The best low-cost structural materials are composites. Composites are a combination of distributed fiber reinforcement, lightweight aggregate, and a binding agent. Ferro-cement is one such composite using metal or synthetic fibers and portland cement, but there are many others. Fiberglass is a common (but toxic) composite. A newcomer in fiber composites is papercrete or fibercrete, which is a combination of pulped paper, or other cellulose-based raw material, and binders such as lime, cement, and/or clay. Sand adds strength and density to these composites, but lightweight aggregates could also be used. Any composite material can be used to create shell structures which we call Composite-Shell construction. Domes 100 ft. in diameter, (and only 10 mm thick!) have been built using portland cement and glass fibers. Using lesser fibers and binders, more modest structures can be built. If the fiber composite is applied to both sides of a thick insulating layer such as straw-clay, perlite cement, or agcrete (see below), a strong "sandwich" is formed. If the insulation layer is made from a structural honeycomb of lightweight fiber composite (filled with insulation), the strength of the shell is even better.
Composite materials can be sprayed (using compressed-air driven sprayers), or poured, or packed directly onto a reusable formwork (using hand methods). Simply pour from a bucket, or apply from a scoop, and spread. Fiber reinforcement options include natural fibers such as wood (paper), bamboo, industrial hemp, sisal, and jute, for low-tech applications, and synthetic, glass, and steel fibers for high-tech applications where greater rigidity is desired. Recycled clothing fibers can be used. Bamboo has been used successfully in place of rebar in many countries for decades. Wood (paper) fiber has been used in Japan for centuries for its strength and beauty. Industrial hemp fiber has tremendous potential where it is available. It is used for architectural molding in North America. Discrete bamboo fibers have tremendous strength, exceeding hemp, and could be the most effective fiber of all if properly processed. Jute (burlap) is also a strong natural fiber. All high silica fibers last a long time if protected from rust or rot.
It may not be such a good idea to build too rigidly, as is common in both industrialized countries and developing countries. Rigid concrete materials are brittle and fail catastrophically during earthquakes. Rebar reinforcement is totally inadequate. Ferrocement structures fare better. But these structures are very heavy. As a result, we now favor very light composites such as fibercrete and agcrete as more forgiving structural building materials. A lightweight foamed composite using paper-hemp-bamboo for fiber reinforcement with cement/lime/clay binder may be an ideal building material. Natural glues may improve flexibility, but some minerals (20-40% by weight) are needed for fire protection.
A suggested composite material is paper fiber and mineral binder. So much waste paper is available, it makes sense to use it. One suggested mix is 60% paper and 40% binder by weight. The binder can be 50:50 portland cement/hydrated lime or 25:25:25:25 cement, lime, clay, and sand. Sand adds strength and density. Liquid soap adds bubbles for a lighter, insulating mix. By volume, these mixes are primarily paper fiber, but will not burn. Various mixes emphasize strength or insulating qualities.
Another composite with even more potential than papercrete is agcrete. This method uses common agricultural wastes such as crop stalks, chipped wood, or any other low density, commonly available lightweight material. It's similar to using perlite, vermiculite, scoria, or other lightweight mineral aggregates. Waste material is simply chopped (up to golf ball size or so), blended, and mixed with cement (and/or lime) just as you would use ordinary gravel. When cured and dry, it makes a very lightweight composite. In France, hemp hurds are used for aggregate and they have been building quite successfully with it. A formula developed by John Stahl is:
Lightweight AgStone Formula:
20 parts chips (any acricultural waste)
Plants which are high in silica include: Hemp Hurds, Western Sorghum, Concho Wheat, Corn, Bamboo, Lantana, Sunflower, and Medusa Head. We're searching for more information and more specifics on high silica plants.
5 parts clay (with sand is OK)
5 parts binders (3 lime, 2 cement)
Depending upon the scale of operations, several types mixers are used. The simplest is a 5 gallon bucket. A 1/2 to 1 hp electric motor with mixing blade is attached to the lid (plywood reinforced) of a 5 gallon bucket and small batches are mixed. A sharp 4-6 inch blade (S-blade is ideal) is mounted 4-6 inches from the bottom. Paper is inserted through a hole, or the operation is stopped and the lid is lifted to add fiber. For larger batches, a 55 gallon drum is cut in half and a gasoline engine from 3-10 hp is mounted on a board. A larger blade of up to 7-9 inches is attached to an extended shaft. Large round animal "stock" tanks, watering tanks, can be used for larger volumes. Shafts are extended by a collar attached to the motor shaft and a length of shaft is added. Replaceable blades are screwed on. For long shafts, some lateral reinforcement may be necessary to avoid excess stress on the motor or engine bearings. Place a bearing near the blade using a frame. Such mixers can be moved from tank to tank for mixing many batches or different ingredients.
Any ordinary cement mixer will work for agcrete. It can even be done by hand with a mixing box and rake. Paddle mixers also work.
To create really strong structures with a minimum of materials, engineering principles should be used. The best method is to create a honeycomb structure between two shells. This creates a highly insulated very strong structure. A low cost method is to apply a thin 1/2 to 1" initial layer to a lower portion of the supporting fabric covering a frame in order to begin the inner shell. Then a honeycomb of material is added on top. Short sections of lightweight plastic pipe, such as smooth, thin, ABS drainage pipe, are used to make circular forms (to create the holes). Common flower pots could also work. Slurry is scooped or poured out over a row of forms, such that 1/2" to 2" of material surrounds the holes and creates a continuous web of honeycomb. This process is repeated with additional rows of lightweight circular forms until the structure is covered in the honeycomb. In 24 hours or more, the cement will set and harden, the circular forms can be reused if carefully removed. Holes can be stuffed with balled newspaper or any lightweight, insulating material, then an outer shell of the desired thickness can be applied. Wasps build paper houses for their young with honeycomb shapes.
This same method of honeycomb building can be used for walls, too. Only the pots are placed reversing the direction with each layer or ABS pipe is used rather than pots. Window frames, door frames, windows, and doors can be cast from this same material. Frames are placed, then the wall is built around them. Beautiful paper windows, such as the Japanese are famous for, can be made by stretching paper on both sides of a window frame with large openings for light. The window is "caulked" into the frame with a bit more of the same material.