Dealing with Lignite in Concrete Prior to Staining

Question:
I have a concrete slab that I want to stain, but the surface is loaded with lignite. Is there anything I can do to eliminate or minimize the lignite?

Answer:
Lignite, or “brown coal,” is a soft low-grade coal with a high water content. It is found in many locations around the country and is often blended with hard coals as a cheap energy source. When aggregates (stone and sand) are mined for ready mix operations, lignite can often contaminate the mix. We see lignite contamination more often in the Mid-South region (Arkansas, Tennessee, Missouri, and Oklahoma). In concrete, lignite appears as brown and black specs on the surface.

Since lignite is a hydrocarbon-based material and integrally mixed into the concrete, it is very hard to remove. Lignite will not take a stain and will become more evident after staining, especially if a lighter stain is used. Using a dark brown or black stain will help camouflage the lignite markings, but this is not always feasible. We have had some success using solvents or degreasers along with aggressive scrubbing to remove surface lignite. If lignite contamination is extensive, consider covering the entire slab with a polymer-modified microtopping. The best advice I can give is to research ready mix suppliers in your area and use one that has a good reputation for sourcing and providing clean aggregates.