Without any doubt, the primary cementing of a gas / oil well is the most critical stage in its life. The cement will make the isolation between zones and will avoid any kind of communication, it will also protect the casing from external fluids or pressures, and it will bring the necessary structural support. When one design is made, a lot of disciplines (chemistry, geology, physics, mechanics, electric, etc) will participate to define the final design. Anyhow, in the completion / production phases of the well, the cement is subjected to higher loads or higher stresses that were designed for the goals above mentioned. For example, when a well is perforated, it is subjected to high dynamic loads and high variations of temperature. Another example is when a hydraulic fracture is performed or when it is converted into a multi-lateral well. A lot of effort was made to improve the techniques in the completion process but not enough to study the way to give the cement column better mechanical properties, and let it improve its strength throughout the well’s life. On the other hand, the criterion to analyze the stability of the cement is through the compression strength that changes with time and composition. The industry accepts that the cement has a very limited capacity to support tensile load. In addition, there is little knowledge about the behavior of a crack under a tensile load and how it behaves as a function of time. In this paper, the behavior of two cementing slurries with different compositions through the use of Linear Elastic Fracture Mechanics (LEFM) [1], the fracture toughness and their variation with time were analyzed. The variation in the mechanical properties was focused in two slurries with different compositions. The difference was the addition of a styrene-buta-diene latex in the cement slurry. The fracture toughness variation through setting time was studied; having been observed a maximum toughness value between 7 and 21 days. All the slurries were mixed under API- Spec10 [2] requirement and all the tests were performed taking into account E 1820-96 ASTM standard [3].