This study has the purpose to investigate the action of Magnesium Chloride (MgC12), which is present in the rocks of the salterns pre-salt layers, when it is incorporated to the Portland cement of G class, during the hardening period of the designated paste to the petroleum well cementation.It was evidenced on the biography review a blank of knowledge about the magnesiums influence on the cement hardening.The information of the study seeks to provide subsidies to technology of the oil wells cementation in salt rocks specific of the pre-salt,where there is the presence of magnesium in the composition.The experimental program has consisted on submitting the G cement paste to different ratios of MgCl2, prepared in laboratory, aiming to evaluate the magnesiums influence on the cement hydration evolution and to correlate with properties on the fresh state.