I will now share with you a true story. There was a time in Richmond, Virginia when a company was marketing a product that was locally referred to as "fly-ash" fill. It was actually a mix of flyash, bottom ash, and lime kiln dust. It worked pretty well, especially for wet subgrades and for building pads/earthwork in the winter (it was very forgiving in wet weather). All of these industrial by-products were stockpiling and this "seemed" to be a good way to deliver to market the product that helped local land developers.
Several new power plants came on line that included stack scrubbers, which produce their own industrial by product - scrubber residue. The company began to mix scrubber residue with the fly-ash, bottom-ash, LKD blend and continued to deliver this to market. Seemingly all continued to go well.
Several months later (after construction completed) a poltergiest appeared at these projects. Buildings began to crack, doors failed to close and walls/slabs began to crack. Needless to say, there was a huge problem!
A cottage industry was born! What was happening. . . .
Here is the summary: Flyash mixtures when blended with lime can be very effective. If, however, there is a source of sulfur (i.e., in the case of the scrubber residue), a damaging reaction occurs in the presence of moisture; the development of ettringite. Any of you concrete guys know about ettringite; however, in a fill pad there is a different dynamic. The ettringite crystals in the pore spaces of the fill create a jacking force that is quite impressive! We measured heave of 6 to 9 inches in some slabs.
My advice: If you are using coal combusion by-products confirm for yourself that the product does not contain appreciable quantities of sulfur. I'd limit the total concentration to under 0.5 percent of the total dry mass. However, to be safe take your odometer and do a swell test and run it for 30 days (i.e., if there is any sulfur in the overall product).
These were HUGE lawsuits!
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¡papá gordo ain’t no madre flaca!