The recycling of basic oxygen furnace (BOF) slag is largely limited by its free lime and free magnesia content leading to highly problematic swelling, preventing civil engineering applications. Moreover the presence of phosphorus is a limited factor for their reuse in iron and steel industrial processes. The results of thermodynamic calculations coupled with cooling laboratory experiments and mineralogical characterisation are presented. The aim of this study was to follow slag crystallization with time and temperature from 1600 °C to ambient temperature using standard tools but also in-situ methods. The predominance of dicalcium silicate (50 wt.%) was highlighted for each type of cooling conditions then calcium ferrites with various proportions. Less total free lime/magnesia is observed for slow cooling conditions (4.5 against 14.9 wt.% in the industrial sample), which is an interesting result for recycling. Experiments are in good agreement with the calculations concerning the nature of the main phases depending on the influence of oxygen. The sequence of crystallisation, the variations in the nature of silicate phases depending on the temperature and the evolution of the phosphorus content are documented. Proportions of low and high phosphorus-bearing calcium silicates are evolving with cooling, which will have to be considered when optimising phosphorus retrieval.