Properties and hydration mechanism of cement pastes in presence of nano-ZnO

Nano-ZnO could delay the setting and the hardening of cement pastes but promote the later cement hydration, which made it an ideal inorganic retarder in cement based materials. In this research, properties and structures of fresh and hardened cement pastes containing nano-ZnO (CZ) were tested. Early hydration of CZ was greatly delayed and its early mechanical performances were decreased. However, the 28 days compressive strength of CZ with 0.1 wt% nano-ZnO was slightly higher than that of blank paste. Nano-ZnO was rapidly dissolved in alkaline environment to form the amorphous Zn(OH)(2) in the early hydration stage, the deposition of Zn(OH)(2) layer on the surface of clinker particles would block the early hydration. With the processing of cement hydration, as-formed Zn(OH)(2) would be further reacted with Ca(OH)(2) to generate intermediate Ca(Zn(OH)3)(2).2H(2)O, which broke the Zn(OH)(2) coatings and promoted the cement hydration in this period. In addition, the coarsening of capillary pores ranged from 5 nm to 50 nm in hardened CZ matrix was observed, which was presumed to be responsible for the autogenous shrinkage reduction of CZ pastes. (C) 2021 Elsevier Ltd. All rights reserved.