Does synthesized C-S-H seed promote nucleation in alkali activated fly ash-slag geopolymer binder?

Abstract

Understanding the effects of nucleation seeds on reaction kinetics and microstructures of alkali activated binders (AABs) is important since it will help us control both early-age and long-term properties. For example, in the calcium-containing AABs, calcium silicate hydrate/calcium aluminosilicate hydrate (C-S-H/C-A-S-H) seeds have been proposed by others to act as nucleation sites to promote formation of geopolymer gel. In this study, we examined the nucleation effects by adding synthesized C-S-H and C-A-S-H seeds into class F fly ash-slag mixes activated both by potassium hydroxide and by potassium silicate solutions. From both calorimetry and ultrasonic wave reflection tests, we found seeding to accelerate reaction of the hydroxide activated binder but decelerate that of the silicate activated binder. Based on further chemical treatments and Fourier-transform infrared spectroscopic characterization, we found the acceleration in the hydroxide binder was due to enhanced formation of C-S-H/C-A-S-H, while the deceleration in the silicate binder was due to decreased formation of K-A-S-H gel despite the enhanced formation of C-S-H/C-A-S-H. Additionally, in both types of binders, we found dissolution of fly ash precursor was enhanced. Evidence in this study suggests that C-S-H seeds act as nucleation sites for further precipitation of C-S-H/C-A-S-H but not for formation of geopolymer gel.