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TECHNICAL PAPER

are not those of a cement or even a slag. activated mKs are Figure 6. (a) Relative setting time (reference: siO /al O = 3.75)
2 2 3
composed of silicon chains integrating aluminum atoms. They and (b) compressive strengths relative to 28 days (reference:
do not have a fixed stoichiometry, their composition depending maximum value of the series), as a function of the siO /al O
2 2 3
on the quantities of silicon (from mK and alkali-silicate) and molar ratio (reactive fraction of mK), for four commercial
aluminum (from mK) initially inserted in the mixture. The practice metakaolins with purities between 50 and 95%. Na O/al O =
3
2
2
consists in targeting performance by varying the proportions 0.9; H O/Na O = 14.5.
of the constituents, which therefore modifies the composition 2 2
of the reaction products. It turns out that the performances 4.3.2 Molar ratio Na O/Al O
2 2 3
obtained depend on the ratios of the oxides in the geopolymer,
+
so an optimum is targeted but it remains difficult to achieve The Na O/al O molar ratio is often set around 1, so that the Na
3
2
2
because of the multitude of compositions of precursors and compensate for the free charges of the aluminum atoms in the
activators available. network. Low ratios often lead to lower reactivity (due to lack
of alkalis), while very high ratios involve an excess of free alkalis
For example, in the case of mK activated by sodium silicates, leading to the appearance of efflorescence (carbonated alkali
one of the ways of ensuring the best mechanical performance in ions).
compression is to fix a certain number of ratios of the different
oxides present (siO , al O , Na O, H O). It should be noted that 4.3.3 Molar ratio H O/Na O
2 2 3 2 2 2 2
the oxides of silicon and aluminum of metakaolin to be taken
into account are those coming from the amorphous phase, The H O/Na O molar ratio is an indicator of the alkali
2
2
since it is generally considered that the crystallized phases (for concentration of the mixture and it sometimes (but rarely, see
example quartz and mullite) do not participate in the reaction. below) allows to control the water content of the geopolymer.
However, it should be taken as low as possible to improve the
4.3.1 Molar ratio SiO /Al O performance of the material.
2 2 3
The siO /al O molar ratio makes it possible in particular to 4.3.4 Mass ratio Water/Solid
2 2 3
control the performances over time. Its increase has the effect
of delaying the setting of the geopolymer (Figure 6a), from a To these molar ratios, it is generally necessary to add a water/
few hours to several tens of hours (3h to 50h for the geopolymer solid mass ratio, in order to control the porosity of the mixture,
made with mK1, i.e. a factor of 16). The evolution of this ratio as the H O/Na O molar ratio is not sufficient to keep the
2
2
(Figure 6b) also leads to a variation in compressive strengths, amount of water constant in the geopolymer. This is due to
the typical optimal values for activated mKs being around 3.6- the fact that the siO /al O , Na O/al O and H O/Na O molar
2
3
2
3
2
2
2
2
4.0, regardless of the mK used (provided that only the reactive ratios are interdependent and it is impossible to fix two while
fraction of the material is count in the ratio - see section 2.1). varying the third one without impacting the Water/solid ratio.

(a) (b)
Figure 6: (a) Relative setting time (reference: siO /al O = 3.75) and (b) Compressive strengths relative to 28 days (reference: maximum value of the
2
3
2
series), as a function of the siO /al O molar ratio (reactive fraction of mK), for four commercial metakaolins with purities between 50 and 95%.
2 2 3
Na O/al O = 0.9; H O/Na O = 14.5.
2 2 3 2 2
The IndIan ConCreTe Journal | JulY 2020 11