This paper presents a study of the cementing efficiency (kd) of fly ash with regards to durability. The investigated variables were the class and amount of incorporation of fly ash, the cement type, the water/cement ratio, and the curing age of the mix. Experimental tests that simulate the penetration of chloride ion in concrete (multiregime method) have been conducted to determine the chloride diffusion coefficient in a non-steady-state. The cementing efficiency has been determined by means of a curve that shows the behavior of the diffusion coefficient of chloride in relation to the water/cement ratio. The results show that the cementing efficiency is strongly influenced by the water/cement ratio. Concerning durability, greater efficiency values than those observed in relation to the compressive strength have been found.