Page 65 - New Trends in Eco efficient and Recycled Concrete

Biomass fly ash and biomass bottom ash 41

Go ´mez-Barea et al. (2009), affirmed that the ashes can be used for soil stabilisa-
tion, as a substitution of cement in concrete, synthetic aggregate, road base and sub-
base, asphalt-based products and even as light bricks.
In the past decade, FA was widely studied, however, there have been few studies
on the use and management of BA.

2.3.1 The design of concrete with biomass ash: general
properties for design

More efforts have to be made to improve the sustainability of the concrete industry.
Replacing traditional aggregates with BA can contribute to this goal. But current
regulations, such as Standard Specification for coal FA and raw or calcined natural
pozzolan for use in concrete (ASTM C618, 2017) about FA for concrete, do not
allow the use of BA for the manufacture of concrete because there is not enough
knowledge about the effects they can have on the properties of concrete.
Until now, only partial replacements of cement by BA has been considered to
maintain the workability, strength and durability of concrete under control.
However, a deeper knowledge of the properties of BA is needed to overcome other
problems. As BA is highly absorbent, the demand for concrete water is high. This
problem of viability can be solved by submitting the fine fraction BA to a grinding
operation to remove the porous elements as well as previously wetting the fine and
coarse fractions of BA in a controlled manner. Regarding long-term behaviuor, hea-
vy metal leaching and exposure to freeze thaw are not problematic, although there
is a susceptibility to acetic and lactic acid attack and perhaps a greater sensitivity to
the alkali silica reaction (ASR).
A lot of research has been carried out to analyse the physical, chemical and
mechanical characteristics of the ashes coming from the combustion of biomass
with the aim of being used in concrete, mainly due to the pozzolanic properties of
some types of BA.
The use of BA in the manufacture of concrete presents an important problem as
it is a very heterogeneous waste presenting a very variable size and with different
percentages of unburnt particles in their composition.
According to Rajamma (2011), the main aspects that influence the characteristics
of BA are:

The properties of the original biomass, that is, if it is of herbaceous origin, wood or bark.
The technology used for combustion.
And whether BA or FA are used.

2.3.2 Influence of BA on concrete’s fresh-state properties:
density and slump
When BA is applied in concrete, the replacement of aggregates by BA can produce
a negative effect on the workability in the fresh state. This is mainly due to the
greater water absorption of BA. However, when it uses fairly high ratios of water