GMWM mixtures with fine schist sand are the ones that
present a regular behaviour, which is probably due to their lower
size mixtures with a more regular distribution of aggregates
throughout the matrix. Similar behaviour has been confirmed by
Teixeira-Pinto when using geopolymeric metakaolin based
binders [15]. That shows how much aggregate size influences
tensile strength in GMWM binders, which is most likely due to
the chemical interaction between the aggregate and the alkaline
activated paste that has no comparison in OPC based systems.
Relations between flexural tensile strength (ft) and compressive
strength (fc) of GMWM binders are shown in Fig. 5.
Statistical correlations allow to conclude that aggregate content
as well as aggregate type influences ft/fc ratio. It seems that
strength results can't be explained by the differences in the
fineness modulus of aggregates, instead it must be noticed that
compositions with higher shrinkage display lower ft/fc ratio,
which agrees well with the previous statement, about the
influence of aggregates in tensile strength reduction.
3.4. Scanning electron microscopy analysis
Figs. 6 and 7 show SEM micrographs of GMWM binders.
The analyses carried out by EDS on the different samples are
displayed in Tables 5 and 6.
In Fig. 6 it is possible to see a scanning electron micrograph
of a GMWM mortar sample, showing the existence of two
different gel phases according to EDS analyses. This finding
confirms the formation of geopolymeric gel (areas X and Y), as
the main reaction product of Al–Si materials in presence of
calcium hydroxide when submitted to high alkaline solutions, as
stated by Alonso and Palomo [12]. High hydroxyl concentration
prevents the calcium hydroxide dissolution, so only small
amounts of calcium will participate in the formation of CSH.
The area identified as Z, has a low C/S ratio which means it
as some sodium replacing Ca2+ in CSH; some authors have
already demonstrated that sodium incorporation in CSH phase
increased as C/S ratio decreases, and therefore they name it as
NSH [36].
Aluminosilicates with similar SiO2/Al2O3 and Al2O3/Na2O
molar ratios were found by Yip et al. [13,14]. Those authors also
found the existence of an optimum of 20% (about 9% of
calcium oxide) slag content in alkali-activation of metakaolin-