Research on the strength reduction mechanism of Cement Kiln Dust (CKD) – Portland cement systems from macroscale and nanoscale

Cement kiln dust (CKD) is a solid waste produced by the thermal decomposition of materials during the production of cement clinker. Theoretically, CKD can potentially be utilized to replace cement materials, which can reduce the expend for natural resources. However, the detailed mechanism of CKD on the source of strength reduction from macroscale and nanoscale is still unclear. This study focus on revealing the strength reduction mechanism of CKD-Portland cement. The results show that the strength reduction is dominated by the sulfate composition contents, which is not relative with the generation of lower elastic modulus C-S-H. And the higher ratio of S/Si is not conducive to enhancing strength. At the microscale, both the crystalline hydrates contend and the corresponding morphology are depended on the CKD dosages. And at the nanoscale, the generated C-S-H is characterized as a stacking structure. The average elastic modulus of generated C-S-H is 33.0 GPa. The relative contents of VLD, LD, HD and UHD C-S-H of the CKD-Portland samples are 4.7%, 22.0%, 37.9% and 35.2%, respectively. Furthermore, the CKD promoted the reaction rate and accelerated the hydration process. This study provides valuable insights into the application of CKD in cement-based material.