Abstract: The compressive strength and porosity of hardened charge sample in a Hermetic Calcium Carbide Furnace（HCCF） was tested，and the formation reactions and mechanism were analyzed by XRD，thermodynamic modeling software， SEM，and EDS methods.As the results showed，the main components of hardened charge were MgO and fixed C，the mass fractions were 70.1% and 20.4% respectively；and the porosity and compressive strength of the sample were 8.55% and 315.60 N at room temperature.During the formation of hardened charge，the reaction between Mg steam and O₂ gas reacted preferentially， and that between Mg steam and CO gas played a dominating role.The contents of MgO derived from the two reactions were 3.2% and 96.8%（mass fraction，both） respectively compared to MgO in total.The formation of hardened charge included two main processes，one was the agglomeration of hardened charge powder，and another was the sintering of agglomeration groups. It could also be found that temperature was the key factor for hardened charge formation.

Key words: HCCF, the hardened charge, abnormal conditions, agglomeration, sintering