Developed by Paul Schwam, a local architect/builder, lava concrete (LC) is a made from a porous, lightweight volcanic aggregate called scoria, which is mixed to form a self-supporting semi-fluid. When cured, it becomes a lightweight stone. Scoria’s porosity holds pockets of water to help cure the cement. Once the water has evaporated, tiny air cavities remain to provide sound and thermal insulation. LC is used for foundations, floors, walls, and roofs. It provides the structure, insulation, thermal mass, and the finish—all in a single and seamless, raw-to-finish process.
Because LC starts as an easily workable semifluid, any shape or detail can be added to architectural elements—planes and curves; textures and details; and accessories, such as fireplaces, furniture, moldings, drip edges, window/door trim, niches, and artwork. Builders can integrate details such as ledgers, beam pockets, utility raceways, and block-outs—eliminating material and labor steps along the way.
LC helps optimize the sequence of construction. Electrical, plumbing, and other utilities install first into empty forms. The structural wall is cast as the final step. The permanence of the concrete and stone structure will remain fresh, even with periodic future upgrades to utilities, internal layout, and furnishings.
After site preparation and installation of subgrade utilities and footings, rebar and forming went directly on top of the concrete foundations. Forms were constructed from treated 4-by-8 plywood with cut-outs for window and door openings. Once utility raceways were installed, pouring was done by three people: one operated the delivery chute; another tended to the formed details, lightly compacting the LC to assure no voids; and the third monitored the mixing machine and conveyors.
The mixing machine was key to the process and specifically designed to combine the cinder sand (scoria) and Portland cement with injected water to form a relatively dry mix. Conveyors delivered the LC to the forms, where it was poured 2 to 8 feet high in lifts. A 24-foot-tall wall may require three to 12 pours.
The lava sand used was a deep red color, from Flagstaff, Arizona. Blended with about 20% Portland cement, the poured fill cures similarly to regular concrete. Sandblasting the surface restored much of the natural volcanic color that was grayed by the cement. Once sealed (water-based Okon was used for Del Fierro), the exterior requires no additional maintenance.
One of the advancements developed during our project was a post/anchor bolt system, which eliminated the need for precast anchors, and greatly simplified the hand-off to other trades. It allowed operations, such as attaching ledgers, to be done simply by pre-drilling and insertion of large bolts. During construction, tests were performed on the strength of the attachment system, under the supervision of structural engineers to comply with the local building department. Windows and doors were installed in the blanked-out openings and secured with polyurethane foam and an LC-based grout.
Due to the complexity and volumetric size of this house, the cost per square foot for the basic shell was about $85. For modest-scale projects, LC can be constructed more cost-competitively.