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The immobilization of wastes in cement suggests that cement compositions can be tailored in terms of pH, Eh and internal chemistry so as to maximize immobilization potential. This work presents some studies concerning the micro structure of calcium aluminate cement (CAC), and silica fume-calcium aluminate cement (SF-CAC) systems, and their resistance to the leaching by natural waters, with a view to assess them for possible uses, such us immobilization of toxic or radioactive wastes. The use of mineral additives such as silica fume, brings to the formation of aluminosilicate phases such as hydrogarnet Ca3Al2(SiO4)3-x(OH) 2x solid solution and gehlenite Ca2Al2SiO7.8H2O, which avoid the harmful so-called "conversion reaction" in CAC. Concretes based on calcium aluminate cement and calcium aluminate cement-silica fume, were submitted to a column leaching test based on water percolation, with a length of 2 years. Fourier-transform infrared spectroscopy (FTIR), and mercury intrusion porosimetry (MIP), were used to examine the bulk properties of the leached concretes. Scanning electron microscopy with energy dispersive X-rays analysis (SEM/EDX) was used to determine the composition of the pastes and morphology of the hydrates. © 2007 Elsevier B.V. All rights reserved.