This paper presents a study of a stabilization/solidification (S/S) process of a tannery waste containing chromium. In this process a mixture of cement, water and the waste is solidified during hydration of the former. Stabilization occurs by the reaction of the chromium components of the waste with the calcium hydroxide formed during the process. A rigid block is obtained with low hazardous of contamination of soil and groundwater after its disposal into industrial landfill. Mixtures were prepared adding dried tannery waste in 10, 15 and 20% of the Portland cement mass. Sodium bentonite and organoclays in 5 % of cement mass were used as additives. Solidified specimens were analyzed after 28 days of hydration time by solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy and unconfined compressive strength analysis. The increase of tannery waste content and the presence of the additives reduce silica polymerization as well as the mechanical strength of the solidified bodies.