Question: When we change from one cement brand to another, we often need to adjust the dosage of our air-entraining admixture. Sometimes a new shipment of cement from the same source also requires dosage changes. Is this normal, or are we doing something wrong?

Answer: It is normal. In a Minnesota study (Ref. 1), researchers prepared mortar samples using six different brands of ASTM C 150 Type I cements, Ottawa sand (20 to 30 mesh), and neutralized vinsol resin air-entraining agent at a dosage of 1 ounce per 100 pounds of cement. The mix-by-weight was 0.5:1:3 for water, cement and aggregate, respectively.Technicians measured the air content using a volumetric meter at a lab temperature of about 70° F. Air contents varied from 5.4% to 10.2% among the six cement brands.Why the variation? Most of it is probably due to differences in alkali content or cement fineness. Cement alkali contents can range from 0.2% to 1.0% (expressed as equivalent Na2O). And at a fixed dosage of air-entraining agent, air content will increase with alkali content. As cement fineness increases, air content decreases at a fixed dosage of air-entraining agent. If you changed from a Type I to a finer-ground Type III cement, you might have to double the dosage rate to maintain the same air content (Ref. 2).It's a good idea to check air content regularly at the plant, but it's especially important when you've changed cement brands or received a new shipment of cement from the same source.

References
  1. Kevin MacDonald, "Effect of cement brand on air-entrainment and setting times," Appendix E in An Evaluation of Concrete Flatwork Durability Problems in Minnesota, ARM of Minnesota, 1998, pp. E-1-E-2.
  2. David A. Whiting and Mohamad A. Nagi, Manual on Control of Air Content in Concrete, Portland Cement Association and National Ready Mixed Concrete Association, 1998, p. 17.