Page 70 - New Trends in Eco efficient and Recycled Concrete

Biomass fly ash and biomass bottom ash 45

The permeability of the concrete is an indication of its performance with respect
to its durability. Permeable concrete allows the entry of aggressive agents such as
chlorides, and this puts the concrete at risk.
Beltra ´n et al. (2014) showed that water penetration increases with the addition of
BBA as shown in Table 2.5.
Concrete is a porous material and incorporating BA increases the porosity, there-
fore, the CO 2 from the air can penetrate through its pores towards the interior,
where a chemical reaction with calcium hydroxide is produced.
Most of the carbonation models consider the amount of cement as an important
parameter, due to its influence on the reaction of CO 2 with Ca (OH) 2 , forming the
hydration of the cement.
This reaction (carbonation) can reduce the pH value of the pore solution to less
than 9. When the alkalinity of the pore solution is lost, the properties of the con-
crete such as strength, permeability, shrinkage and resistance to chemical and physi-
cal attacks will be affected by carbonation.
As the pore index is related to the particle size, it has been shown that the con-
crete manufactured with BA of smaller particle size have a low carbonation com-
pared to those that use medium or coarse particle size.

2.3.5 Technological properties: thermal and acoustic insulation
The characteristics of thermal insulation and fire resistance are favoured, with
respect to the products with ashes of the BA combustion, when ash or slag is used
that come from processes such as combustion, pyrolysis, gasification or other ther-
mal treatment processes using biomass, together with various binders and a minor
proportion of additives, materials with high thermal and acoustic properties, are
obtained.
The use of BA, mainly bottom ones, as a substitute for coarse aggregate for the
development of porous concrete with sound absorption properties has been the
main subject of several investigations carried out by the authors in recent years
(Arenas et al., 2013; Leiva et al., 2012).
The properties of a sound-absorbing, porous concrete composed mainly of BBA,
showing that porous concrete recycled with ash presents properties similar to other
conventional materials used for acoustic absorption applications (Leiva et al.,
2012). In addition, the origin of BBA (pulverised coal combustion, co-combustion
or gasification of coal and pet coke, biomass combustion) does not affect the

Table 2.5 Durability properties of hardened concrete (Beltra ´n et al., 2014)
Age With 0% With 3% With 6%
BBA BBA BBA
Water penetration (mm) 28 36.3 61.5 75.3
Chloride penetration (mm) 28 10.3 19.7 21.8
56 18.1 23.1 27.2