Page 72 - New Trends in Eco efficient and Recycled Concrete

Biomass fly ash and biomass bottom ash 47

aluminosilicate three-dimensional network in form gels CSH (Puertas et al., 2000),
CASH (calcium aluminosilicate hydrate) (Rashad, 2013) or NASH (sodium alumi-
nosilicate hydrate) (Ismail et al., 2014).
These gels make concrete with similar or higher strengths to that of OPC sys-
tems (Part et al., 2015). Alkali-activated systems are also denominated in the litera-
ture as:

Alkali-activated cements, such as slags due to its content in calcium (Palomo and
Palacios, 2003).
Inorganic polymer, obtained from industrial by-products such as coal FA, GBFS (granu-
lated blast furnace slag) or mine tailings and contaminated soil (Sofi et al., 2007).
Hydro-ceramic or low temperature inorganic polymer glass, obtained from the reaction
between metakaolinite in an alkaline sodium silicate solution that leads to an amorphous

silicate at temperatures below 100 C(Rahier et al., 1996).
Recently, geopolymers have attracted a considerable amount of attention with
respect to OPC because of its behaviour as a binder with a high compressive
strength at early ages, improved workability, reduced permeability, increased dura-
bility, resistance to acid attack, reduction of plastic shrinkage cracking and excel-
lent fire resistance, in addition to the large amount of energy demanded, among
other environmental and economic benefits (Duxson et al., 2007; Liew et al., 2017;
Provis and VanDeventer, 2009; Davidovits, 1991). Thus, geopolymer concrete pro-
motes sustainable and innovative use of waste materials leading to faster concrete
production, with the reduction of the curing time and construction costs, minimising
the maintenance and increasing the service life of construction projects (Liew et al.,
2017). In this sense, the mixture of geopolymer binders and aggregates of natural
sources (Albitar et al., 2018; Tennakoon et al., 2017), lightweight (Islam et al.,
2017, Novais et al., 2018), recycled aggregates (Nuaklong et al., 2016), or even
crumb rubber (Park et al., 2016), among others, is presented as an opportunity to
improve sustainability of the industry of concrete (Liew et al., 2017).

2.4.2 Limits and opportunities of biomass ash to produce
geopolymer concrete
The use of biomass ashes in the manufacture of geopolymer concrete or alkali-
activated cementitious composites results in a promising sustainable concrete where
the use of OPC can be eliminated totally. Biomass ash geopolymer is considered to
be the latest contribution to the scientific field of studies on geopolymers.
In the literature, recent studies regarding the utilisation of rice husk ash (RHA)
or palm oil fuel ash (POFA) as biomass ashes to produce geopolymer concrete in
partial substitution of the traditional precursors (FA, GGBFS or MK) can be found.
The chemical composition and physic characteristics of RHA and POFA are sum-
marised in Table 2.6. In particular, it can be seen that the chemical composition in
most of the BA studied conform to the ASTM C618 (ASTM C618, 2017) with
regard to the maximum content in sulphuric anhydride (#4.0%), maximum value
of loss of ignition (#10.0%) and minimum content in pozzolanic compounds