Thanks, Chem Geek.
So I guess that in come cases (with everything else being equal), I could imagine that the salt concentration in a non-SWG pool could spall natural surfaces much like SWG can.
Next question: Is this limited to natural stone, or do concrete-based pool decks (concrete, fired pavers, etc.) have same risks, too?
Concrete, brick, ceramic and glass will fair much better against salt and water versus many types of natural stone. Slate, sandstone, limestone and other soft stones are especially vulnerable to damage from salt.
There are many options for concrete coping materials - precast molded concrete coping, poured-in-place cantilever coping, etc. In general terms, man-made surface materials will generally last longer than natural materials. Brick, ceramic and glass are more dense and less porous compared to concrete.
Concrete is typically harder/stronger than natural stone (thus resists the forces of salt expansion better than stone). Salt may leach out of concrete as efflorescence, but this doesn't necessarily translate into spalling and breaking the concrete with the same likelihood compared to natural stone.
Concrete and brick can be sealed with a variety of relatively low-cost products to waterproof the substrate, providing an exceptionally high level of protection against salt and other water-based effects (e.g. freeze/thaw spalling or cracking, staining from dissolved metals in water, etc.).
Natural stone can be sealed, but the products are typically much more expensive compared to concrete sealers. One gallon of the stone sealing products from StoneTech or Dry-Treat costs as much as five gallons of sealer for concrete sealing products such as Endur-O-Seal Hydra Lok and comparable products.
Glass and most ceramic materials are waterproof to begin with, so the risk of spalling due to salt expansion during evaporation/drying is essentially zero. You don't need to seal glass or ceramic because these materials are waterproof (and typically won't absorb any sealer, nor be penetrated by salt water), but the grout between tiles can be sealed using most grout or cement sealing products. Thus, 'regular' waterline tile works fine in saltwater pools.
Any materials that remain submerged year-round with no wet/dry cycle don't necessarily need to be sealed/waterproofed, as the threat from salt-driven, expansion-based spalling is specific to the drying cycle. When dissolved salt dries (the water carrying the salt evaporates), the salt will crystalize, where the crystals grow larger than the original volume of water inside the capillaries within the stone, potentially breaking off chunks of the surface of the stone. This is very similar to freeze/thaw damage where water inside a substance freezes, the water turns to ice which demands a greater volume. If the material remains wet and never allowed to dry or freeze, the material isn't at risk from these crystallization forces.
Any porous materials that are exposed to frequent wet/dry cycles or freeze/thaw temperature changes should be sealed, as a best practice. Examples include beach entries into pools, waterfalls, and coping. Any porous material that is only exposed to splash-out water but not year-round water is at a lower risk, thus decking materials are at a lower risk versus coping, and coping at a lower risk versus the materials around waterfalls, and the highest risk is at the waterline (which is why ceramic or glass tiles are most often used at the waterline).
In summary, yes, concrete is vulnerable to salt, but less so compared to natural stone. Sealing products for concrete tend to be lower cost than similar products for natural stone.
