Heat release characteristics of lime and time-dependent rheological behaviors of lime-activated fly ash pastes

The lime-activated fly ash pastes are widely used to prepare lightweight aerated concrete and achieving effectively control on their time-dependent rheological behaviors is the key to obtaining aerated concrete with a uniform pore structure. Considering the digestion of lime releases a large amount of heat, which affect the rheology of pastes, this study first investigated the heat release characteristics of lime to provide a basis for regulating the rheology. The results show that less water content and higher water temperature lead to rapid and more heat release of lime, the hydration activity of lime obtained by calcination at 900–1000 °C is ideal, gypsum and sucrose have a strongly inhibition effect on the heat release of lime. The rheological experiments show that the lime-activated fly ash pastes follow the Bingham rheological model. Generally, the viscosity and yield stress are kept at a certain level before 30–40 min curing. While, less water content and higher lime content promote the viscosity and yield stress to increase and also shorten the thickening time. The highly exothermic effect of lime obtained by calcination at 900 °C results in a rapid increase in the viscosity and yield stress compared to that at 1000 °C. 15% gypsum additions have a significant inhibition on the development of rheology due to the inhibition on the lime hydration. Interestingly, the viscosity and yield stress of the paste with 2% sucrose decrease with time before 40 min curing, and then gradually increase with time. Thus, gypsum and sucrose are effective candidates for regulating the exothermic rate and heat release of lime, and also provide an effective path for the regulation of time-dependent rheological behaviors of the lime-activated fly ash pastes.