Mechanisms of CLDH seeding on hydration kinetics of slag-based geopolymer: Towards aluminosilicate cement phase engineering

Abstract

Geopolymer and related chemically activated materials are more sustainable aluminosilicate cementitious materials that have less natural materials consumption and CO2 emissions. An intrinsic challenge comes from the lack of sufficient crystalline phases in the binder, which leads to the suffering of volume instability and other issues for this type of cements. This study proposes phase engineering concept, for the first time, and investigates the stargate of using active crystal seeds. Calcined layered double hydroxides (CLDH) was used as a type of seed to understand the impact on hydration kinetics and microstructure of slag-based geopolymer. Through various characterization techniques, it is revealed that during the hydration process of slag, CLDH adsorbs OH− ions and other anions from the solution, leading to a rapid recrystallization and formation of hydrotalcite (Ht) phase. The generated Ht phase serves as nucleation sites, accelerating the formation of C-A-S-H amorphous gel and more Ht phase. Ht nanocrystals consistently exists in the binder, at least in the studied period, together with C-A-S-H nanocrystals and more ordered gel phases. Low modulus of silicate activator is found to enhance the nucleation effect of Ht. This research provides valuable insights into the interactions and mechanisms governing the hydration kinetics and phase evolution in geopolymer, and proves the effectiveness of phase engineering via seeding.