Vermiculite Concrete Mix
This article contains information about swimming pool floors made from a vermiculite concrete mix. Vermiculite is a mineral with very specific properties that allow for a unique concrete application that is often used on the floors of some (vinyl liner) swimming pools. Where normally portland cement is mixed with sand to make mortar, it is possible to substitute some of the sand aggregate with a vermiculite aggregate. Doing this will have a dramatic affect on certain physical properties of the concrete.
So what is vermiculite anyway? Vermiculite is a hydrous phyllosilicate mineral. It is basically like a popcorn made from a mineral in the mica family. When heated, vermiculite expands greatly and takes on a light and fluffy texture. Most people are not familiar with vermiculite except that they have heard that vermiculite contains asbestos. This is not exactly true. There was a period of time where vermiculite containing asbestos was used to insulate homes. Horticultural grade vermiculite, like used for gardening or as an alternative aggregate in concrete, does not contain asbestos and is not considered dangerous. The problem is not that asbestos is added to vermiculite. It is that a naturally occurring form of asbestos called tremolite-actinolite can sometimes contaminate vermiculite.
Vermiculite with greater than 1% asbestos content is considered contaminated, where asbestos with less than 1% asbestos is considered, for all intents and purposes, to be "free of asbestos". This is an important distinction that should encourage anyone working with enough vermiculite to cover a pool floor with concrete to take extra PPE steps, most specifically, to use a respirator when mixing.
How strong is vermiculite concrete? - Adding vermiculite to a concrete mix will substantially weaken the overall strength of the finished concrete. Sand and gravel are used for concrete mixes because they are cheap, and they make a very strong finished product. The more sand you substitute with vermiculite, the weaker the concrete will become. This is why you would typically not see a vermiculite floor poured by itself, but instead vermiculite concrete is added over a stronger base layer of regular concrete as part of the "two stage bottom" process used on some vinyl pools. In most areas grout (mortar) based pool floors are the most common, by far, but there are some advantages to a modified vermiculite concrete floor.
What does vermiculite concrete feel like? - Vermiculite concrete is not as strong as regular concrete, and it does not feel like regular concrete when you tap it, or jump on it. While hard, a vermiculite concrete will be noticeably less hard than, say, a concrete sidewalk tile. This slightly softer, almost cushioned feel, provides enhanced durability to vinyl liners as it limits the sandpaper effect that can happen when a liner drags over a full strength concrete floor that has become rough due to poor finishing or ingression of water.
Insulation value of vermiculite concrete - Vermiculite concrete can have up to ten times the insulation value of traditional concrete made from sand, gravel and cement. Using vermiculite concrete on the floor of a pool will increase the thermal efficiency of the pool and limit the amount of heat that escapes the water into the floor of the pool. The ground is always cool at the bottom of an eight foot deep hole and so it makes sense to improve the insulation of the pool in this area, however the majority of heat loss in a pool does not happen through the floor, but through the surface of the water. This is why using a pool cover is so important if you want to limit heat loss from your water.
What does vermiculite concrete look like? - Vermiculite concrete can be hard to identify because the physical properties of the concrete, like strength, will vary depending on the mix proportions that were used. This can make it hard to definitely identify vermiculite modified concrete in some cases. What is most noteworthy is that vermiculite concrete has a shiny, gold-like shimmer to it and this will be the easiest way to confidently identify a vermiculite floor.
How do you patch vermiculite concrete? - If you have an existing vermiculite concrete pool floor that requires patching then it is important to patch with a similar concrete mix on top. Using a regular strength mortar patch on a vermiculite concrete floor would very likely result in the patch delaminating and shaling. This would look bad underneath of a liner, as well as create a danger to the liner from the sharp edges of the patch breaking apart as the patch fails. If you are unsure as to how much vermiculite to use to recreate the unknown mix that the floor was originally completed with, you are better off to err on the side of too much vermiculite as opposed to not enough. Too much will make the patch weaker than you want, but not dangerous to the liner since the concrete is weak and will compress. Not enough vermiculite could mean that the patch is too hard, and this is where the danger for sharp pieces of patch shaling off exist.
How to finish vermiculite concrete - Finishing of vermiculite concrete (the process of troweling it smooth and flat) is much more forgiving than traditional concrete mixes. Normally with concrete the act of troweling raises water in the concrete to the surface...too much water raised and you will compromise the quality of the finish. Vermiculite holds water extremely well. This is why it is so widely used for gardening applications. When in a concrete mix this serves to hold the moisture needed for actuating the concrete and it is much more resilient to overtrowling. Vermiculite however is very sticky and tends to hang up and drag on the trowel. A small (near tiny) addition of liquid dish soap to the mix can help to alleviate the friction when finishing and make the mix less sticky.
Grades of vermiculite - Vermiculite comes in grades based on how fine it has been processed to be. Since concrete for pool floors is not the primary use for most commercially available vermiculite, you need to be aware of what grade you are using. In theory, all grades can be mixed in concrete with reasonable success, however the finer graded vermiculites are better for the purposes of mixing concrete. The larger chunks with coarse graded vermiculite is good for holding water in a potting soil mix...but less good for making a smooth and flat concrete surface. Coarse vermiculite is about the size of 4mm or 5mm pebbles which would be about the amount of eraser you see sticking out of a pencil. The pieces for fine graded vermiculite would be closer to the diameter of the pencil lead instead of the eraser.
