Preparation and formation mechanism of pure phase Ca2Al-layered double hydroxides nanosheets synthesized by a T-type microchannel reactor: Application as hardening accelerator for mortar

CaAl-LDH has its special application such as crystal seeds in mortar to improve the early strength. But it is hard to prepare pure phase CaAl-LDH by traditional coprecipitation method and to avoid the byproducts like aluminum hydroxide and calcium carbonate. In this paper, pure phase Ca2Al-LDH nanosheets could be obtained by controlling Rb/s (mole ratio between base and salt solution) of 1.5 using a T-type microchannel reactor. The so-obtained Ca2Al-LDH nanosheets had a two-dimensional structure with lateral size of 30-100 nm and thickness of 2-3 nm. By correlating characterizations of pH titration with zeta potential, detailed information was obtained during the formation process of LDH prepared by the microchannel reactor and traditional coprecipitation, respectively. Tracking zeta potential of LDH prepared by microchannel reactor revealed that Ca2Al-LDH nanosheets were formed through one-step reaction between salts and base solutions in microspace. While for the case of traditional coprecipitation method, results revealed LDH formed through three stages: Al(OH)3 formation, mixing of trace Ca2+ into Al(OH)3, and their transformation into LDH. As a hardening accelerator for mortar, a dosage of only 0.35 wt % Ca2Al-LDH nanosheets prepared using microchannel reactor exhibited greatly enhanced performances in both the early tensile strength and compressive strength of the mortar, which increased by 62% and 36%, respectively.