Q: We use a 10-mm brown river aggregate in decorative exposed aggregate finishes. From time to time, what appear to be rust stains become visible (usually after some time) from the coarser angular stone that is found in minor quantities in the aggregate. Is there a way of halting and cleaning up this problem? We assume that the stains come from what we understand to be ironstone.

A.: In many cases the staining is from pyrite in the aggregate. You can remove the stains with a dilute solution of iron-stain remover (or muriatic acid, although it's not as controllable as the proprietary solutions). But to prevent the staining from reoccurring you have to remove the offending aggregate particle. Drill the single aggregate particle out of the concrete using a masonry bit and then epoxy or cement a new aggregate particle into place.

Ironstone (which is ochre rather than pyrite) is very common in some areas (such as Northern Alberta). Limits of 0.8% to 1.0% by mass in the coarse aggregate are used to limit popouts, but they are uneconomical to completely avoid. The local ready-mixed concrete industry in Alberta is trying to develop a tolerance standard, but this will vary from location to location. One popout per driveway panel (about 10x10 feet) has been suggested. A more subjective approach is: "If you can count them quickly, they aren't a problem."

As far as identifying the problem aggregate, a petrographic analysis would be needed. This technique (especially using the Petrographic Number developed by the Ministry of Transportation, Ontario) provides a lot of useful information at a reasonable cost. You can also get a test done using scanning electron microscopy. (One lab that we've been told can perform such a test with only a small sample is DCM Science Labs, Wheat Ridge, Colo.)

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 question and answer in this Problem Clinic entitled "Rust in exposed aggregate" was based on ARI concrete construction forum postings.