Salt Water pools and Sandstone/Flagstone

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red-beard

red-beard

Gold Supporter
May 27, 2019
1,624
Houston, TX
Pool Size
25000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite Pro (T-15)
I converted my pool 2 years ago to a Saltwater pool. I keep the salt between 2800-3000 ppm. I keep the CSI slightly negative, and I run about 30-50 ppm Borates. I am not experiencing any more (or less) sandstone/flagstone deterioration than before converting. My "flagstone" has been disintegrating since I put it in. And the first 13 years my pool was "conventional" and not TFP!

What is the real scoop on this? I see others in my neighborhood complaining. And I see some websites claiming saltwater damages sandstone and marble tile. They claim it is from the salt being absorbed by the porous stone. I understand it getting in, but does this break the bonds in the sandstone? If this is real, can we get down into the chemistry on this?
 
It's the water destroying the sandstone, not the salt. Period! Sandstone is not always the right material for around a pool. It's just layers of compressed sand, and water is instrumental in "de-layering" it. That's all that is going on. Like yours, my beautiful sandstone was coming apart long before I converted to SWG. Some sandstone is better than others for a pool. It all depends on where it was "harvested."

The other consideration: all pools are salt water pools! (Eventually.) Chlorine, acid and humans all leave salt behind as a byproduct, and that salt builds up over time. Even if salt was the problem with the stone (it's not!), you'd have the problem whether you have an SWG or not. Converting to a saltwater/SWG pool is safe for your pool and its equipment (assuming you only let the salt level get so high).

There are products that can reduce the de-layering. That has worked in my pool. It's not cheap. And it's not sealer (which will not stop the de-layering). It penetrates the layers and glues them together (that is wildly paraphrasing, but that's the layman's description).
 
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Water. I too have sandstone coping. It flakes, it dissolves / melts into sand in the pool. It is cheap "stone" that cheap builders like to use, because it's cheap.
Get a good mason to advise on products to treat it to prevent it from degrading.
 
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Dirk said:
Sandstone is not the right material for around a pool.
Click to expand...

My 3" thick flagstone looks the same now as it did the day it was installed. There have been some minor areas of bedding layers that have come off, but nothing significant. Flagstone can be used for coping .... but it's the type of flagstone that matters.

Texas flagstone is known to be of inferior quality compared to stone that comes from other geographic locations (I know, for all you big-hat Texans it's hard to hear that you're inferior at something ... :p ). Arizona flagstone is very high quality and used in and around pools all the time here. So what matters is where the stone comes from.

Sadly, there's not much you can do. As @Dirk alluded to you can apply sealing products to try to mitigate loss of the flagstone layers, but then you will be wedded to using those products on a regular basis (typically every 2 years, give or take). Or, you can wait until the next time you renovate the pool and go for a different material.

Sorry, no easy answers here ...
 
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@JoyfulNoise is right--our local flagstone is of poor quality. There are better quality stones available, but most builders are going to choose the cheapest thing possible to save money & maximize profit margin. That usually means local stone.

Most of the flagstone we get here comes from either local TX or Oklahoma sources and it is pretty weak sandstone. It's naturally lacking in cementitious minerals, so it just isn't all that hard of a sandstone & erodes quickly.

In my case, I haven't sealed it--I just sweep away the new flakes at the beginning of the season and take it as weathering. It's been there over 30 years, so it fits. Except for one flag I had to have replaced on the corner of my spa. The "new" stone has almost completely dissolved over the last decade. It's just the nature of the material.

I'm planning a remodel in a couple of years & when I do that, I will be using cast concrete coping--not only because I want something that holds up better, but because I want something I can reliably put brass cover anchor points in as I want a winter cover to keep some of the leaf trash out. I don't trust that the crummy flagstone I have would hold up for the anchors.
 
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Just want to repeat. Typical stone sealers will do nothing for the flaking/sanding problem. There is probably some benefit to sealing the stone, perhaps it sheds some of the water that is causing the problem, but that's not the solution I was referring to. I can't remember the name of the product, but I put in a call to my stone guru to see if he can enlighten us. He's now a TFP member... Oh @mcleod...
 
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Dirk has emailed me this thread and I've read the entries. I'd like to add a thought or two. For background, I was in the stone preservation and restoration business for 45 years. I am not intending to diagnose a specific situation and am speaking broadly. I also addressed the issue on dirks pool deck.
  • Water always deteriorates stone (think Grand Canyon). Sometimes quickly, sometimes slowly. Saltwater. Freshwater. It's all water.
  • Sealers, the penetrating type (sometimes called impregnating) can efficiently repel water to slow down the process of degradation. Penetrating sealers with a high percentage of "solids" (the active repelling chemistry) can enhance the protection factor, creating a strengthening by adding more stuff into the stone. In my experience, this approach is still a short-term fix.
  • The approach I've taken (and used at Dirk's) is to use a chemical consolidator or densifier, applied in a determined number of saturating coats, followed then by a penetrating sealer. Consolidators create a chemical matrix that binds with the mineral make-up of the stone, making it denser and stronger. This first step enhances the performance parameters of the penetrating sealer and simplifies the maintenance of the stone going forward for a number of years. How many years depends on the number of variables in any specific situation.
  • The processes, while not simple, could be done by a competent DIYer.
McLeod
 
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Thanks for the answers. Knowing that all pools are saltwater it didn't make sense. But there is this belief out there that it is the salt that does it. My sandstone has been disintegrating for 15 years! And as I said, no better nor worse than before. This is why I've planned for my eventual remodel with some sort of rough or roughened granite.
Actually, I'm surprised the Diamond-Brite has held up as well as it has.
 
