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Application of alkali-activated industrial waste 359

As Izquierdo et al. (2009) described, geopolymers were introduced by Joseph
Davidovits in 1972, when he was looking for inorganic-polymer technologies. He
proved that some kinds of inorganic materials could be polycondensed, like organic
polymers, by means of the chemical reaction between alumina-silicate oxides and
alkali silicates, producing polymeric Si O Al bonds. The network obtained con-
sists of SiO 4 and AlO 4 tetrahedra, linked alternately by sharing all the oxygen.
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The Al in fold IV combinations demands the presence of cations in the frame-
work to balance the negative charge. For this purpose, temperatures lower than

100 C, high pH values, concentrated alkali and atmospheric pressure are required.
The three-dimensional amorphous to semi crystalline silico-aluminate structures
resulting from the hydrothermal conditions to form calcium silicate hydrate gels
(Puertas et al., 2000), calcium aluminosilicate hydrate (CASH) (Rashad, 2013)or
sodium aluminosilicate hydrate (NASH) (Ismail et al., 2014) are named geopoly-
mers. The empirical formula of geopolymers is:

M n 2SiO 2 Þ 2AlO 2 UwH 2 O
ð
z
n
where M is a cation such as potassium, sodium or calcium; n is the degree of poly-
condensation and z is 1, 2 or 3.
According to Davidovits (1991), geopolymers can be used in several industries,
among which civil engineering, automotive, aerospace, metallurgical and plastic
stood out. They can be used in pure form to store toxic chemicals or radioactive
wastes, with fillers for special concrete or in moulds for thermoplastics and in rein-
forced applications such as moulds or tools.

13.1.3 Role of precursors and alkaline activators
In the development of geopolymers, it is necessary to apply these main
components:
Precursors: The first part of the AAM production process is carried out from a
natural solid material based on aluminosilicates. This material is prepared at differ-
ent temperature ranges to facilitate reactivity. The precursor is the main component
in the production of polymers and is obtained from natural materials, waste and by-
products. Zeolite, MK, volcanic ash, granulated blast furnace slag, calcined shale
and other industrial waste have been used as precursors.
Activators: The precursor element must be activated by a second component
based on an alkaline solution. The geopolymer pastes are usually prepared by using
three types of alkaline activators: sodium hydroxide, sodium silicate and sodium
carbonate.
Geopolymerisation: This process begins with the dissolution of alumina and sil-
ica from the precursor combined with the alkaline activator. As described by
1 21
Nikolov et al. (2017), after initial removal of surface metals such as Na and Ca ,
2
the alumina-silicates are attacked by OH ions. Since Al O bonds are weaker than
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Si O bonds, Al will enter the solution first as Al(OH) complexes (Duxson and