This paper is part of a research project where the performance as addition to the Portland cement of a residue from an oil company is assessed in order to establish viability of building materials. This waste –called Fluid Catalytic Cracking residue (fcc) – is derived from fluidized-bed, catalytic cracking units and its composition is primarily determined by alumina and silica. The hydration process of cement pastes adding fcc is analyzed but results are compared against metakaolin addition. Ordinary Cement Portland (opc) pastes were elaborated using 10 and 20 percent of fcc and mk as a weight cement replacement. The pozzolanic activity of additions and hydration products type were determined by X-ray diffraction (xrd) and thermo gravimetric analysis (tg/ dtg). The main phases in the fcc-blended cement system were calcium silicate hydrates (csh gel), calcium aluminate hydrates (cah) and calcium aluminosilicate hydrates (cash), akin to products based on metakaolin blended pastes.