Shrinkage Mitigation Mechanism of Calcined Clinoptilolite Zeolite in High-Strength Cement Mortar

Shrinkage behavior is one of the vital properties of concrete that would possibly induce cracking and structure failure, especially in high-strength cementitious composites. Internal curing has been proven to be an effective solution for shrinkage mitigation. In the present study, natural clinoptilolite zeolite is adopted to evaluate the shrinkage variation of high-strength mortar. Modification by heating with target temperatures of 100°C, 500°C, and 700°C is applied to investigate the physical and mineral change of clinoptilolite zeolite. Hydration, strength, and shrinkage of cementitious composites containing natural or preheated zeolite are conducted. Results show that moderate zeolite dosage (10%–20%) exhibits a nucleus effect in cement hydration, inducing earlier hydration peaks. Calcined zeolite gains considerable strength at later ages, due to the filling effect of the pozzolanic reaction products. Modification of calcination improves the mitigation effect on shrinkage. The increased amount of pores over 35 nm is responsible based on theoretical calculation of equilibrium equation. Such a pore coarsening phenomenon contributes to the enhanced internal curing capability of calcined zeolite, not only benefiting high-strength mortar shrinkage control, but also mitigating the over-time internal relative humidity drop.