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For sandstone, the most durable is a quartzitic sandstone, which consists of interlocking quartz grains, cemented by silica.

You should inquire about the strength of the sandstone, which a good supplier should provide.

The stone’s data sheet should provide details about the compressive strength, the flexural strength, the porosity, the density, the hardness, the wet and dry Static Coefficient of Friction etc.
 
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ASTM C616 classifies Quartz-Based Dimension Stone into three categories: Sandstone (Type I), Quartzitic Sandstone (Type II), and Quartzite (Type III). The stone is classified by the following physical properties: absorption by weight, density, compressive strength, modulus of rupture, and abrasion resistance.

Sandstone is the weakest. I think that regular sandstone is not a good choice in most cases. It tends to be too weak and does not hold up well.

Quartzitic sandstone is very strong and durable. It is composed of grains of Quartz bonded by Silica. I think that the best choice in most cases for decks is a quartzitic sandstone. It has a good texture, a good slip resistant surface, and very good weatherability.

Quartzite is stone that was originally sandstone but has been heated and pressurized in the earth into a harder and more durable stone than sandstone. During heating and pressurization, the grains of sand are melted together. This tends to give quartzite a slicker surface. In cases of water immersion, especially with a salt pool, I think that a true quartzite is usually a better choice.

Tennessee Crab Orchard Stone is usually a quartzitic sandstone. It is sometime referred to as a quartzite, but I don't think that it is a true quartzite. Many stone suppliers will be able to tell you the physical properties and composition of the stone, and to which classification the stone belongs. If there is any question, you might want to consult with a stonemason.

You want a denser stone that has less open pore space. You should get a stone with a density of at least 150 pounds per cubic foot and a water absorption by weight of not more than 3%. Water absorption can cause freeze/thaw damage, and creates more damage from salts and other chemicals.

http://www.clevelandquarries.com/archit ... .php#specs

http://www.astm.org/Standards/C616.htm

http://www.cmc-concrete.com/CMC%20Flyer ... esting.pdf

ftp://ftp.ebuild.com/woc/M950530.PDF

http://isse.utk.edu/ccp/projects/natura ... dstone.pdf
 
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The right sandstone is one of the best choices for a pool deck.

The best stones are very durable, they have a good texture that is not rough and not slippery and they do not get as hot as some other types of decking, especially the lighter colors.

Ask a good stone supplier or a good mason about which stones are the best and get samples and data sheets on each stone.

One of the best tests for a stone is a scratch test.

If the stone is easily scratched, it is probably weak.
 
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Dirk said:
It's the water destroying the sandstone, not the salt. Period! Sandstone is not always the right material for around a pool. It's just layers of compressed sand, and water is instrumental in "de-layering" it. That's all that is going on. Like yours, my beautiful sandstone was coming apart long before I converted to SWG. Some sandstone is better than others for a pool. It all depends on where it was "harvested."

The other consideration: all pools are salt water pools! (Eventually.) Chlorine, acid and humans all leave salt behind as a byproduct, and that salt builds up over time. Even if salt was the problem with the stone (it's not!), you'd have the problem whether you have an SWG or not. Converting to a saltwater/SWG pool is safe for your pool and its equipment (assuming you only let the salt level get so high).

There are products that can reduce the de-layering. That has worked in my pool. It's not cheap. And it's not sealer (which will not stop the de-layering). It penetrates the layers and glues them together (that is wildly paraphrasing, but that's the layman's description).
Click to expand...
What did you use to stop the delayering, please?
 
JamesW said:
For sandstone, the most durable is a quartzitic sandstone, which consists of interlocking quartz grains, cemented by silica.

You should inquire about the strength of the sandstone, which a good supplier should provide.

The stone’s data sheet should provide details about the compressive strength, the flexural strength, the porosity, the density, the hardness, the wet and dry Static Coefficient of Friction etc.
Click to expand...
I was reading on the Niagara escarpment. It was interesting to find out that the dolomite is a form of salt/silica fused sandstone. And that stuff is VERY hard. No question, the sandstone used around here is cheap stuff. I am looking to go with roughened granite when I rebuild this pool. But maybe a prime sandstone would be better.
 
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Prosoco makes very good quality stone treatment. Hard to get though unless the have a local reseller otherwise you have to ship their stuff and it ain't cheap ...
 
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