Conventional cement production is one of the most carbon-intensive processes in construction, and most of the alternatives to it exist as material-science papers rather than things you can actually stand in front of. Artist Yvonne Mak’s Sandbag Castle closes that gap by building a castle out of them.
The piece floats in the Dommel river, beside the Van Abbemuseum café in Eindhoven, Netherlands — hundreds of concrete modules shaped like stacked sandbags, flying red warning flags bearing a custom coat of arms, in the half-serious, half-defensive posture of a fortification. What makes it more than a sculptural joke is the material list: the modules are cast from over ten different low-carbon concrete variants, including calcined clay, alternative binders, geopolymer concrete, recycled aggregates, and self-healing concrete — a working catalogue of the industry’s actual decarbonization options, rendered as a single walkable (or in this case, river-bound) object.
Mak’s stated aim is to demonstrate existing and emerging options that reduce the CO₂ emissions of concrete construction — not a single silver-bullet material, but the range of what’s already buildable. The piece was produced with TBI as main partner alongside specialized material and technology contributors, and was submitted through designboom’s reader-submission DIY feature rather than commissioned by a gallery or institution, which fits its slightly guerrilla, structure-first framing.
There’s something pointed about the choice of form. A castle under siege is usually defending something fragile; here the thing under threat is the construction industry’s status quo, and the “defenders” are the alternatives.