Page 8 - July-Open-Access-2020

TECHNICAL PAPER

(a) (b)
Figure 7: Examples of changes in the proportions of the geopolymer constituents (mK + sodium silicate + water) and of the Water/solid mass
ratio as a function of the Na O/al O or siO /al O molar ratio, keeping the two other molar ratios constant, including the H O/Na O ratio
2 2 3 2 2 3 2 2
(data from ).
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This is illustrated by Figure 7, which shows the evolution of the ratios insignificant, in contrast to activated aluminosilicates.
proportions of the geopolymer constituents (mK + sodium It is therefore quite common to express the activator as a
silicate + water) and the Water/solid mass ratio as a function percentage relative to the slag, since the slag is generally almost
of the molar ratio Na O/al O or siO /al O , while maintaining completely reactive. Furthermore, the more the activation rate is
2 2 3 2 2 3
constant the two other molar ratios, including the H O/Na O increased, the more effective the activation (Figure 8a). However,
2 2
ratio. Note that even when keeping the H O/Na O ratio it can be noted that the mechanical behaviors can be very
2 2
constant, this systematically leads to a variation in the Water/ different from Portland cements:
solid ratio. It has been shown that the Water/solid ratio has, as - The lack of activator (too low proportion) is very penalizing
for Portland cements, a very significant effect on the mechanical at young age, especially at 1 and 2 days. Figure 8a shows
performance of geopolymers (Figure 4). that more than 3% Na O is necessary to expect a minimum
2
of strength at 1 day.
4.4 Formulation of alkali-activated binders with
high calcium content - On the other hand, the short-terms increase in strength
can be remarkable, as attested by the value of 38 mPa at 1
activated slags are very different from geopolymers due to day when 8% of Na O is used.
2
their calcium content, allowing them in particular to produce - Performance at 28 days can reach values as high as
C-a-s-H. Thus, the presence of calcium makes the use of molar
Portland cements.
activators can nevertheless vary within the same category, so it
is necessary to specify their composition. Thus, sodium silicates
are defined by their SiO /Na O molar ratio and several ratios
2 2
are commercially available, with more or less silica and alkalis.
It is therefore obvious that the effectiveness of a silicate, for a
given quantity, will depend on its composition. The use of two
simultaneous activators can sometimes improve the behavior of
the mixture. Figure 8b shows the effect of adding 5% CEm I to
the activated slag of Figure 8a. It can be seen that the strength
at 1 day is greatly improved for mixtures with a low Na O
2
content (5 to 10 mPa against 0 when there is no cement), but
that performance at high Na O content is degraded at all ages.
2
It is therefore advisable to formulate the alkali-activated binders
according to the desired applications, which may or may not
require performance at a young age. Furthermore, it is very likely
Figure 8: Effect of the alkali silicate level (expressed as % Na O relative
2
to the precursor) on the compressive strengths of an activated slag, at that the durability of these materials will be impacted by the rate
1, 2 and 28 days. (a) 100% GGbs (b) 95% GGbs and 5% CEm I of formation of the reaction products, like in Portland cement
(data from ) applications.
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12 The IndIan ConCreTe Journal | JulY 2020