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392 New Trends in Eco-efficient and Recycled Concrete

Figure 13.24 Relationships between compressive strength of BFS/SCSA activated pastes
and thermogravimetric mass losses (Moraes et al., 2017). SCSA, Sugarcane straw ash.

silicate derived activator by means of a new source of silica. RHA is usually chosen
as an alternative silica source.
Bouzo ´n et al. (2014) described the first successful example of alternative RHA-
derived activator, by refluxing RHA with NaOH solution. The resulting activating
reagent was an alkaline aqueous suspension, as the mixture was not filtered. The
suspension was used for activating spent FCC residue. The authors demonstrated
that grinding RHA is not required for yielding a good activating suspension, and
when compared RHA activated geopolymers and sodium silicate ones, they were
very similar in strength. The new alternative route led to a 50% reduction in CO 2
emission compared to the geopolymer synthetised with a mixture of commercial
waterglass and NaOH (Mellado et al., 2014).
Several complementary studies have been also reported. Mejı ´a et al. (2013) pre-
pared activating solutions by reacting NaOH and RHA, however when they were
used for activating BFS or FA, an important decrease in strength was obtained.
RHA-KOH activating system was also reported for MK-geopolymers (Villaquira ´n-
Caicedo et al., 2017): in this case, the compressive strength for plain MK activated
sample and the geopolymer with the RHA-derived activator was close to 8 MPa.
RHA alternative silica source was beneficial to the insulating behaviour: lower ther-
2
mal conductivity (0.17 J/m K) and thermal diffusivity (0.16 mm /s), which was attrib-
uted to the elevated porosity (35% open porosity) (Villaquira ´n-Caicedo et al., 2015).
Tchakoute ´ et al. (2016a,b) also studied MK-geopolymers. Different activating
systems were prepared by reaction of NaOH and RHA at 80 C for 2 h: SiO 2 /Na 2 O

molar ratios were 0.31, 0.47, 0.62, 0.78, 0.93, 1.09 and 1.25. Additional research in
MK geopolymer activated with RHA-NaOH was developed (Kamseu et al., 2017;