Page 57 - New Trends in Eco efficient and Recycled Concrete

Biomass fly ash and biomass bottom ash 33

Figure 2.5 Schematic of biomass thermal power plant-based on fluidised bed combustors
(Hinojosa et al., 2014).

In general, BBA present an appearance similar to a fine natural and dark sand,
because the burnt process is stopped before the final consumption of the biomass,
and it is obtained a mix of ash with unburned particles of biomass.
On the other hand, BFA is obtained in industrial combustion and gasification

installations at temperatures commonly between 800 and 1600 C. This type of ash
is powdery and is extracted from filters through which the combustion vapours are
released.
BFA presents a high content in alkali constituents (Na, K) and less alumina con-
tent (Al 2 O 3 ) than coal FA traditionally used as a pozzolanic additive in cement for-
mulations (Rajamma et al., 2009; Rajamma, 2011). The composition of BFA
depends mainly on its origin varieties from woody to herbaceous and other natural
resources.
Wang et al. (2008) compared the properties of BFA from co-firing with coal FA
in concrete, and they concluded that BFA from co-firing biomass with coal within a
certain blending ratio (25%) should be considered in concrete.
Both BFA and BBA can be applied in the manufacture of mortar and concrete
with some limits. Due to their properties in both kinds of ash, these ashes present
some appropriate properties for the manufacture of cement-based materials, but the
high content of sodium and potassium make their use difficult because they could
cause shrinkage and deformation problems in the concrete matrix. However, BFA
can be used in agriculture as a fertiliser, which is a lower application than their use
in concrete.
Cuenca et al. (2013) studied the possible use of BFA as a filler in self-
compacting concrete and Go ´mez-Barea et al. (2009) studied the addition of BFA to