Fibre Reinforced Facade Encyclopedia: Composition
Low alkali cement is cement with a reduced alkali content, and in glass fibre reinforced concrete it is shorthand for a second durability strategy: rather than only arming the glass fibre against the matrix, reduce the aggressiveness of the matrix itself. It is a legitimate approach, but the term is used loosely and covers two very different things, only one of which meaningfully changes the environment the fibre sits in.
Reducing a Portland cement from 0.9 to 0.5 percent alkali equivalent barely moves the pore solution pH, because calcium hydroxide buffers it at about 12.5. A calcium sulfoaluminate cement, which produces no free calcium hydroxide, sits closer to pH 10.5.
At a glance
| Property | Typical value | Note |
|---|---|---|
| ASTM C150 low alkali definition | below 0.60% Na₂O equivalent | Na₂O plus 0.658 times K₂O, by weight |
| Typical Portland cement alkali | 0.4 to 1.0% Na₂O equivalent | Varies with clinker source |
| Portland pore solution pH | 12.5 to 13.5 | Buffered by calcium hydroxide |
| Calcium sulfoaluminate pore solution pH | about 10.5 to 11.5 | No free calcium hydroxide produced |
| Calcium aluminate cement pore solution pH | about 11.4 to 12.5 | Conversion behaviour must be managed |
| Metakaolin replacement | 10 to 25% of cement by weight | Consumes calcium hydroxide by pozzolanic reaction |
| CSA setting time | 5 to 40 minutes | Much faster than Portland; needs retarder control |
| Cost relative to Portland | roughly 1.5 to 3x | CSA and calcium aluminate cements |
| Effect on fibre requirement | None | Alkali resistant glass is still mandatory |
Two meanings of the same phrase
In cement standards, low alkali has a formal definition. ASTM C150 sets a limit of 0.60 percent alkali equivalent, calculated as the sodium oxide content plus 0.658 times the potassium oxide content. That specification exists to control alkali silica reaction, in which alkalis in the cement react with reactive silica in the aggregate and cause expansion and cracking. It is an aggregate compatibility limit and it has almost nothing to do with glass fibre durability.
The reason is that the pH of the pore solution in a hydrated Portland cement is not set by the sodium and potassium content alone. Hydration produces calcium hydroxide, which is only moderately soluble but is present in large excess, and it buffers the pore solution at around pH 12.5 whatever the alkali equivalent happens to be. Moving a cement from 0.9 to 0.5 percent alkali equivalent is worth having, and it does reduce the sodium and potassium concentration in the pore water, but it does not take the fibre out of a strongly alkaline environment.
The routes that genuinely lower matrix alkalinity
- Calcium sulfoaluminate cement: hydrates to ettringite and monosulfate rather than to calcium silicate hydrate plus calcium hydroxide. Because it produces no free calcium hydroxide, the pore solution sits around pH 10.5 to 11.5 and the second, physical ageing mechanism of calcium hydroxide crystallising into the fibre bundle is largely removed
- Calcium aluminate cement: also free of calcium hydroxide, with a pore solution around pH 11.4 to 12.5, but subject to conversion of its hydrates over time with an associated strength loss, which has to be designed for
- High pozzolan replacement: metakaolin or silica fume at 10 to 25 percent consumes calcium hydroxide by pozzolanic reaction, lowering both the amount available to crystallise and, modestly, the pH
- Low alkali Portland clinker: a marginal gain as described above, worth specifying but not decisive
Of these, calcium sulfoaluminate cement is the one that changes the picture rather than shading it. Several proprietary glass fibre reinforced concrete systems are built on it for exactly this reason.
The trade-offs
Calcium sulfoaluminate cement sets very fast, often within 15 to 40 minutes, which is workable in a controlled factory but unforgiving. It needs its own retarder and superplasticiser package, since admixtures optimised for Portland cement do not always behave the same way. Its early strength is high and its long term strength gain is flatter. Supply in Australia is limited compared with Portland cement, which affects both price and lead time, and the colour differs, which matters where integral colour or a matched panel run is required. The long term field record is shorter than the seventy years behind Portland cement, so durability arguments rest more on accelerated testing and less on age.
Calcium aluminate cement carries the additional complication of conversion, in which its initial hydrates transform to a denser, lower volume form over months or years, increasing porosity and reducing strength. It is a known and designable effect, but it must be designed for rather than discovered.
Pozzolan replacement is the low risk option. Metakaolin at 10 to 25 percent is a normal part of a modern mix, costs relatively little, improves density and surface, and requires no change in process.
It does not replace alkali resistant glass
This is the point that has to be stated plainly. Reducing matrix alkalinity is a supplementary strategy. Alkali resistant glass fibre with 16 percent or more zirconia remains mandatory in glass fibre reinforced concrete, and the GRCA specification does not offer a lower alkalinity matrix as an alternative to it. E glass fibre in a calcium sulfoaluminate matrix is still a specification failure, because pH 11 is still strongly alkaline and because a panel outlives the certainty anyone has about its matrix.
The sensible position for a specifier is to require the zirconia content as a non-negotiable minimum, then treat pozzolan content, polymer modification and cement type as the additional durability measures that let a manufacturer justify the long term design values they are using.
Fire behaviour
All of these are inorganic cements with no polymer backbone, so none of them compromises the ability of 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. As always the classification belongs to the tested product at its tested composition, and a change of binder is a change of product that needs its own test evidence.
Standards and references
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- ASTM C150 (Portland cement, including the 0.60% alkali equivalent low alkali option)
- AS 3972 General purpose and blended cements; AS 3582 supplementary cementitious materials
- BS EN 14647 (calcium aluminate cement) and BS EN 1170 series (GRC test methods)
- BS EN 15422 (specification of glass fibres for reinforcement of mortars and concretes)
- AS 1530.1 and NCC 2022 Volume One C2D10 (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
Does low alkali cement mean alkali resistant glass is not needed?
No. Alkali resistant glass with at least 16 percent zirconia remains mandatory in glass fibre reinforced concrete. Reducing matrix alkalinity is a supplementary durability measure, never a substitute for the fibre specification.
What does low alkali actually mean in a cement specification?
In ASTM C150 it means below 0.60 percent alkali equivalent, calculated as sodium oxide plus 0.658 times potassium oxide. That limit was written to control alkali silica reaction with aggregate, not to protect glass fibre.
Why does reducing the alkali content not lower the pH much?
Because hydrating Portland cement produces calcium hydroxide in large excess, and that buffers the pore solution at around pH 12.5 regardless of the sodium and potassium content. Lowering pH meaningfully requires a cement that produces no free calcium hydroxide.
What is calcium sulfoaluminate cement?
A cement that hydrates to ettringite rather than to calcium silicate hydrate plus calcium hydroxide, giving a pore solution around pH 10.5 to 11.5. It sets fast, costs more, has limited supply in Australia and a shorter field record, but it genuinely changes the fibre environment.
Is a lower alkalinity matrix worth specifying?
It is worth asking a manufacturer what their durability strategy is and what long term design values they justify from it. Pozzolan replacement and polymer modification are the low risk measures; a change of cement type is a bigger decision with cost, colour and supply consequences.
Assessing a GRC durability submission? ShapeShell-RC is produced with alkali resistant glass to the GRCA specification and batch tested for MOR and LOP. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.