cheap waterproof adhesive

Some friends and I are building a small demonstration of porous concrete for an environmental fair and I would like to make a mixture that will serve as a mock-up of the actual material. My initial plan was to mix up some gravel and crushed stone aggregate with construction adhesive, dyed if necessary, to make sort of a rice krispy square consistency that could be spread into the small clear cube which is made of lexan plastic. My main reservation about the construction adhesive is that the adhesive is really tacky and might form lots of threadlike strings as the particles move apart. I was also considering grey silicone latex caulk. The final material does not really have to be very strong, but it has to be waterproof so we can demonstrate the porosity by pouring water through it.

Anybody have any thoughts about what kind of adhesive/binder would be good to try?

There are plenty of waterproof adhesives at places like Home Depot and Lowes. I’d suggest getting a good epoxy – they’ll set by chemical reactions, and don’t have to dry out, so they stay as good adhesives even in water.

Heck, a lot of common adhesives are good under water once they’ve set. White glue (like Elmer’s Glue-All) and clear cement (like Duco) work well, but they only have limited flexibility.
Have you considered using a Hot Glue Gun? Flexible, and the “glue” doesn’t require drying to set, and it’ll set quickly, as soon as it cools.

Why not visit a pet store and pick up an airstone of a suitable size and shape? You can probably find a large selection online. Here is a 6" one, for example. They look kind of like concrete, and are specifically constructed to be highly porous since they have to pass air at a fairly good flow rate.

How big a cube? What size aggregate and/or crushed stone?
You might try a water base polyurethene. Dip the DRY aggregate in the poly, drain off the excess, and place in the cube. Cure in a WARM oven.
Clear epoxy, with 1 to 2 hours setting time could be used the same way. The long set time would allow for dipping/coating and draining.

Most folks are looking for something to seal concrete blocks such as “Ugli-Paint”

The cube is about the size of a 20 gallon fish tank. It will have a hollow plywood box in the center so as to reduce the final weight, so there will only be aggregate around about the outermost 2" of all visible sides. Poly, elmers glue, or other clear adhesives would work, but we would like the adhesive/binder used to be opaque and gray so the final material will more closely resemble concrete.


You might try a water base polyurethene. Dip the DRY aggregate in the poly, drain off the excess, and place in the cube. Cure in a WARM oven.

Ms. Doubt already had a bad experience with a cylinder head in the oven, so I don’t think she’d be too happy about me baking a big concrete and polyurethane cake!

But it’s trivial to make clear adhesives opaque! Just mix gray powder into it! You don’t need a lot. Carbon black works great if you want it black. I’ll bet you can use graphite or some similar dry lubrication powder, or cement dust or something to get clear epoxy/hot glue/whatever gray.

Hard is making something opaque clear.

Oh, that’s a lot bigger than I was visualizing. Well, if you’re looking for a waterproof, opaque grey adhesive, I’d go with something like a casting resin. You can get them in a huge assortment of colors; there’s bound to be one that’s suitable.

Sorry about the hijack, but I gotta hear this.