GRG vs GRC —
and Where Mineral Cast Fits

GRG and GRC get quoted as if they were two grades of the same product. They aren't. One is a gypsum system for the inside of a building; the other is a concrete system built to stand outside in the weather. Reading them as interchangeable is where a specification goes wrong. This guide separates the two on the axes an architect actually resolves at design stage — base material, environment, weight, and the kind of surface each one carries — and then names the case neither system is designed to serve.

What Is GRG?

GRG is Glass Reinforced Gypsum: a gypsum plaster cast around a glass-fibre matrix — the modern form of fibrous plaster. The fibre does the structural work, letting a thin gypsum shell span a large hollow moulded shape — a barrel vault, a dome segment, a deep cornice run, a column casing — without cracking under its own reach. It takes fine plaster detail well, which is why it is the traditional material for moulded interior ornament at scale: cornices, ceiling roses, run mouldings, produced from reusable moulds in repeat.

It is a dry-interior material — GRG is not rated for wet rooms or exterior exposure. Its economy is in repetition: cast a mould once, run the moulded form many times. That is a different job from a one-off relief authored for a single wall — held to a specified depth and register, documented, and delivered made-to-order — which is the case we return to below.

What Is GRC?

GRC is Glass Fibre Reinforced Concrete: a cement-and-sand matrix reinforced with alkali-resistant glass fibre. Swap the gypsum binder for cement and the material changes character entirely. GRC is weather-durable and impact-durable, which is why it belongs on facades, exterior cladding, soffits and site furniture. It reads as concrete — a coarser, harder surface than plaster — and it is markedly heavier: at a 15 mm section a GRC panel is on the order of 39 kg/m², several times the weight of a timber panel of the same thickness. That mass is the point outdoors and a liability on an interior partition.

Read this way, the choice between them is not a contest. It is one question answered first — where does the surface live? — and everything else, fixing, finish, format, follows from the answer.

The Case Neither Was Built For

Both GRG and GRC are reinforcement systems, and both are built around repetition — a moulded form or a cladding unit, cast to a catalogue and run in quantity. That is a real problem and they solve it well. A design-led feature surface poses a different one: fidelity — rendering a specific, authored relief, at a chosen depth and register, across a whole wall without the design drifting from panel to panel. Not a stock moulding run many times, but one motif, made for one project.

That is the gap a bespoke cast relief panel is made to close. At Studio Luminant the relief is not a by-product of the reinforcement — it is the specification. The panel is cast on your concept, not a catalogue's, and the same authored design can run across a room in the right material for each location: Lumina PMAG™, a polymer-modified alpha gypsum, for interior walls and ceilings; Lumina PUCOMP™ where the surface meets wet areas, exterior exposure, or complex curvature. One design, one finish schedule, two materials underneath — so a lobby feature wall and the covered-terrace surface beyond it read as one continuous idea rather than two suppliers' approximations of it.

Lumina PMAG™ is over 95% mineral by mass and cast by vacuum casting against a 3D-printed mould master — the route that lets it carry a crisp, project-specific relief line rather than a stock profile. It arrives the way a specifier needs it to: in large format, moisture-resistant, ceramic-smooth, finished in artisan finishes, and documented as a BIM object in IFC format for the specification set.

The Geometry a Rigid Tool Can't Cut

There is one more axis that separates a made-to-order cast surface from every milled or moulded alternative, and it only matters once the others are settled. A router or a rigid press works line-of-sight: it can only reach what it can withdraw a tool from cleanly. A cast surface has no such limit, because the mould is flexible silicone. It releases captive geometry — undercuts, woven passes, a relief that folds back on itself — that a rigid tool physically cannot cut and a rigid mould cannot demould.

For most projects this is a detail. For the project that hinges on a specific tactile idea, it is the whole reason to cast rather than mill. The mould, not the gypsum, is what releases the captive form — which is why, past this point, comparing reinforcement systems stops being the useful question.