AbstractA comparison is made of the properties of magnesium oxides prepared from basic magnesium carbonates and Indian and Greek rock magnesites. the density increased with burning temperature and the reactivity decreased. the effect of this on the properties of oxychloride cements is shown.Iodine number is shown to be a good index of reactivity and specific surface (air permeability) to indicate the amount of chloride solution needed to make a workable mix with an oxide.