Study on hydration kinetics of geopolymer synthesized with recycled concrete and brick powder

Concrete and brick waste accounted for the majority of construction and demolition waste. In recent years, the use of recycled concrete powder (CP) and recycled brick powder (CP) to prepare geopolymer binders gained significant attention. This study investigated the hydration characteristics of geopolymer made from BP and CP using isothermal calorimetry and low-field nuclear magnetic resonance (1H NMR) techniques. The results showed that the incorporation of BP or CP reduced the heat release during geopolymer hydration. The degree of hydration of the geopolymer binder initially decreased and then increased as the addition of BP or CP increased, while the nucleation rate rose with the increasing content of BP or CP. Additionally, the study revealed that the geopolymer reaction process consisted of four distinct stages. The addition of 10% CP increased the rate of gel water production during the acceleration stage of the geopolymer reaction, While the incorporation of 10% BP had a smaller effect on the acceleration stage but prolonged the duration of the deceleration stage. However, when the recycled powder content reached 50%, the four-stage reaction process was no longer evident. The compressive strength test results showed that the compressive strength of the geopolymer increased and then decreased with the increase in BP dosage, with the highest strength observed at a dosage of 10%, reaching 54.1 MPa. In contrast, the compressive strength of the geopolymer doped with CP decreased as the CP replacement increased.