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For many, building the mixer is the most intimidating part of papercrete. Fortunately, McCain has started a mixer-building sideline (see "Sources").

FORMULAS

Since the most common mixer design has a capacity of 200 gallons, most papercrete builders think in terms of 200-gallon batches. Usually, people add one 94-pound bag of Portland cement to a mixerload. Half a bag will work fine, but the slurry will dry more slowly and won't be as hard when it does. Some people add two bags per mixerload, particularly if the papercrete is to be used close to the ground, for roof panels or for floors.

While there's no one right recipe, the basic formula for a 200-gallon batch is: 160 gallons of water, 60 pounds of paper, 94 pounds of cement and 65 pounds of sand (about 15 shovelfuls). The sand adds thermal mass, reduces flammability, makes the slurry pack down better for a denser, stronger block and helps prevent cracking when papercrete is used for stucco. Although the paper makes up only 27% of the dry ingredients by weight, it constitutes well over half the volume of a finished block.

When making roof panels, leave out the sand for an even lighter weight material with maximum insulation value.

Incidentally, as we said earlier, slurry is 85% water by weight. A lot of this water drains out immediately after the papercrete is poured. The rest evaporates as the slurry dries, leaving behind the millions of tiny air pockets that account for papercrete's lightness and superior insulation properties.

As for fidobe, the ideal formula depends on your dirt. It pays to make up small batches in a kitchen blender before you go into serious production. Vary the dirt to paper ratio and see what proportion seems best. But keep in mind there is a trade-off: the more dirt you add, the heavier and stronger your block will be, but the less insulation it will provide. Our personal choice is a 4:1 ratio (dirt to paper, by weight). This gives a strong block that is reasonably lightweight, with a substantial R-value. For a 200-gallon batch, we mix together 160 gallons of water, 60 pounds of paper and 240 pounds of dirt.

The clay content of the dirt should be anywhere from 30% to 100%. With regular adobe, too high a clay content causes cracking. Not so with fidobe, since the paper fibers hold the block together.

FLAMMABILITY

Unless you add enough nonflammable material to the mix, both papercrete and fidobe will burn, slowly and without flame, like a charcoal briquette. We've found that papercrete made with a 4:1 ratio (cement to paper, by weight) will not burn, and that fidobe made with 3 parts dirt to 1 part paper will not burn. So we don't consider flammability to be much of an issue. Besides, a nonflammable stucco inside and out will cut off the oxygen supply to the wall, preventing burning.

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