Ca₄Co₂Fe₂O₁₆Si₄

Architectural potential

A cobalt-iron-modified calcium silicate fired at 1100 °C, 50 MPa, with a recorded density of 0.35 g/cm³ that flags the piece as a porous or foamed cast rather than a dense binder. Against Portland cement the embodied CO₂ is down eighteen per cent and energy down eighteen per cent, so the environmental argument is genuine though modest. At 50 MPa the piece sits firmly in the non-reinforced non-structural-topping register: pavilion wall infill panels, landscape retaining-wall blocks in garden rooms, floor screeds in retrofit carpentry over timber sub-floors, prefabricated cladding panels on light steel-frame additions, and cast plinths for outdoor sculpture. The cobalt-iron colouring is the specification difference — the client is buying a blue calcium-silicate binder for visible cast work rather than a plain grey one, which suits work where the binder is the finish and no render goes over it. The caveat is cost: €42.51 per 1 kg workshop batch brings the cobalt load into play at binder scale, an order of magnitude more than the plain white-grey sibling, and the low recorded density suggests a foamed or air-entrained mix that needs verification for exterior freeze-thaw work before any exposed landscape use.

Material character

A saturated deep cobalt blue carried by the calcium-silicate matrix, reading more muted than a cobalt-blue soda-lime glass but clearly more saturated than a conventional pigmented concrete — the iron co-doping warms the blue slightly into a sea-green direction under raking light. At 0.35 g/cm³ and 120×120×197 mm the slab firms single-handed — a softwood register despite its block-sized footprint, the weight telling of a porous or foamed cast. Surface cold-pressed chalky, typical of ambient and low-fire calcium-silicate bodies; edges cold-cut, kerf whitened with carbonation powder. Fully opaque through its depth. Distinctly cooler in colour and noticeably lighter in the hand than the neutral Ca9Ce1O1Si6 sibling of the same family.