Influence of temperature on rheological properties during early-stage geopolymerization

Metakaolin geopolymers have gained much interest as a large-scale, 3D printable material. It is well known that increasing temperature can expedite the geopolymerization reaction, but little is known about how the temperature variability of the printing environment can influence the rheology of fresh geopolymer pastes. In this study, the influence of temperature on the viscosity of potassium geopolymer pastes was investigated under constant shearing at rates of 25, 50, or 100 s(-1), yield stress measurements, and oscillatory motion. The temperature range examined was 5 degrees C-55 degrees C, in systematic 5 degrees C increments. It was found that temperatures above 30 degrees C resulted in lower starting viscosities compared to colder temperatures, but eventually exhibited an exponential increase in viscosity as the geopolymerization chemical reaction became dominate. In addition, a higher shear rate delayed, but did not stop, the exponential increase in viscosity from occurring. Yield stress values also reflected an upward trend with increasing temperatures after a 30-min temperature soak. Lastly, oscillatory measurements indicated that viable printing times for 50 degrees C or above were as little as 50-60 min total and were compared to Vicat needle testing. Overall, the influence of temperature on rheological properties could be used to manipulate the geopolymer viscosity for optimum printing conditions.