I find this information very helpful when considering using grout. You might want to skip the grout, and just do foam/expanding foam/paint/sealant, unless you want to allow it to cure and exhaust the CaOH. I found this post on dendroboard about a week ago. The user who posted it has contributed a lot to this website for over 6 years, and I trust them. VivariumWorks, they also have a website VivaroumWorks.com Anyways, here is their post I found. It is a bit long, but worth the read. I'll post the link to the thread at the bottom.
Hey, its a long and complex topic that isn't talked enough in detail so I've summed up some the key issues with using grout and offered some reasons why you are seeing what you are, how to fix it, and how to cure it. Hope this help and good luck on the build! Grout is a mixture of various metal oxides for colorants, sand of various sizes, and portland cement (the glue). Due to it's specific intended purpose it isn't designed to be super strong as most of it's strength comes from the tiles on either side of it, and why most non-sanded grouts are recommended only for small thin sections and the 'sanded grout' for the larger sections/spaces as the large sand they use acts as a sort of aggregate and stabilizer. It is designed and used in a much higher water/cement binder ratio than standard cement (which uses the same 'glue') and sometimes will contain various additives to assist with binding to other cement/rock materials, but typically results in a very high water/cement ratio. The higher the w/c ratio, the more water takes up space in the rock matrix when drying/curing. The excess pore water evaporates out and the result is a low strength material due to: low total binder concentration, high water concentration, and in our case a thin layer. So how this is relevant to us is that, yes what you are seeing is to be expected. Grout when used for our purposes, as a layup material over foam to make backgrounds/fake rocks, is to be used in multiple layers. First a base layer, usually a dark color, then followed up with one or two detail and color layers. With each layer you loose detail of the rock look you carved out of the foam, so less is better so as to not lose all of your hard carving work, but you still need a thick enough layer to be strong. Its basically finding what will be strong enough to work for your application and leave it at that. So yes you'll want to put one or two more layers on top if it's just for dart frogs. You can also try lowering the amount of water you mix with the grout so it will brush on thicker. Curing will be important. It just drying won't be enough. It will need to stay moist (not submerged) for a minimum of 28 days. This so the dicalcium silicate in the portland cement can fully hydrate. This reaction is very slow and takes a full 28 days to happen, and even then it never really stops, just slows down to a very small amount. The issue for us is that as the cement hydrates it puts out CaOH (calcium hydroxide or lye). This is crazy basic and will shoot your water and soil's pH sky high and kill your plants and base burn your frogs. So the issue here is that when you cure your grout, it needs to cure wet, but not submerged, for a full 28 days to let the reaction fully happen and leach out as much as possible into water you'll toss. Just because your pH is good on day 8 or 10 because you poured acid over it, as it's falsely believed will help, the pH will rise again from day 10-28 as the rest of the dicalcium silicate and other cement products continue to hydrate and kick out CaOH. So make your background, put it into your tank, and before you put anything into it pour water over it, leaving a pool of water in the bottom of the tank, and cover the top with plastic wrap and leave it for a month. You want water to soak up into your grout layer from the bottom by wicking. It would be good to occasionally change out the water in the tank as you want a strong gradient pulling the CaOH from the grout's pore water and into the waste water. Some people, myself in the past, have thought to simply trap the CaOH inside the grout by sealing it. All sorts of stuff has been suggested and used. Epoxy, polyester/'fiberglass' resin, silicone caulk, waxes of all sorts. None of this works long term. The reason for this is that the CaOH and water will find a way. It wants to due to diffusion, and unless you have a perfect seal, it will find a way, and leach the strong base into the water/soil and slowly poison the tank/water. Obviously there's a million other factors in play, and not everybody will see the same effects as the organic acids in the soil can help to counteract the leaching out base, but the simple fact is that it can, will, and has killed many many many an animal/tank. So toss out the old idea that you can seal it quick and skip the month long curing process. It sucks that we have to do this, but that's the trade off. You want a cheap, rock-looking, easy to buy, waterproof, laminating material? You go with grout. BUT, you have to do it right or your plants and critters (and subsequent pocketbook) will pay for it down the road. http://www.dendroboard.com/forum/par...lp.html#/enter