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Application of alkali-activated industrial waste 399

amorphous gel, such as alumina-silicate gel. According to previous studies, the
industrial by-products used to make AASm and AASc should be rich in alumina
31 41
(Al ) and silica (Si ), such as ground granulated blast furnace slag, FA, etc., can
be activated by a mixture of sodium hydroxide with sodium silicate or potassium
hydroxide with potassium silicate (Barbosa et al., 2000) and used as binders
(Douglas et al., 1991; Brough and Atkinson, 2002; Puertas and Ferna ´ndez-Jime ´nez,
2003).
Alkaline activators play a fundamental role in the hydration of binders. Due to
their characteristics (Table 13.5), mortars and alkaline activated concretes can show
higher mechanical resistance results than those obtained by a concrete made with
OPC. The alkaline cements are divided into two groups depending on the nature of
their cementitious components (CaO SiO 2 Al 2 O 3 system):
Type 1: high calcium oxide content;
Type 2: low calcium cements.
One of the main elements to consider in the precursor materials is the calcium
content. The blast furnace slag activated by alkylation shows high calcium content
and is dominated by C A S H (type 1) (Provis et al., 2015; Richardson et al.,
1994), on the other hand, precursors with low calcium content such as MK or FA
generate an alkali aluminosilicate (N A S H) (Type 2) gel with a highly cross-
linked (Provis et al., 2005, 2015; Richardson et al., 1994)(Fig. 13.30).
Therefore, the physical and chemical properties of alkaline cements must be con-
sidered in order to manufacture AASm and AASc, considering the nature of the
raw materials. The mineralogical characteristics and the chemical composition of
the raw materials are determined in the properties of the final product. According to
the literature, the highest values of compressive and flexural strength are obtained
with AAMs whose starting ([SiO 2 ]/[Al 2 O 3 ]) reactive ratios range from 2 to 4
(Chindaprasirt et al., 2012; Criado et al., 2010; Garcia-Lodeiro et al., 2014).
In this way, the alkaline activators can be mixed with aggregates and water to
produce AASm and AASc in the same way that commercial mortars and concretes
are produced. Consideration must be given to aspects such as the design of the mix-
ture, the intended application, geometry, etc., for the correct choice of the
precursor.

13.3.2 Rheology of new mortars and concrete

Rheology is defined as the study of the fluidity and deformation of matter. The rhe-
ological behaviour of mortars and concretes must be studied to understand their
consistency and workability (Tattershall and Banfill, 1983). The rheological behav-
iour affects the internal microstructure which affects the mechanical behaviour and
the durability of the material (Aguirre and de Gutie ´rrez, 2013).
According to the literature (Puertas et al., 2005; Banfill, 2005), Portland cement
pastes and mortars behave like Bingham fluids (Eq. (13.1)): that is, where the slope
is the plastic viscosity (μ) and the y-intercept the yield stress (τ o ), which means