The incorporation of waste and industrial by-products into concrete has been one of the alternatives to reduce the world consumption of cement and, consequently, to make it a more eco-friendly material. Therefore, and knowing that durability plays an important role in the life cycle of the construction materials, this work investigates the durability of cementitious materials with incorporation of fly ash (FA) when submitted to environments containing simultaneously the aggressive agents Cl^- and CO₂.

The increase in the amount of aluminates provided by cement substitution by FA is one of the factors that leads to a decrease in the free chlorides content since the aluminates react chemically with the chlorides binding them. On the other hand, Ca(OH)₂ present in the matrix from cement hydration is consumed due to the pozzolanic reactions, facilitating the advance of the carbonation front. In this context, it is important to know the behaviour of cementitious materials containing FA in environments subjected simultaneously to chlorides and carbonation.

Mortar specimens containing 0, 20, 40 and 60 %wt of Portland cement CEM I 42.5R replaced by FA were moulded. After 90 days of curing, half of the specimens were introduced into the carbonation chamber (20 ºC, 55 % RH and 4 % CO₂) where they remained for 15, 60 and 90 days. The other half was protected with plastic film during the same period. After, the specimens were subjected to the chloride diffusion test by migration.

According to the obtained results, there was an increase in the chloride diffusion coefficient of the mortars submitted to the combined action for all percentages of FA incorporation. However, this increase was most evident for the higher percentages of cement replaced by FA. This fact may be related to the increase in the large capillary pores caused by carbonation and to the fact that the carbonated concrete has its chlorides binding capacity reduced.