INFLUENCE OF CARBONATION ON THE CHLORIDE ION DIFFUSION COEFFICIENT IN FLY ASH CONCRETE

The incorporation of fly ash (FA) in cementitious matrices has been frequently used in order to make the matrix more resistant to the action of chlorides. On the other hand, it is known that Ca(OH)(2) existing in the matrix is consumed by the pozzolanic reactions, which makes easier the carbonation front advance. Given that the chloride ingress and carbonation are the two main causes of degradation in reinforced concrete, we speculate about the behaviour of FA concrete when the structure is submitted simultaneously to chlorides and carbonation. This work studied the influence of carbonation on the chloride migration coefficient in FA concrete. For this, specimens with 0% and 40% replacement of cement CEM I 42.5R by FA were casted with water/binder 0.55 and 0.50 respectively. After 90 days of curing period, half of samples were subjected to accelerated carbonation (20 degrees C,55% RH and 4% CO2) for 1, 2 and 3 months. The other half was protected with plastic film during the same period. Non-steady-state migration tests, according to LNEC E463 (Portuguese specification), were performed with specimens subjected to both experimental conditions. The results show that, for these conditions, the carbonation has a direct influence on chloride diffusion coefficient, increasing it. For FA concrete samples this effect is more evident. In these cases, the carbonated samples studied showed a diffusion coefficient up to two times higher than noncarbonated ones. The increase in large capillary pores, caused by carbonation, can be responsible for the increase of chloride penetration into concrete subjected to combined action.