Petruchi's thread -- Pool floor stains

[petruchi]

Hi folks! Thanks for the great site.

I look after the pool at a youth camp. We (for the first time in 7 years) have drained the pool for a thorough clean and to change out the pool lights. We had hoped to also clean up some odd stains on the pool floor around the water outlet vents. We have tried pretty well everything, with no effect. The only thing that has worked is scraping the floor with a knife at a very flat angle, which is laborious and painstakingly slow. I've attached some pics of the floor stains here:

http://imgur.com/a/3pI1I

The amazing thing to me is that a solution of muriatic acid has no effect on the stains whatsoever. Zilch. When the acid solution moves off the stain onto cleaner plaster, it begins to bubble and let off steam like you'd expect, and whitens it up nicely.

These stains have been on the pool floor for 6 years, pretty well since the pool had opened. There was a lot of construction in that first year, and the thought is that some fine construction dust had got into the system and stained the floor. Not sure what the pool chemistry was like at the time, but my guess is that it was still being worked out.

Any help? Thanks in advance for reading!