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New admixtures, supplementary materials lead to one problem—incompatibility Is your cement to blame? What should you recommend to your customers? - Rachel J. Detwiler and Ella Shkolnik, Contributing Authors

You are the concrete supplier on a large paving project. The concrete exhibits considerable delays in setting, poor strength development and ultimately severe cracking of the pavement. Why is this happening? You are a tech services representative for a cement company. You receive reports from the field of intermittent problems with early stiffening when your cement is used with a Class C fly ash and a water reducer. Is your cement to blame? What should you recommend to your customers? Your paving operations were going along fine, and then you had to switch cements because of a shortage. Now you can’t place the concrete before it starts to stiffen. Is there something wrong with the new cement? You specify materials carefully and make sure they all conform to the relevant AASHTO standards, yet somehow a particular combination results in setting problems—too slow or too fast, consistently or intermittently. Maybe you’ve used this cement successfully for the past three years, and now that you’ve started using Class C fly ash, you’re having setting problems. Is it because of the fly ash? Maybe not. You may be experiencing incompatibility. Incompatibility is not a new problem, but we seem to be seeing more of it now than in the past. Although we don’t admit it, we tend to treat concrete as an “abuser-friendly” material. We know what ACI and other authorities recommend as good practice, but competitive bidding and tight schedules can make compliance difficult. There are new admixtures on the market, and we use more of them than before—in combinations of two, three or more. We also make greater use of supplementary cementitious materials: fly ash, slag and sometimes silica fume. These materials are all beneficial when used appropriately, but the more ingredients in the concrete, the greater the likelihood of an unexpected interaction between them.

Sulfate solutions In essence, concrete sets because of the reaction of the C3A (tricalcium aluminate) in the cement with water. Gypsum and other sulfate-bearing components are added to the cement during grinding to control the reaction and prevent flash set. Setting, in effect, is a race between the sulfates and the aluminates (C3A in the cement and sometimes in Class C fly ash). If there is enough sulfate in solution at the right time, the hydration of the C3A is controlled; if not, the concrete experiences flash set. What affects the performance of the sulfates? In various investigations of setting problems in the field, we have seen the following: •

Not enough sulfates in the cement, sometimes because of limitations imposed by the state DOT;