What Is A Concrete Pool?
There is a lot of confusion among laymen about the terminology of concrete pools. In South Africa, older pools are usually constructed with gunnite, which is dry concrete mix that is sprayed from an industrial gun onto a steel mesh structure – hence gun-nite. Gunnite is very useful for creating curved shaped pools that were so popular last century. Gunnite leaves a rough texture which is often finished with marbelite, which is just like normal grey plaster but made with fine marble dust instead of sand aggregate and thus forms a less porous seal than conventional plaster does. But it is not completely water tight. There is often a little seepage through marbelite pools.
Marbelite slowly erodes under low pH conditions. The leaching marble pushes pH up, which prompts conventional pool owners to add pool acid. They do this to maintain a pH of 7, under which conditions chlorine is effective. It is very difficult for pool owners to dose pools acurately or slowly. Dumping in a large dose of acid drops pH quickly and this can erode marbelite. And so the cycle goes on… This feedback loop results in the pitting of old marbelite lined pools and brittle cracked pipes in chlorine pools.
Often, marbelite is sealed with fibreglass when it starts to erode, so one can have a gunnite, marbelite and fiberglass pool, all in one!
The Use Of Concrete In Natural Swimming Pools
Concrete natural pools can be constructed from reinforced brickwork (masonry) or shuttered concrete, waterproofed with crystallizing additives like Xypex and Penetron. Masonry pools can be waterproofed with cementicious sealants, the best of which combine with the cementicious structures beneath to form a monolithic structure. It is important when working with cementicious sealants to make sure the various layers are chemically compatible with one another, and to make sure that they are applied at the right time to ensure proper bonding when curing.
Once a cementicious shell has been constructed, it is best to finish the shell in an appropriate finish which is biologically neutral and provides a non-slip texture.