Page 403 - New Trends in Eco efficient and Recycled Concrete

358 New Trends in Eco-efficient and Recycled Concrete

Initially, natural precursors such as kaolinite, metakaolin (MK) (Davidovits,
1991) or natural pozzolans (Kani et al., 2012) were used. Industrial by-products,
such as coal burning fly ash (FA) (Izquierdo et al., 2009), granulated blast furnace
slag (GGBFS) (Lecomte et al., 2006) and spent catalyst residue from fluid catalytic
cracking (FCC) (Rodriguez et al., 2013) have been incorporated into experimental
studies during the past decade.
Moreover, other alternative raw materials were recently used as by-products
from other industries or directly from other waste streams, such as municipal solid
waste incinerator bottom ash (MSWI-ba) (Zhu et al., 2018), rice husk ash (RHA),
sugarcane bagasse ash (SCBA), palm oil fuel ash (POFA) and wood ash (WA)
(Paya ´ et al., 2017). Even sewage sludge ash (SSA) (Istuque et al., 2016) has been
used in the manufacture of new inorganic polymeric materials as a filler or as a sub-
stitute for traditional precursors.
Since the use of conventional alkaline activators can significantly increase envi-
ronmental impacts and economic cost, alternative alkaline solutions based on
sodium silicate (water glass) (Passuello et al., 2017; Puertas and Torres-Carrasco,
2014) and shell chemically-modified RHA (Bouzo ´n et al., 2014; Passuello et al.,
2017) has been used in order to reduce the potential reduction of CO 2 emissions by
up to 63% (Mellado et al., 2014).

13.1.2 Alkali-activated materials (AAMs): a definition
Geopolymers consist of a polymeric Si O Al framework, similar to zeolites,
being structures produced by tetrahedral alumina-silicate units’ condensation, with
alkali metal ions that balance the charge associated with tetrahedral aluminium.
Geopolymers are synthesised from a two-part mix consisting of an alkaline solution
and solid aluminosilicate materials and the reactions occur at ambient or slightly
elevated temperature (Feng et al., 2012).
Although, there is still some ongoing debate about terminology related to AAMs
and geopolymers, both terms are commonly used in the literature in an interchange-
able way and, occasionally, even as synonymous (Luukkonen et al., 2018).
These alkali-activated systems are less frequently known as:
1. Alkali-activated cements: binders containing large amounts of calcium such as slag
(Palomo and de la Fuente, 2003).
2. Inorganic polymers: predominantly obtained from industrial waste materials, such as coal
FA, granulated blast furnace slag (GBFS), mining waste and contaminated soil (Sofi
et al., 2007).
3. Hydroceramic or low-temperature inorganic polymeric glass: due to the reaction below

100 C of a dehydroxylated clay (metakaolinite) suspended in an alkaline solution of
sodium silicate leading to an amorphous silicate (Rahier et al., 1996).
AAMs are attractive materials that can be used instead of, or partially substitute,
OPC, and offers denser and more compact materials with higher mechanical
strength and an appropriate durability in its hardened state, an acceptable early-age
rheology and a reduced environmental footprint (Provis and Bernal, 2014).