Controlling phase transfer of recycled coal-series kaolin waste used as superior admixture

• The relationship between the phase transfer of CSK and the performance is revealed. • The phase contents of active and inactive components can controllable regulation. • Optimal mechanical performance of cement is achieved by tailoring reaction kinetics. • The mechanism for maintaining performance is also revealed. Valorizing industrial solid wastes into high-performance construction materials is essential for achieving a circular economy. Coal-series kaolin (CSK), a widespread aluminosilicate by-product of coal mining, shows strong potential as a supplementary cementitious material. However, its effectiveness depends critically on its thermal transformation, which remains challenging to control precisely. In this study, we establish a direct relationship between the calcination kinetics of CSK, its phase evolution, and the resulting mechanical performance of cement composites. By regulating the thermal decomposition process, we selectively promote the formation of highly reactive metakaolin while suppressing the crystallization of inert mullite. This kinetic-driven strategy yields an optimal calcination temperature of 800 °C, at which CSK-blended cement retains 94.8 % of the mechanical strength of the reference sample. The findings provide a kinetic framework for utilization of CSK, converting problematic waste into a value-added resource, and paving the way for more sustainable construction materials.