Calcification is a key step in the newly proposed calcification-carbonation process, which involves the recycling of alkali (Na2O). In this study, the mechanism of calcification of red mud with lime (CaO) was elucidated thermodynamically, and the factors influencing the calcification were studied experimentally. The results showed that sodium aluminosilicate hydrate (Na2O center dot Al2O3 center dot xSiO(2)center dot(6-2x)H2O) can be calcified with Ca2+ in red mud slurry, though not so with Ca(OH)(2) and CaO. Furthermore, calcification is mainly a solid-liquid reaction involving the transformation of sodium aluminosilicate hydrate to calcium hydrogarnet. Temperature increased the rate of CaO hydration and favored the calcification reaction. For reaction time equals 2 h, liquid-solid ratio equals 4, and the amount of lime added to dry red mud equals 30 wt%, at temperatures >160 degrees C, the content of Na2O in the calcified slag reached <1%. When the reaction time reaches 90 mins, the content of Na2O in calcified red mud slag became stable. (C) 2017 Elsevier B.V. All rights reserved.