Aluminothermy used for the spot welding of large iron structures is based upon the fact that:

Aluminothermy used for the spot welding of large iron structures is based upon two facts (i) As compared to iron, aluminum has greater affinity for oxygen. hence, aluminum acts are reducing agent.(ii) Reaction between iron oxide and aluminum is exothermic. High temperature is involved in the reaction.Aluminum reduces iron oxide to iron. Fe2​O3​+2Al→2Fe+Al2​O3​