Yvonne Mak's floating Sandbag Castle tests ten low-carbon concretes

A floating castle built from more than ten experimental low-carbon concrete mixes now sits in the Dommel river beside Eindhoven's Van Abbemuseum Café. Visual artist Yvonne Mak's Sandbag Castle measures five meters wide and three meters high, and its hundreds of stacked, sandbag-shaped concrete modules do the arguing for her: conventional cement has an environmental cost, and the industry already has alternatives.

The installation doesn't just depict sustainable concrete. It is made of it. Mak cast the work in over ten distinct low-carbon formulations, developed with main partner TBI and a group of specialized material and technology contributors.

The recipe list reads like a materials conference program:

• calcined clay
• alternative binders
• geopolymer concrete
• recycled aggregates
• self-healing concrete formulations

Together, the mixes demonstrate existing and emerging structural alternatives designed to cut the CO₂ emissions of concrete construction. A castle, in other words, doing double duty as a lab bench.

Why sandbags?

The sandbag forms aren't decoration. Cast in concrete, they reference temporary flood barriers, pointing at how extensively societies rely on concrete for protective infrastructure. The material that causes the problem is also the material we reach for when defending against it. Mak lets that irony float, literally, in a river.

Red warning flags top the castle, borrowing the visual language of beach safety warnings. Each flag carries a custom coat of arms with symbols for water, bridges, flora, and hands — a compact emblem of where nature, infrastructure, and human intervention meet.

What does the installation change?

Nothing structurally, and that's rather the point. Sandbag Castle functions as a public demonstration that low-carbon concrete variants already exist and can carry real weight, in a civic waterway, next to a museum café. The project puts geopolymer and self-healing mixes in front of an audience that usually meets concrete only as backdrop.

With TBI involved as main partner, the material experiments have a route beyond art. Developed alongside specialized material and technology contributors, the formulations target the CO₂ emissions of concrete construction — a sector where every percentage point of binder substitution counts.

The installation sits in Eindhoven, Netherlands, and images of the work are credited to Yvonne Mak. For now, the castle floats as proof of concept: the alternatives to conventional cement are no longer hypothetical. They're stacked in a river, holding their shape.