If you are not familiar with vermiculite concrete mixes it would be a very good idea to do some test batches to get a feel for how it performs. It changes concrete completely and is a totally different process than regular concrete or mortar applications. For example, you need to be aware of water content in your mix even more so than with regular concrete mixing...and it is already really important with regular concrete mixing!
The vermiculite will absorb an incredible amount of water, and so you will most likely bind up your first mixes fighting with the water content. Furthermore the vermiculite can even pull water away from your mix and steal away the catalyst that your concrete needs to actuate properly. The solution to most of these water woes is to pre-soak your vermiculite in water so it is completely saturated before you add it to your mix. Trying to use dry vermiculite in a concrete mix would be a lot more challenging to get consistent mix results.
How To Make Vermiculite Concrete?
I have never heard of a ready mix concrete batch plant that offers a vermiculite concrete mix. Perhaps there are some places that offer this, but for the most part if you plan to have a two stage bottom on your pool, or use a vermiculite concrete mix for something, you will need to mix it yourself. Mixing vermiculite concrete is tricky and not really suitable for standard barrel style concrete mixers as the vermiculite is too sticky and will not tumble in the drum properly. The same can be said for drill and whip concrete mixers in that the vermiculite mix is too sticky to spin properly.
The best way to mix vermiculite concrete is with a paddle style or corkscrew style mortar mixer. These mortar mixers are a lot more money that a regular barrel style concrete mixer, so the average handyman is very unlikely to have access to this machine. If you are looking to buy or rent the perfect machine for mixing vermiculite concrete, this is what you are looking for. Typically anything from 2.5 cu/ft up to 9 cu/ft in size would be decent, with the larger sized machines being more suitable for large batching.
Vermiculite patch mix recipe
1 part type "general use" portland cement
1 part clean, washed, sharp sand (brick sand)
2 parts (by volume) hydrated vermiculite
10% liquid content weldbond or white wood glue
* one part just means an equal amount by volume, so a part could be equal shovels full, or containers like a bucket - just so long as you observe the approximate ratio described here the mix should be usably good.
This patch mix will make a concrete that is heavily modified with vermiculite, but still enough sand and cement content to be usefully strong as a finished product. The addition of the glue to the mix enhances the already sticky nature of vermiculite concrete. This will help the patch to adhere but will make finishing the patch even more difficult from the concrete dragging on the trowel from friction. This is why you would not use the glue for mixing vermiculite concrete for an entire floor pour, for example.
Vermiculite large batch mix recipe
1 x 94 lb. bag type "general use" portland cement
1 x 5 gallon bucket washed, sharp sand (brick sand)
1 x 4 cubic foot bag hydrated vermiculite
2% liquid content acrylic additive (this is a retarder to help slow initial set, and improve workability)
* the addition of a tiny amount of liquid dish soap added to the mix right before you finish mixing will enhance the finishing characteristics by reducing the friction and drag of this otherwise sticky concrete mix.
** another common mix would be to skip the sand completely and double the amount of vermiculite used. This would provide a very soft surface for the pool liner to go over, but the concrete in and of it=self will be very weak. If the vermiculite layer is supported by a regular strength concrete under it, then you can get away with using this mix variation with no sand at all, for optimum insulative enhancement of the pool floor.
If you have not worked with vermiculite concrete mixes before then you are likely to overmix yours. Unlike regular concrete and mortar mixes which have a "more is better" fundamental characteristic when it comes to spinning or mixing, vermiculite will begin to break down if you mix it too much. The goal with a vermiculite mix is to batch it as quickly as possible while still having the different ingredients thoroughly dispersed throughout. However, once you feel that the concrete is mixed well enough, minimally, then you would give it a tiny shot of dish soap, mix it for ten more seconds, and then it is ready to be placed and trowled right away. Mixing too much causes the vermiculite to almost deflate, which will dramatically reduce the size and coverage for your batches.
This is a 4 cubic foot bag of vermiculite available from Amazon. This would also likely be available local to you from landscape supply stores and some building centers. Just be sure to check the grade to get fine or medium grade at most for the best concrete mix.
This is an example of a general use portland cement. The type, or name, of the cement may differ depending on where you are located geographically. In my area, type 10 denotes regular, or general use, as goes type G, type GU, type N (normal). Simply ask for the most common portland cement and you should not have any trouble finding the right one.
Weldbond universal adhesive is very similar to white wood glue, and when added to a concrete mix will result in enhanced bonding characteristics. This is extremely useful for making bonding slurry, as well as adding directly to patch concrete to help make sure new concrete patches adhere as well as possible to the old concrete surface.
This liquid acrylic concrete additive has a number of positive benefits when added to a concrete mix such as improved workability, slightly retarded initial set time, stronger finished product, minimized cracking, enhanced bonding...all good things for a concrete mix. This should be a regular go-to product for almost any concrete mixing application you do.
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