Interaction between sulfate and chloride solution attack of concretes with and without fly ash

In this paper, the two sets of concretes under attack of erosion solution of sulfate and chloride salt were investigated. The one set is the plain concrete without fly ash addition. The other set is the concrete with 20% and 30% of fly ash addition, respectively. The corrosion solution includes three types: 3.5%NaCl, 5% Na₂SO₄, and a composite solution of 3.5%NaCl and 5% Na₂SO₄. In addition, two corrosion regimes were employed in this study: naturally immersion (stored in corrosion solution for long duration), drying-immersion cycles. The damage process of the two sets of concretes was systematically investigated under the above three types of corrosion solutions and two corrosion regimes. The interaction between sulfate and chloride salt was also quantitatively determined. The experimental results shown that a presence of sulfate in the composite solution increased the resistance to chloride ingress into concretes at early exposure period, but the opposition was observed at latter exposure period. For the damage of concretes, a presence of chloride in the composite solution reduces the damage of concrete caused by sulfate. Addition of fly ash may significantly improve the resistance to chloride ingress into concretes and the resistance to sulfate erosion when a suitable amount of fly ash addition and low water-to-binder (W/B) was employed. Studies of the different corrosion regimes indicate that concretes stored in corrosion solution for about 850 d, the changes in relatively dynamic modulus of elastically (RDME) could be described by three stages: linearly increasing period, steady period, and declining period. Whereas for drying-immersion cycles, an accelerated trend could be found. The changes in RDME included an accelerated decreased stage, linearly increased stage, and then a slowly decreased stage, finally accelerating failure stage. In order to elucidate the above experimental results in a microscopic scale, the mechanism was also investigated by the modern microanalysis techniques.