Page 74 - New Trends in Eco efficient and Recycled Concrete

Biomass fly ash and biomass bottom ash 49

The industry of edible rice produces rice husk as a waste material which is also
used as fuel in boilers and power generators due to its high calorific power. RHA
generated contains high percentage of silica (. 90%), hence it is a
suitable cementitious material (Lim et al., 2018) and a feasible precursor for geopo-
lymerisation (Suksiripattanapong, et al., 2017). Consequently, it has been incorpo-
rated in geopolymer concrete due to its pozzolanic effect (Kabir et al., 2017).
Particularly, RHA possesses a low amount of alumina compound which makes it
worse as precursor.
Finally, another highlight is that the variability and heterogeneity in the proper-
ties of these biomass ashes make them difficult to obtain geopolymeric products
with homogeneous behaviour. That is the reason why it is necessary to deepen its
study, which is still in a very early stage.

2.4.3 Stabilisation of biomass ashes using geopolymerisation:
leaching characteristics
The ashes from combustion in power plants contains heavy metals that are consid-
ered as first pollutants in the world (Wang et al., 2017) as these heavy metals cause
very serious world environmental problems. It has been scientifically proven that
geopolymers can effectively solidify or stabilise heavy metal ions (Zhang et al.,
2008; Guo et al., 2014). Besides that, heavy-metal pollutants can be locked into the
three-dimensional network structure of geopolymers. According to Sun et al.
(2014), geopolymer solidification presents many other advantages, such as: (1) geo-
polymer’s permeability coefficient being very low, thus it can effectively prevent
the infiltration of pollutants elements; (2) in the synthesis of geopolymer Al (III) is
comprised of a four-fold coordinated atom allowing Al0 4 the ability to negatively
charge, (3) so it can absorb the positive charge of heavy metals; and finally, (4)
geopolymers entails simple processing durable and resistant to weather. These find-
ings are available in a few studies, such as that conducted by Wang et al. (2017) in
which geopolymer showed a degree of solidification of Pb(II), Cd(II), Mn(II) and
Cr(III) of 99.9%. Yunsheng et al. (2007) also reached an immobilisation efficiency
of 98.5% in a slag-based geopolymer mortar when heavy metals are incorporated in
the geopolymeric matrix in the range of 0.1% 0.3%.

2.4.4 Properties of geopolymer concrete produced
with biomass ash

The recent references founded in the scientific literature related to the investigation
of geopolymer binders with the incorporation of biomass ashes indicates that the
use of geopolymer contributes to the improvement of the waste management prob-
lem in several industries, in addition to the environmental problems associated to
the construction sector with respect to the manufacture and use of OPC (Part et al.,
2015).