Understanding the role of silicate concentration on the early-age reaction kinetics of a calcium containing geopolymeric binder

Understanding the effects of silicate concentration of activator solution is critical for developing cost effective geopolymeric binders, and controlling their setting and hardening behavior. However, in calcium containing geopolymers, effects of silicate concentration of activator solution on the amount and nature of reaction products at different ages are still unclear. In this study, we examined class F fly ash and slag binders activated by both silicate and hydroxide solutions. Faster hardening from ultrasonic wave reflection and faster reaction from calorimetry were seen in the binder activated by silicates than that by hydroxides. Subtraction of infrared spectra, aided by selective chemical treatments, indicated the main reaction peak in calorimetry curves correlates with formation of calcium silicate hydrate/calcium aluminosilicate hydrate (C-S-H/C-A-S-H) of the same Ca/Si ratio in the hydroxide activated binder but formation of both C-S-H gel and potassium aluminosilicate hydrate (K-A-S-H) gel in the silicate activated binder. In additional tests, the main reaction peak of the silicate activated binder was delayed as silicates in the activator solution became less polymerized, further confirming contribution of K-A-S-H formation towards the main heat peak in calorimetry curve. Scanning electron micrographs showed products grew from surface into pore space in the hydroxide activated binder but within interstitial space in the silicate activated binder.