Two series of sinters have been made in a sintering pilot plant. The starting material consisted of two industrial ore mixtures. Sinter structure was studied using electron microscopy, with special attention being paid to the SFCA (silicoferrite of calcium and aluminium) phases, which confer to the sinter an optimum behaviour in the blast furnace. It was confirmed that these phases become more abundant as the sinter basicity increases. The magnetic susceptibility and mechanical strength of the sinters were determined. It was confirmed that a correlation exists between these parameters and basicity, which makes it possible to select from each series of sinters the optimum basicity sinter corresponding to the start of stabilisation of magnetic susceptibility and the appearance of free lime in their structure.