The cement industry is among the leading reasons for CO2 emissions. Therefore, it is critical to increase the incorporation of “Supplementary Cementitious Materials (SCMs)” which help partially substitute clinker. However, conventional SCMs are not abundant, and their availability is increasingly limited including fly ash and slag. Under these circumstances calcined clays were developed. Also, their combination with limestone and gypsum in “limestone calcined clay cement LC3” systems present one of the viable solutions owing to their availability and pozzolanic reactivity when thermally activated. This review focuses on the examination of the quantitative link between the calcined clay grade, the reactivity assessing methods and the mechanical strength development of LC3 systems as a subsequent result. The study discusses the efficacy and enhancement of the “Rapid, Relevant and Reliable (R3)” reactivity assessment tests. Furthermore, a comparative study of mechanical results data across different clay grades shows a significant and critical grade paradox. Even though LC3-50 mixes require a minimum calcined kaolinite content of around 40% to achieve the Ordinary Portland Cement (OPC) mechanical performance, there is only a marginal benefit exceeding 60–65%. This ceiling is linked to microstructural inhibition because the rapid C-A-S-H development in high-grade clays prematurely diminishes clinker hydration kinetics due to pore refinement. Consequently, the most effective and optimal approach for maximizing both mechanical strength and material availability is strategically utilizing abundant medium-grade kaolinitic clays.