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Application of alkali-activated industrial waste 363

0.2. The reached compressive strength was higher than 50 MPa after 7 days and
higher than 70 MPa after 180 days. Higher reactivity was found for slag samples
ground under CO 2 atmosphere as shown through calorimetric and compressive
strength tests.
High magnesium nickel slag (HMNS) is produced from pyro-metallurgical pro-
cess of high-magnesium nickel oxide ores. Slag tested by Zhang et al. (2017)
showed 52.3% of SiO 2 and 29.6% of MgO. HMNS is mainly amorphous (Yang

et al. 2014), as shown by the broad peak in the range 2Θ 5 25 40 , and only traces
of quartz and calcium silicate were found. The high concentration of magnesium
causes some problems in a mix made with OPC. However, the design of new
HMNS-based geopolymer binders mixed with coal FA showed potential to being
reused as a construction material.
Mixes containing 0% 60% of HMNS were activated with sodium silicate solu-
tion (silicate modulus Ms of 1.2, 1.4, 1.6 and 2.0). The highest compressive strength
at 28 days of curing was found in Ms 5 1.4% and 20% HMNS sample, reaching
more than 60 MPa (Fig. 13.2). The compressive strength reached in samples with
Ms 5 2.0 was significantly lower (less than 30 MPa).
Furthermore, CO 2 emission analysis showed that geopolymer pastes have to be
considered as much more sustainable than conventional Portland cement because
CO 2 emission obtained for 20% HMNS (with Ms 5 1.4), was about 0.22 t per tonne
of paste, while for a tonne of Portland cement paste (w/c ratio of 0.42) it was
0.58 t.
The mechanical and deformational behaviour of these geopolymers were also
investigated at high temperature (Yang et al., 2017) and showed that the substitu-
tion of FA by HMNS mitigates the dehydration process and a lower shrinkage is

obtained for temperatures below 250 C. Apparently, the geopolymeric gel formed

Figure 13.2 Compressive strength of fly ash/HMNS pastes after 28 days of curing:
dependence with the silica modulus (Ms) of the activator (Zhang et al., 2017). HMNS, High
magnesium nickel slag.