⁷Al and ²⁹Si MAS nuclear magnetic resonance and electron energy loss spectroscopy). Mechano-chemical activation enhanced early age (<12 hours) reactivity for both clays. Evaluating early age reactivity by unit mass of anhydrous meta-kaolinite explains how surface-adsorbed moisture results in underperformance of mechano-chemical activation at later ageing times. External surface area alone does not predict reactivity acceleration well - edge : basal surface area of meta-kaolinite is proposed as a more relevant factor that governs early age performance of mechano-chemically activated clays. The structure-property-performance relations of mechano-chemically activated meta-kaolinites are explained through interactions of kaolinites' intrinsic mineralogical characteristics (i.e. initial particle size, aspect ratio, structural order) and extrinsic processing effects (i.e. intensive milling on structural order and physical characteristics).