3.2. Compressive strength
Compressive strength of alkali-activated mine waste mud
(GMWM) binders is shown in Figs. 1 and 2. Plain geopolymeric
mine waste mud paste was named PCP. GMWM binders using
schist (Sc) fine aggregates with an aggregate/binder ratio of 0.5
were named SC-AG/B 0.5. Similarly when limestone (LS) or
granite (GR) aggregates were used they were named respectively
LS-AG/B 0.5 and GR-AG/B 0.5.
GMWM based binders showed very high strength at early
ages, even higher than alkali-activated mixtures made with more
reactive aluminosilicate raw materials like metakaolin [16]. It
seems that strength level is due to calcium hydroxide and iron
oxide which provides extra potential nucleation sites [34,35].
Compressive strength of GMWM binders made with schist
sand are very similar when aggregate/binder ratio varies from
0.5 to 1.5 (Fig. 1). That assumption is not valid to an aggregate/
binder ratio of 1.7 mostly because to keep a similar workability
more water has been added to the mixture. Therefore when
H2O/Na2O molar ratio is increased to 11.4 it leads to a strength
decrease. Previous studies about geopolymeric mine waste mud
mortars showed that H2O/Na2O molar ratio is a very significant
parameter being closely related to compressive strength.
Compressive strength increases due to less water and hence
less porosity in the gel. And also when H2O/Na2O molar ratio
decreases due to higher NaOH concentration [28]. Increasing
NaOH concentration provides better dissolution of silicate and
aluminate species leading to increase inter-molecular bond
strength. The adverse effect reported by Lee et al. [34] related to
reduction in strength due to excess of alkali has not been
confirmed. This subject requires further investigations. However
it must be a subject related to setting rate, because faster
rates give less time for dissolution leading to more unreacted
particles and strength reduction, so it may be expected that those
authors used more reactive material (fly ash and kaolin) than
mine waste mud used in the present work