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360 New Trends in Eco-efficient and Recycled Concrete

2
Provis, 2008). Detached Si tetrahedra will now become easier for OH attack and
this results in units and oligomers, containing SiO groups.
Initially there is a preferential dissolution of aluminium, but very soon an equi-
librium is produced due to the release of the silica and aluminium, so that both ele-
ments are released at similar speeds. By the accumulation of silica and alumina in
contact with the products, dissolved coagulation is favoured, producing polyconden-
sation. Finally, an aluminosilicate gel is formed that is sedimented to produce an
amorphous product (Nikolov et al., 2017).

13.1.4 Performance of AAMs cement and concrete: rheology,
mechanical behaviour and durability
The excellent performance of cement and concrete with AAMs is attributed to the
highly refined pore network, forming a denser C A S H phase in calcium of the
geopolymer paste, finding a distinct correlation between the microstructure formed
during the curing process and the mechanical and durability properties of
geopolymers.
In the fresh state, rheological behaviour of these new binders is hardly affected
by the ratio of solid to liquid (S/L), the temperature of geopolymerisation and the
activating solution concentration, solid content and temperature being the most
influential. Moreover, type and particle shape of precursors strongly affect the vis-
cosity and the paste yield stress. Particularly, it should be noted that non-spherical
particles, like those of MK, increase the values of the rheological parameters
(Provis et al., 2010).
AAMs tend to show superior strength compared to OPC materials. Despite the
influence on calcium-containing raw materials (Xu and van Deventer, 2003) and
curing treatments (Palomo et al., 1999; Van Jaarsveld et al., 2002) or chemical acti-
vator applied (Part et al., 2015), due mainly to its dissolving power during the syn-
thesis of the geopolymer process, at an optimum concentration, these characteristics
play a vital role in the mechanical properties of AAMs.
Moreover, finer geopolymer source materials achieve higher degrees of reactiv-
ity and vigorous geopolymerisation due to their higher specific surface area, which
in turn will have higher development of initial strength.
Furthermore, due to their inorganic composition, geopolymers are intrinsically
fire resistant and exhibit excellent thermal stability far superior to traditional OPC
(Barbosa and MacKenzie, 2003). Nevertheless, AAMs have a somewhat limited
record of durability in service and this is one of the main limiting factors which
affects the common use of these as binders (Provis and Bernal, 2014). Park et al.
(2015) attributed a high influence of the effect of chemical activator and curing
regime in the dimensional stability and durability of AAMs and found an indirect
relationship between both parameters with the mechanical strength exhibits.
Ridtitud et al. (2011) associated the increase in shrinkage to the lower strength
development and curing temperature. However, Aydin and Baradam (2012) studied
the effect of curing on shrinkage, concluding that AAMs show higher values of