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Application of alkali-activated industrial waste 369

depicted. Taking into account workability, setting time and compressive strength,
the proposed optimal dosage was Ms 5 1.00, 4.5% Na 2 O and s/s ratio lower than
0.375. High values of shrinkage (1.5% 3% in pastes) were obtained in 50% RH.
Nath and Kumar (2017) studied the synergistic role in FA/SiMn slag mixtures.
They found an improvement of mechanical strength with increasing SiMn slag,
which was attributed to reactive CaO in the slag, and co-existence of two types of
gel: C A S H and N (C) A S H. Fly ash sample activated with 6 M NaOH
solution reached less than 5 MPa after 28 days of curing, while when increasing the
slag content, this strength raised to 25 MPa for 20% FA 80% SiMn slag.
Ladle slag (LS) or electric arc furnace slag (EAFS), is a by-product from the
refining process of steel and is rich in calcium compounds. The behaviour of EAFS
has been studied in geopolymers made with a binary blended precursor, MK
(Bignozzi et al., 2013; Murri et al., 2013; Lancellotti et al., 2014) or with FA
(Niklioc et al., 2016; Murri et al., 2013). Bignozzi et al. (2013) studied mixtures of
MK with LS replacements in the range of 0% 100%. The EAFS had a high per-
centage of calcium (CaO, 54.5%) and also contained 16.4% SiO 2 , 11.1% Al 2 O 3 and
8.7% Fe 2 O 3 . From the mineralogical point of view, it contained γ-Ca 2 SiO 4 , olivine,
gehlenite and mayenite. The mixtures were activated by means of NaOH/water

glass and cured at 20 C in a 55% RH environment. The best results in compressive
strength were obtained for MK/LS mixtures 40/60, 30/70 and 20/80, reaching
40 50 MPa. They have less porosity than MK-richer samples. Niklioc et al. (2016)
studied the mechanical and thermal properties of FA-based geopolymers by incor-
porating EAFS. The slag was added in the range of 0% 40%. A mixture of amor-
phous N (C) A S H gel along with N A S H gel was obtained with the
presence of slag. The 70/30 sample displayed the highest strength, close to 33 MPa.
This geopolymer was tested under high temperature treatment and it showed a 32%

reduction in strength at 400 C and 80% at 800 C. Binding gel crystallised at

800 C, as shown in Fig. 13.7, and gehlenite was the result of the bulk
crystallisation.
RM (bauxite waste) is obtained as waste in the industrial production of alumina
(Bayer process). This alumina becomes the raw material for obtaining aluminium.
This waste presents high alkalinity since in the process the bauxite is treated with
NaOH at 240 C and high pressure (1 6 atm) for dissolving aluminium oxide from

the ore. The chemical compositions of several RM samples are summarised in
Table 13.2. In general, SiO 2 ,Fe 2 O 3 ,Al 2 O 3 and Na 2 O are the main oxides; however,
the variability in the percentages is very wide. Thus, iron oxide content usually is
the highest in the RM samples and Na 2 O is in the 6% 15% range. The main crys-
talline phases were haematite, boehmite, rutile, cancrinite, katoite, calcite and
quartz.
In general, RM is blended as binary systems with different mineral precursors:
MK (Haijjaji et al., 2013; Kaya and Soyer-Uzun, 2016), blast furnace slag (Ye
et al., 2014), FA (Nie et al., 2016), coal gangue (Geng et al., 2017), RHA (He
et al., 2013) and silica fume (SF) (Hairi et al., 2015).
Haijjaji et al. (2013) activated a mixture of MK/RM with sodium hydroxide and
sodium silicate. The mixtures were prepared with the following target in molar