Hydration Characteristics and Mechanical Properties of Cement-Based Materials Modified by Calcined Zeolite and Montmorillonite

Montmorillonite and clinoptilolite zeolite were used as representative materials to prepare calcined clay-cement binary cementitious materials in order to study the effect of calcination treatment on the activation of clay minerals and the activity difference between layered and framed clays in this research. The influence of different calcined clay content (2%, 4%, 6%, 8%, 10%) on the fluidity, compressive strength, microstructure, phase change, and hydration heat of cement-based materials were analyzed. The calcined clay improves the fluidity of cement-based materials as compared with the uncalcined group. The addition of calcined montmorillonite (CMT) improves the development of mechanical strength, and the optimal compressive strength reaches 85 MPa at 28 days with 8% CMT. However, the activity of calcined clinoptilolite zeolite (CZL) is weak with few reaction sites, which slightly reduced the mechanical strength as compared to the blank sample. The addition of CMT changes the microscopic morphology of hydration products such as C-S-H and C-A-H, leading to the formation and transformation of ettringite in the early stage. It promotes the gradual polymerization of Si-O bonds into SiO-Si bonds simultaneously, which accelerates the early hydration process. However, CZL acts mainly as a filling function in the cementitious system. In brief, CMT as an admixture can improve the mechanical properties of cement, but CZL has little effect. This work provides a guideline for the applications of calcined clay in cement, considering the influence of clay type on workability and mechanical strength.