Fibre Reinforced Facade Encyclopedia: Composition
The cement matrix is the binder phase of glass fibre reinforced concrete (GRC, GFRC): Portland cement, graded fine silica sand, water, a superplasticiser and usually a polymer and a pozzolan. It holds the glass fibre in place, transfers load into it, gives the panel its compressive strength, surface and fire performance, and slowly changes the composite over decades. Understanding that last point is what separates a durable panel from one that becomes brittle.
A typical sprayed mix is one part cement to one part sand by weight at a water to cement ratio of 0.30 to 0.35. There is no coarse aggregate at all; the largest particle is about 1 mm.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Cement to sand ratio | 1:1 by weight | Some facing mixes run 1:0.5 |
| Water to cement ratio | 0.30 to 0.35 | Low; superplasticiser makes it workable |
| Sand grading | 0.15 to 1.0 mm | Washed, dried, rounded silica sand |
| Sand silica content | above 96% SiO₂ | Low soluble salts and clay content |
| Pozzolan replacement | 0 to 25% of cement by weight | Metakaolin, silica fume or fly ash |
| Acrylic polymer solids | 5 to 7% of cement by weight | Where the mix is polymer modified |
| Pore solution pH | 12.5 to 13.5 | The environment the fibre must survive |
| Matrix compressive strength | 50 to 80 MPa | At 28 days, sprayed mix |
| Matrix density | 1,900 to 2,100 kg/m³ | Sprayed and roller compacted |
What is in the mix and why
The matrix has no coarse aggregate. A panel skin of 10 to 20 mm sprayed around chopped glass fibre cannot accommodate stone, so the aggregate is a washed, graded, dried silica sand between about 0.15 and 1.0 mm. It must be high in silica, low in clay, low in soluble salts and consistent, because a fine mix has a large aggregate surface area and any contamination shows up as efflorescence, colour variation or set problems.
The water to cement ratio is kept at 0.30 to 0.35, far lower than ordinary concrete, to give high strength and low permeability. A mix that dry would be unsprayable, so a polycarboxylate superplasticiser disperses the cement and restores workability without adding water. Pozzolans, most often metakaolin, replace 10 to 25 percent of the cement and do two things: they refine the pore structure and they consume free calcium hydroxide, which matters for the reason set out below. An acrylic polymer is added in most modern mixes, covered on the polymer modified GRC page.
The cement itself is a general purpose Portland cement to AS 3972, usually white or off white where colour is architectural, since integral colour and oxide pigment both depend on the cement being pale enough to take them.
What the matrix does to the fibre over time
This is the part that is usually glossed over. Fresh Portland cement produces a pore solution at pH 12.5 to 13.5 and, as it hydrates, precipitates crystalline calcium hydroxide. Alkali resistant glass fibre with 16 percent or more zirconia resists the chemical attack from that alkalinity, and that problem is genuinely solved. A second mechanism is not chemical at all.
Each glass strand is a bundle of hundreds of fine filaments with spaces between them. In a young panel those spaces are open, the bundle can slide and stretch, and fibres pull out progressively when the matrix cracks, absorbing energy. Over years, calcium hydroxide crystallises into those interstitial spaces, filling them and locking the filaments together. The bundle stops behaving as a flexible rope and starts behaving as a rigid rod, and load concentrates on individual filaments instead of being shared. The composite keeps most of its first crack strength, the limit of proportionality, but loses strain capacity and a substantial part of its modulus of rupture, and it fails in a more brittle way.
How the matrix is adjusted to slow it down
- Pozzolans: metakaolin or silica fume at 10 to 25 percent replacement react with calcium hydroxide and convert it to calcium silicate hydrate, so there is less of it available to crystallise in the fibre bundle
- Acrylic polymer: at 5 to 7 percent solids by weight of cement the polymer film occupies the interstitial space itself and cushions the filaments
- Low water to cement ratio: a denser matrix slows ion transport through the panel
- Low alkali and calcium sulfoaluminate cements: reduce the alkalinity of the matrix rather than arming the fibre, covered on the low alkali cement page
- Adequate curing: proper hydration produces a dense matrix; a poorly cured panel is permeable and ages faster in every respect
None of these replaces the requirement for alkali resistant glass. They are all secondary strategies layered on top of a fibre that already meets the 16 percent zirconia minimum.
Curing and what the matrix needs
An unmodified mix requires about seven days of moist curing after demoulding at 16 to 24 hours, because cement hydration needs water and a thin section loses it fast. A panel that dries out early is permeable, weak and prone to shrinkage cracking. Polymer modified mixes form a film that retains internal moisture and allows air curing instead, which is why almost all production mixes are polymer modified: the curing shed is the constraint, not the chemistry.
Drying shrinkage is a matrix property and matters at panel scale. Typical values are 0.1 to 0.2 percent, which is why movement joints are designed into a facade rather than relied on to be small enough to ignore.
Fire behaviour
A cement matrix contains no polymer backbone to burn, which is what allows a glass fibre reinforced concrete product to be tested and classified as non-combustible. ShapeShell-RC holds an AS/NZS 1530.1 result of Not Deemed Combustible from Ignis Labs. The classification belongs to the tested product at its tested composition and thickness: polymer content, pigments, coatings and any stud frame or insulation behind the skin are assessed separately.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- BS EN 1169 (factory production control) and BS EN 1170 parts 1 to 8 (test methods)
- AS 3972 General purpose and blended cements
- AS 1478.1 (chemical admixtures for concrete) and AS 3582 (supplementary cementitious materials)
- ASTM C1228 and ASTM C1230 (GFRC test methods, US)
- AS 1530.1 and NCC 2022 Volume One C2D10 (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
What is in a GRC mix?
Portland cement and fine graded silica sand at roughly one to one by weight, water at a 0.30 to 0.35 ratio, a polycarboxylate superplasticiser, usually an acrylic polymer at 5 to 7 percent solids by weight of cement, often a pozzolan such as metakaolin, and alkali resistant glass fibre. There is no coarse aggregate.
Why is there no gravel in GRC?
The skin is only 10 to 20 mm thick and is sprayed or premixed around chopped fibre. Coarse aggregate would not fit, would disrupt fibre distribution and would wreck the surface finish. The largest particle is about 1 mm of silica sand.
Does the cement matrix attack the glass fibre?
Chemically, yes, which is why alkali resistant glass with at least 16 percent zirconia is mandatory. A second effect is physical: calcium hydroxide crystallising into the spaces between filaments stiffens the fibre bundle over years and reduces strain capacity, which is why design uses aged property values.
Does the matrix make GRC non-combustible?
It is what makes non-combustibility achievable, because there is no polymer backbone to burn. The classification itself still has to be earned by testing the finished product; ShapeShell-RC is Not Deemed Combustible to AS/NZS 1530.1, tested by Ignis Labs.
What cement should be specified?
A general purpose Portland cement to AS 3972, white or off white where the panel carries integral colour. Pozzolan replacement of 10 to 25 percent is common and beneficial. Low alkali and calcium sulfoaluminate cements are a separate durability strategy with their own trade-offs.
Reviewing a GRC mix design or a durability submission? ShapeShell-RC is batch tested for modulus of rupture and limit of proportionality to the GRCA specification. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.