Q.: We have some concrete that has developed rust stains over a period of about a year. It appears to be caused by the aggregate. Are there any solutions to this besides changing aggregate sources?
A.: The problem may relate to the presence of iron in either the coarse or fine aggregates. If the staining is derived from the aggregates, either coarse or fine, it is most likely coming from the decomposition of iron sulfide pyrite or marcasite. Even small amounts can cause serious staining. Interestingly, iron sulfide in the body of the concrete usually doesn't decompose. Decomposition occurs near the surface where the mineral is exposed to air and water. A petrographer could examine a sample of the stained concrete and tell you whether the aggregate is likely to be the source of staining. Petrography performed on the aggregate (ASTM C 295) can tell you how much of the aggregate contains minerals likely to cause stains.If this is the case, alternative aggregates should be used. The other alternative is to use a concrete sealer on the exposed surfaces. Any of the major admixture companies can provide assistance in the use of these products.
Also, be sure to check on the fertilizer that the people use. Some fertilizer includes iron, and when the granules get wet on the concrete they leave rust spots. We have heard of reports of this in Las Vegas.
Aggregate Research Industries
Aggregate Research Industries web site (www.aggregateresearch.com) includes topic-specific forums where forum members can pose questions and respond, creating an interactive discussion group. The questions and answers in this Problem Clinic were based on ARI concrete construction forum postings. Forum members who responded to these questions include such industry experts as Bob Neal, Laura Powers, Bill Abbott, and Teck Chua.