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Application of alkali-activated industrial waste 377

Figure 13.12 Compressive strength development for alkali-activated red clay brick waste
1
with different w/m/r ratios (w: water/precursor ratio; m: concentration of Na in the
activator; r: silica modulus of the activator) (Reig et al., 2013a).

Robayo et al. (2016) also studied the influence of different dosage parameters in
the activation of RCBW. They studied the activation with NaOH (2% 10% Na 2 O
in respect to RCBW) and obtained moderated strength, even for high temperature

curing (70 C). Fig. 13.13 shows the strength evolution for these pastes. They dem-
onstrated that the addition of sodium silicate enhanced the strength rate, with an opti-
mum content of SiO 2 /Al 2 O 3 5 6.62 and Na 2 O/SiO 2 5 0.12 molar ratios (compressive
strength 55 MPa for 25 C curing temperature) and with SiO 2 /Al 2 O 3 5 7.1 and Na 2 O/

SiO 2 5 0.12 molar ratios (compressive strength 68 MPa for 70 C 24 h pre-curing

temperature and 27 days of 25 C curing). Tuyan et al. (2018) found an optimum

alkali-activator concentration for a maximum strength when 10% Na 2 O and 1.6 silica
modulus ratio were used (Fig. 13.13). For the selection of the optimum results, the
energy consumed in the high temperature curing process was taken into account.
Sassoni et al. (2016) studied RCBW geopolymer for masonry repointing. They
adjusted the chemical composition of the geopolymer by the addition of sodium alu-
minate, sodium silicate and sodium hydroxide as reagents for preparing the activator.
The use of sodium aluminate improved the efflorescence process and microstructure
densification. From the point of view of the potential compatibility of these geopoly-
mers with historic mortars, intermediate SiO 2 /Al 2 O 3 ratio systems (0.6 1.4) were
appropriate because of their porosity and water permeability behaviour, showing that
some systems had similar properties to historic lime-based mortars.