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TECHNICAL PAPER

particularly effective (in general more than hydroxides) because 3.2.3 Alkali-sulfates
they already contain silica in solution, which makes it easier to
initiate the formation of reaction products. The setting can be These activators are partially effective for slag, but practically
fast and the development of strength too, so that it is possible ineffective for aluminosilicate precursors because of their low
pH. They must therefore be used in combination with another
to reach more than 20 mPa in 1 day. maintaining the rheology activator, or with an additive to increase the pH (clinker for
is nevertheless difficult beyond 30 minutes (mainly with GGBS), example).
unless additives are used. Furthermore, these activators are
highly corrosive and can be difficult to use elsewhere than under
4. FORMULATION OF ALKALI-ACTIVATED
supervised conditions (precast for example).
BINDERS AND GEOPOLYMERS
alkali-silicates are generally in the form of aqueous solutions, – What are the parameters that must be taken into account
more or less viscous, composed of siO , m O (m = Na or K) and
2 2 for the formulation, other than the quantities of binders
water. They are characterized by the modulus siO/m O and
2 and water?
their water content. The most common are sodium silicates,
– should we systematically use molar elemental ratios to
mainly for cost reasons (KOH is more expensive than NaOH).
The higher the modulus, the less alkalis, so that the moduli target a required performance?
around 3 (used in detergents for example) are not enough
4.1 Quantity of binder
alkaline to activate the precursors correctly. The reduction of
the modulus to values of 2 and less is then preferable to allow It is usual for concretes made of Portland cement to fix a
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good activation of the system. However, it can be difficult to quantity of binder (ex: 300 kg/m ), which makes it possible to
commercially produce stable sodium silicate solutions with target given performances, either in strength or in durability to
one or several aggressions. This is possible because cements
moduli less than 1.7 (precipitation of silicate compounds in the have performances guaranteed by the standards (strength at
solution), unless the quantity of water in the product is greatly young age and 28 days for EN 197-1). Thus, a given cement
increased. This is generally little practiced because it is not must make it possible to achieve mortar strength at a given age
economically viable (little active material and a lot of water), in by fixing a cement content and a water/cement ratio.
addition to the fact that the active phases (siO and Na O) would
2 2 In the case of alkali-activated materials, there are many
be too diluted and this would lead to putting too much water difficulties in using the same approach:
in the mixture, which is very harmful for performance (section
3.2). One way to lower the modulus of sodium silicates is to use – The variety of precursors makes systematic generalization
them after drying (for example by spraying). modulus 1 sodium difficult, because the reaction products are different
metasilicates are therefore commercially available, but these depending on the precursors and the same class of
precursor may contain very variable amounts of active
have a high cost.
material. Thus, it is difficult, during the development of a
formula, to fix the quantity of binder in kg per m , as for
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3.2.2 Alkali-carbonates Portland cement concretes.
The main advantage of alkaline carbonates is that they are not – In addition, one can wonder about the fact of counting or
corrosive, therefore less dangerous to use than alkali-hydroxides not the activator in the binder. The amount of activator is
and silicates. available in solid form, they are also cheaper than often far from being negligible, since it can reach several
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alkali-silicates. available in solid form, they are also cheaper tens of kg/m (in dry extract) in the case of metakaolins
activated by silicates . What happens when the alkali-
[6]
than alkaline silicates. However, they are essentially effective
with precursors such as GGbs, according to a delayed soda silicates is dissolved in water?
production mechanism , because their equilibrium pH in – The performance of a precursor totally depends on the
[9]
water is too low to directly attack the glass network of the slag. nature of the activator used, so that a given quantity of
They must therefore be used in large quantities to improve the binder will have very different performances depending on
reaction kinetics, unless they are combined with a more alkaline the precursor-activator pair.
activator (hydroxide, silicate or cement/clinker). Therefore, it is unlikely that the quantities of binder given in
European concrete standard EN 206 are representative of
Carbonates appear to be ineffective in activating aluminosilicate the values necessary to achieve mechanical performance or
precursors because they are unable to provide the pH necessary durability. Without the possibility of generalization on the
for the destruction of the glass/amorphous network of fly ash or quantity of binder, it seems that a performance-based approach
metakaolins. is more suitable in order to achieve the objectives set.

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