This paper discusses a multidisciplinary approach used to determine the cause of a severe and erratic retardation problem. The problem involved random loads of ready-mixed concrete having extended set times of up to 40 h. Eight samples of a Type I/II portland cement, one Class F fly ash, two cylinders, a core, and a sample of Type D water-reducing admixture were evaluated using chemical, microscopic, and physical analyses, and mortar performance tests.
Six samples of the cement were tested for set time performance using a modified ASTM C 403 method. The set times of control cement-sand-water mixes ranged from 2:31 to 5:12 (h:min). When the cements were combined with only 33 mL/100 kg (0.5 oz/100 lb cement (cwt) of admixture, the difference in set time increased dramatically, 2:34 to 7:16 (h:min).
Analysis of the cement, fly ash, concrete, and admixtures did not indicate the cause of the problem. However, the sum of the individual compounds plus the LOI was unusually low, 97.5 to 98.5%, suggesting that one or more components had not been identified in the cement samples. Further analysis revealed the presence of approximately 1% zinc in all the cements. Zinc compounds are known to be powerful retarders of cement hydration.
An extensive investigation—conducted to determine why the cement samples had a wide range of initial set times and yet had a consistent 1% zinc content—indicated that two interrelated factors were responsible. First, cement samples extracted with dilute solutions of the admixture indicated that more zinc could be leached from the cement with the longest initial set time as compared to the faster setting cement. Second, the two cements had different microstructures, which could have corresponded to a variation in their zinc availability and thus the extent to which the hydration reaction was delayed.
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