2.2. Mineralogy of Geopolymer Binders
shows XRD spectra of the precursor and geopolymer binders. Patterns are similar, being montmorillonite and quartz, the crystalline phases having been detected. The partial dissolution of montmorillonite upon geopolymerization, revealed by the FTIR analyses and confirmed by QPA results (Table 2
, and the increase in the amorphous phase are the most relevant observations. However, the fraction of montmorillonite that dissolves is small for all the activator modules used, as observed by other authors 
. Quartz peaks remain almost unchanged, indicating its non-reactive character. The analysis confirms the low dissolution of the crystalline phases by the activating solutions. The amorphous hump at 18–30° (2θ
) in the SOBE precursor becomes more prominent and slightly shifts to 20–35° (2θ
) with the geopolymerization progress 
XRD of raw material and geopolymer cements as function of Na2
/NaOH mass ratio of activating solution.
Table 2. Quantitative crystalline phase composition as derived from the Rietveld refinements *.
||Phase Composition (wt%)
||82.4 ± 0.2
||17.6 ± 0.3
||81.8 ± 0.2
||18.2 ± 0.2
||81.3 ± 0.2
||18.7 ± 0.3
||81.7 ± 0.2
||18.3 ± 0.5
||80.5 ± 0.2
||19.5 ± 0.3
2.3. Bulk Density, Total Porosity and Water Absorption of Geopolymer Binders
The values of bulk density, total porosity and water absorption of the geopolymers are shown in Figure 3
. As expected, bulk density and total porosity or water absorption values followed an opposite tendency when the Na2
/NaOH mass ratio of the activator changed. G-SOBE-1:1 specimens have a bulk density of 1828 kg/m3
, total porosity of 20.7% and water absorption of 4.35%. As the Na2
/NaOH mass ratio decreases, density decreases while total porosity and water absorption increases: G-SOBE-1:4 geopolymers show density = 1453 kg/m3
, total porosity = 37.9% and water absorption = 14.9%. Samples prepared with lower Na2
/NaOH mass ratios have more water. The water/binder ratio increases from 0.71 in G-SOBE-1:1 to 0.81 in G-SOBE-1:4 specimens; the removal of water will generate porosity 
. Inadequate amounts of binder precursor and alkali solution results in less efficient dissolution of the precursor, with consequent creation of pores in the geopolymer matrix and the formation of a less homogeneous structure 
. The ultimate removal of excess water upon drying will also create porosity. A decrease in the Si/Al molar ratio tends to generate less dense structures as a consequence of slower geopolymerization, according to the degree of reaction data 
. So, we have physical and chemical/reactive contributions to the observed tendencies.
Bulk density, total porosity and water absorption of geopolymers after 28 days of curing as functions of the Na2
/NaOH mass ratio.
2.4. Compressive and Flexural Strength of Geopolymer Binders
shows the compressive strength of samples cured for 7 and 28 days. Values ranged from 10.1 to 17.9 MPa (7 days) and 15.7 to 28.9 MPa (28 days). An increase in the Na2
/NaOH mass ratio enhances resistance, in direct relationship with higher compactness. Figure 4
shows a linear correlation between compressive strength and bulk density. The enhancement of soluble Si, by using an activator with a higher modulus, extends the geopolymerization process and the formation of the N-A-S-H aluminosilicate gel responsible for the consolidation of the geopolymer matrix and the development of mechanical strength 
Compressive strength of samples cured for 7 and 28 days.
Relationship between bulk density and compressive strength of geopolymers cured for 28 days.
The flexural strength values of the samples are shown in Figure 5. Between distinct formulations the changes follow the same trend in mechanical resistance after 28 days curing. However, differences after 7 days are minor, and sample G-SOBE-1:4 shows the maximum value (2.2 MPa). Progress with curing age is now more expressive, and resistance after 28 curing days is three to four times higher than at 7 days. Interestingly, it was observed that flexural strength values are only about three times lower than corresponding compressive resistances in samples cured for 28 days.
Flexural strength of samples cured for 7 and 28 days.
2.5. Thermal Conductivity
The geopolymers showed thermal conductivity values in the range 0.30–0.41 W/mK (Figure 6
), being lower in samples less dense (the ones prepared with a lower Na2
/NaOH mass ratio). As expected, there is an inverse relationship between thermal conductivity and porosity 
. In general, geopolymers exhibit lower thermal conductivity values than Portland cement (1.5 W/mK) 
, due to the existence of pores in the microstructure 
Thermal conductivity of samples cured for 28 days.
2.6. Microstructure of Geopolymer Binders
SEM micrographs at different magnifications of geopolymer binders cured for 28 days are shown in Figure 7 and Figure 8. At low magnification (220×, Figure 7) a homogeneous, dense and compact morphology can be observed in all samples. In any case, non-homogeneously distributed pores and some microcracks are visible. As expected from the density/porosity values, G-SOBE-1:1 samples seem more compact.
SEM micrographs (220× magnification) of geopolymers cured for 28 days. (a
) G-SOBE-1:1; (b
) G-SOBE-1:2; (c
) G-SOBE-1:3; and (d
SEM/EDS micrographs (2000× magnification) of geopolymers cured for 28 days. (a
) G-SOBE-1:1; (b
) G-SOBE-1:2; (c
) G-SOBE-1:3; and (d
As the Na2
/NaOH mass ratio decreases, more pores formed due to water evaporation during curing 
are observed, possibly due to an increase in the water/binder ratio (Table 2
At higher magnification (2000×, Figure 8), sodium aluminosilicate gel with a spongy and globular morphology is visible in all samples. Its elemental chemical composition reveals the dominance of silicon, aluminum and sodium (zone 1 EDS analysis), as expected. In addition, unreacted SOBE particles with angular shapes are also visible. EDS analysis (zone 2) shows an abundance of silicon and aluminum, and smaller quantities of potassium, iron and magnesium. It can be observed that as the amount of sodium silicate decreases the geopolymers contain more unreacted SOBE particles and possess a higher porosity.