Prepare the forms you are going to use for the hypertufa bench. Heavy cardboard boxes will work. You will need to place cinder blocks, bricks or heavy wood along the outside walls of the box so the box walls will not bulge when the hypertufa is poured.
Line the forms with a layer of plastic. This will keep the hypertufa from adhering to the form.
Wear rubber gloves, or other waterproof gloves, a dust mask and protective glasses when mixing and working with hypertufa.
Combine two parts Portland cement, one and one-half parts sifted peat moss, two parts perlite, one-half part contractor's sand and a handful of fiber mesh in a wheelbarrow or large container. Mix thoroughly the dry ingredients for the hypertufa, using a cement trowel, garden trowel or shovel. The dry mix must be thoroughly and evenly mixed.
Add water to the hypertufa mix slowly and in small quantities, using a trowel, shovel or garden hoe to mix it. The consistency of your hypertufa depends on the particular project you are doing. For a bench, it needs to be firmer than if you were molding a piece to be carved. The ideal ratio is 1:1 or 1:1.5 hypertufa and water; in other words, equal parts of hypertufa and water, or slightly more water. How moist the peat moss is and how high, or low, the humidity is plays a factor in how much water is needed to make the hypertufa mix damp but not crumbly or oozing with water.
Test the hypertufa mix periodically for proper consistency by taking a handful and squeezing it in your hand. The hypertufa should stay together in a ball shape, with a little water squeezed out between your fingers. When you have mixed it to this consistency, let the mix rest for 10 minutes to firm up, and then remix, or stir, the hypertufa before filling the forms.
Fill the hypertufa bench forms to a thickness of 4 to 5 inches. Use shovelfuls of hypertufa to fill the forms, stopping occasionally to tamp the hypertufa down with a piece of 2-inch by 4-inch lumber.
Mist the surface of the hypertufa in the forms with water in a spray bottle, then place the filled form in a large black plastic trash bag and seal the bag with a twist tie. This is the curing process that will allow the hypertufa to slowly dry without cracking or crumbling later. After 24 hours have passed, open the bag and test the hypertufa by scratching the surface. If your fingers are dusted with hypertufa, mist the surface and reseal the bag. Your hypertufa is not cured until the surface is hard enough not to produce dust when scratched. Check the forms every two to three days. Depending on the temperature and humidity, curing can take from five to 28 days.
Carefully remove the cardboard forms from around the hypertufa when it has completely cured. Assemble the hypertufa bench by placing the block legs upright, and then place the seat piece on top of the legs. You can epoxy the seat to the legs for more stability.