Rehydration Activity of High-Temperature Calcined Recycled Sand Autoclaved Aerated Concrete

Autoclaved aerated concrete is an excellent thermal insulation wall material, but with a large amount of waste. This paper describes the high-temperature activation and rehydration activity of waste cement–lime–sand autoclaved aerated concrete (SAAC) and discusses the high-temperature phase transition of SAAC. SAAC calcined at 750 °C was confirmed to be a metastable and amorphous state, which could hydrolyze Ca2+ ions with reactivity in water. The conductivity curve demonstrates that the concentration of ions in the suspension decreases rapidly at 150–250 min, and the hydration reaction turns dominant at this time. The hydration heat curve also displays a hydration exothermic peak at 2.5 h. In addition, the conductivity measurement of suspension and the exothermic measurement of hydration reaction proves that SAAC calcined at 750 °C has a hydration activity and can rehydrate with SiO2 in the system. The rehydration activity was verified by replacing 30% cement in the standard test block with calcined SAAC because the calcined SAAC at 750 °C has high hydration activity, and its activity index reached 89.58%. Fly ash is a commonly used cement admixture at present. Hence, the SAAC calcined at 750 °C and the fly ash were used to replace 30% of the cement in the cement test block, respectively. The results of this comparative experiment vividly showed that the reaction activity of SAAC calcined at 750 °C was higher than that of fly ash. Therefore, according to this research, SAAC has activity after calcination at 750 °C and can be hydrated again.