Things You'll Need
Polystyrene foam blocks or boards
Polystyrene foam cutter
Copper wire or metal fasteners
Acrylic fortifying cement
Let the boulder sit outside without the plastic for another month or two to weather it and dry it completely before installing it at your pool. For added strength and stability, the boulder's cavity can be filled with insulating foam or even compressed plastic bags.
When applying the hypertufa, do so on a calm day outdoors. If working indoors, turn off all fans and close windows. A breeze can cause the hypertufa to dry out too quickly.
Turn your swimming pool into a tropical lagoon with imaginative faux boulders and rocks to deliver an unspoiled feel. Building the boulders is a time-consuming task, so make sure to allow plenty of space and time during mild weather. A successful project requires creativity, the right materials and attention to detail. If you cure the hypertufa, a concrete-like substance, properly, the faux rocks will be waterproof.
Design the rock you wish to build. Cut polystyrene foam with the polystyrene foam cutter into arced pieces that can fit together to build the rough outline and size of the rock. Make it a few inches smaller than your design to allow room for applying the hypertufa later.
Attach the pieces together with metal pins or short lengths of copper wire to form the basic outline.
Cut smaller pieces of polystyrene foam to make the outline irregular, adding bumps or bulges or other shapes. Attach with pins or wire.
Press steel mesh onto your boulder, bending it so that it conforms to the underlying shape. Use pliers to twist copper wire to apply it to the mesh, then push the wire into the polystyrene foam to hold the mesh firmly in place. Too much is better than too little here; the mesh must hold its shape during later steps.
Glue small chunks of polystyrene foam, about 1 inch thick, to the metal mesh all over the boulder and allow the glue to dry. These are to keep the chicken wire from simply collapsing onto the mesh.
Attach the chicken wire to the foam blocks by twisting copper wire around the chicken wire and pushing the other end of the wire into the foam. This is a good time to bend the chicken wire to design crevices and valleys in the "rock."
Making and Applying Hypertufa
Mix the dry elements of hypertufa thoroughly in a wheelbarrow, using the trowel. These elements are 1 part Portland cement, 1 part builder's sand and 2 parts peat moss. All can be found at building supply stores or garden centers.
Add acrylic fortifying cement to the mix, following manufacturer's directions.
Add water slowly to the mix, stirring with the trowel, until the mixture resembles thick mud. Immediately spread 3 inches of the hypertufa over the rock form with the trowel. If desired, use the point of the trowel to trace crevices, lines and pit-marks into the hypertufa.
Spray the final product with a mist of water and immediately wrap it in black plastic, folding the edges of the plastic under the bottom edges of the boulder. It is important that the hypertufa not dry too quickly, or it will be unstable when fully dry.
Remove the plastic once or twice a day for two to three days to mist with water. At the end of this time, the hypertufa will have set. But you're not done yet.
Cure the hypertufa. The best method is to move it into the sun, remove the plastic and spray with water, then put the plastic back on. The heat of the sun will turn the sealed plastic into an automatic mister. If that's not possible, continue to remove the plastic each day to mist the hypertufa. Continue this procedure for at least a month.
Pete Hisey has been a writer and editor for over 25 years, primarily in the business-to-business field. He has expertise in many areas, including retail, consumer electronics, banking, dining, agriculture and entertainment media.