pH neutralizing media on high efficiency boiler

Does anyone in here know where a person could buy PURE calcium carbonate to use as a pH neutralizing media for the condensate from a high efficiency boiler?

Until recently I've been using limestone quarried right here in Manitoba that's approximately 90 percent calcium carbonate. The problem is that the remaining 10 percent that doesn't dissolve in the condensate forms a kinda "fuzzy" film on the surface of the limestones that holds a zone of stagnant water around each stone, making the neutralizing filter ineffective.

So, I have to take the limestone out of the filter and clean the stones to remove that fuzzy undissolved stuff each month.

If I could find a source of pure calcium carbonate in 1/2 inch "clean" size, then I wouldn't have to do that chore every month cuz everything would dissolve. I've been told about crushed coral from ocean reefs, but they crush it to such a small size that I'm concerned my filter would just back up and I'd have condensate spilling out of my boilers onto my boiler room floor.