mixtures with fine schist aggregates an increase in aggregate
content leads to an increase in tensile strength, however
mixtures with an aggregate/binder ratio of 1.5 show a strength
decrease due to an increase in H2O/Na2O molar ratio, from 10.3
to 10.8 when water had been added to the mixture to keep a
similar workability. The same happens in mixtures with granite
aggregates. However when using limestone aggregates the
results are random with maximum tensile strength occurring at a
low content of aggregates and for 7 days curing, decreasing
after that day, but the mixture with an aggregate/binder ratio of
0.75 present lower strength at 7 days curing, rising with curing
time. The explanation may rely on the fact that strength
evolution is somehow affected by shrinkage, which is currently
associated with the presence of lime in alkaline activated
binders. As it can be seen in Fig. 4 from shrinkage behaviour of
mixes with granitic aggregates (GR-AG/L 0.5 and GR-AG/L
1.0) which has the same water content but a very different
shrinkage level.
Tensile strength begins to decrease after 14 days of curing in
all cases, except for mixtures with coarse granitic aggregates
with an aggregate/binder ratio of 0.75, although it is not clear at
that stage why that happens. Besides it must be noted that
shrinkage shows a maximum at 14 days curing, so tensile
strength decrease may be due to shrinkage cracks near the
aggregates.