Q. We're having a problem with lignite (coal) in the sand we use to make concrete. There's no cost-effective way to remove it, and it floats to the surface in concrete slabs. Water causes it to swell and form surface blisters after the concrete cures. The effect is only cosmetic, but owners complain and we'd like to solve the problem for our customers. Is there an admixture that would keep the coal in suspension and prevent it from floating to the slab surface?
A. Anti-washout admixtures (AWAs) might solve your problem. They're water-soluble polymers that thicken the cement paste by physically binding the mixing water in concrete. Most consist of microbial polysaccharides, such as welan gums, or polysaccharide derivatives, such as hydroxypropyl methyl cellulose or hydroethyl cellulose. AWAs decrease concrete slump, so you might need to add a high-range water reducer (HRWR) to counteract that effect without reducing strength or durability. Since the cost of both an AWA and HRWR may make this solution too expensive, an alternative approach is to use concrete with a maximum slump of 4 inches. Fewer lignite particles will float to the surface in lower-slump concretes.