To immobilize heavy metals and metalloids contained in some wastes, solidification / stabilization (S / S) process has been tested and used to reduce the leaching of contaminants before their disposal in the landfill. Cement is the most adaptable binder currently available for this immobilization. A growing number of works have dealt with this issue, mainly using ordinary Portland cement (OPC), some of its phases or OPC modified aluminum aluminate, calcium sulfate and / or additives. In this study we will immobilize two different arsenic salts (sodium arsenate heptahydrate (Na2HAsO4,7H2O) and Sodium arsenite (NaAsO2)), include in three different cement matrices: ordinary Portland cement (OPC), high alumina cement (HAC) and low alumina cement rich in iron oxide content (LAC). Currently it has been reported that CACs show potential advantages when used to encapsulate certain toxic and radioactive wastes, due to their high initial mechanical resistance, avoiding the chemical attack and preventing the abrasion. In previous research the capacity of the CAC inclusion for toxic metals was excellent, in this way, these mortars perfectly encapsulate the toxic metals, which makes the use of HAC and LAC suitable for waste management, as has been demonstrated in the leaching tests. The aim of this work is finding the best conditions to encapsulate the As trough different tests as the leaching test and isothermal calorimetry.