Sulphide, sulphate and sulphuric acid corrosion of concrete in laboratory tests

Portland cement (PC) concrete is generally a highly durable structural material. Nevertheless, certain chemical actions and aggressive environments in a livestock building can cause deterioration and total collapses of structures have occurred long before they have reached their design life. The sulphide and sulphate resistance of three replicates of eight different reinforced concrete mixes were investigated in a laboratory study in which one half of the 48 specimens were half submerged in a sodium sulphate solution (20,000 ppm SO42-) and also exposed to hydrogen sulphide gas (1,000 ppm H2S). The other half of the 48 specimens was subjected to hydrogen sulphide gas only. The mixes included PC concrete with W/CM ratios of 0.4 and 0.5 and six mixes with cement replacements of slag, fly ash or silica fume, all with water/cementitious material (W/CM) ratio of 0.4. After 23 cycles of testing over about 36 months, the electrochemical potential results and visual inspection of the reinforcing bars indicate that the PC concrete with 0.5 W/CM ratio was the least resistant against steel corrosion. Corrosion of the concrete was more critical than that of the steel. All treatments containing silica fume performed much better than PC40. Treatments that contained fly ash performed worse than plain PC concrete with the same W/CM ratio. Sulphate resistant cement concrete was more resistant than Type 10 Portland cement concrete, in both sets of tests. In general the samples that were exposed to hydrogen sulphide and sulphate corroded at a higher rate then those exposed to the H2S gas only. In subsequent tests 6 of the 8 mixes were exposed to 7% sulphuric acid for about one year. Preliminary results indicate the greatest mass loss for the concrete with 0.5 W/CM ratio and very similar amount of loss for the 5 mixes with W/CM ratio of 0.4. The least amount of mass loss was experienced by the mix with sulphate resistant cement.