Cardboard is disposable. Rammed earth is ancient but weak in tension. Neither is taken seriously as a structural material.
Cardboard tubes packed with rammed earth achieve compressive strength comparable to cement-stabilized rammed earth — at less than one-third the cost and one-quarter the carbon footprint. The rammed earth core enhances the structural performance of cardboard more than tenfold. With carbon fiber reinforcement, the composite reaches performance comparable to high-performance concrete.
The mechanism is confinement. Compressive strength in brittle materials is often limited not by the material's intrinsic resistance to compression but by lateral expansion under load — the Poisson effect pushes material sideways, initiating cracks. A tube that confines this lateral expansion transforms the failure mode from brittle splitting to ductile yielding. The material inside doesn't change; the geometry around it changes what “failure” means.
Cardboard provides exactly this confinement. It resists hoop stress — tension around the circumference — which is what prevents lateral expansion. A material associated with fragility and disposability becomes a structural confinement system when given something to embrace.
The gap between “weak” and “strong” is often not a gap in composition but in boundary conditions. Soil inside a cardboard tube is the same soil outside it. The tube doesn't make the soil stronger — it prevents the soil from failing in the mode that would have destroyed it. Compressive strength, in this case, is a geometry problem, not a materials problem.