Ca₄O₁₆Si₄V₄

Architectural potential

A vanadium-doped calcium-silicate in the low-CO₂ binder register, sitting at the non-structural end of the family — 50 MPa of compression at 3.34 g/cm³ and an eighteen-per-cent CO₂ cut against Portland. The visible-absorber chemistry is the architectural hinge: the V⁴⁺ sites pull visible light into the body as heat rather than reflecting it, making the slab a real-colour solar-gain collector rather than a passive binder. Natural placements are south-facing thermal-wall infill panels, cast trombe-wall slabs behind glazing in passive-solar retrofits, absorber panels for solar-preheat air chambers, and warm-toned cladding on walls where daytime heat storage is the point. At the 1000 g batch scale and 5000 g workshop ceiling, prefabricated cladding panels and thermal-wall blocks both come within reach. Against Portland concrete the argument is a modest environmental improvement combined with a functional role Portland does not carry — absorbing rather than reflecting the visible spectrum. The caveat is cost: €51.75 per 1000 g batch is an order of magnitude above conventional cement, so specification stays project-scoped — thermal-mass pieces in passive-solar architecture — rather than general pour. The 1100 °C vitrification fire also keeps the kiln in the supply chain.

Material character

The 85×85×41 mm slab reads a subtle white-grey with a faint vanadium-blue undertone emerging in shadow — a cooler cast than standard Portland, closer to a low-iron Portland white. At 3.34 g/cm³ the piece is firm-handed and sits two-handed without strain, a weight register between cement mortar and fired clay tile. Surface fires to a partially vitrified skin, slightly glassy under raking light where the flux has run. Edges cold-cut, kerf-white. Against pure Portland concrete the body reads denser and slightly darker, and under direct sun the slab warms perceptibly faster than an ordinary cement test piece — the visible-absorber signature in hand.