Q: In a recent CC article (“Two Percent Cement” by Joe Nasvik, November 2011), you discussed a unique concrete mix that had only 2% portland cement. Did the researchers address the issue of passivation of the reinforcement to prevent corrosion? With such a low amount of portland cement, how can an engineer be assured that the pH will be high enough, for say 50 years, to passivate and thus protect steel reinforcement from corrosion?
A: We are told by Kevin MacDonald at Cemstone that developed the mix that the pH of the pore solution is about 12.5. This level will readily passivate the steel. The critical Cl to OH ratio is present in the pore solution, and examinations have shown abundant Ca(OH)2 throughout the sample. The fly ash used does contain some dicalcium and tricalcium silicate and so is hydraulic in its own right (cementitious), although the researchers only relied on the fly ash for its pozzolanic activity. We’ll still need the required concrete cover over the steel of 3 inches for concrete in contact with earth, 2 inches for #6 and larger rebar, or 1 1/2 inches for smaller bars and welded-wire reinforcement.