Page 66 - New Trends in Eco efficient and Recycled Concrete

42 New Trends in Eco-efficient and Recycled Concrete

with respect to cement (W/C), workability is not the real problem. To obtain an
adequate workability, W/C ratio of 0.60 0.65 is recommended. In addition, the use
of a superplasticizer can easily improve the workability (Beltra ´n et al., 2014).
Previous studies showed that the density of concrete, mortar or soil cement mix-
tures depends mainly on the density of the aggregates used (Matias et al., 2013).
In general, the data for dry bulk density decreases with the use of BA (Rosales
et al., 2017) slightly as Portland cement is replaced by more BA due to the lower
density of BA compared to the ordinary cement.
The decrease in the apparent density shows an increase in the porosity of the
samples, therefore, the water penetration increases due to the hydration products
and pozzolanic materials that fill the voids in the cement paste (Wongkeo et al.,
2012). The use of highly porous industrial by-products, such as BA, significantly
increases workability due to its high water uptake.

2.3.3 Physical-mechanical properties of mortars and concrete
with BA

Different studies have shown that biomass ash, depending on its composition and
combustion process, does not always have a pozzolanic character. However, many
types of BFA have pozzolanic properties similar to coal FA, for example, sugarcane
straw, rice husk, wood and wheat straw (Martirena et al., 2006; Yu et al., 1999;
Naik and Kraus, 2003). Some of these types of ash have been used as a mineral
admixtures, showing a good mechanical behaviour, in addition to reducing the use
of natural resources used for construction (Kumar and Patil, 2006). However, bio-
mass ash from olive trees has a low content of oxides (Al 2 O3, SiO 2 , and Fe2O 3 )
that provide pozzolanicity and a high CaO content. This composition could limit
the use in cement-based materials (Vassilev et al., 2010).
Some researchers have evaluated the use of BBA for the manufacture of mortar
(Gemelli et al., 2004; Maschio et al., 2011; Da Luz Garcia and Sousa-Coutinho,
2013; Modolo et al., 2013; Carrasco et al., 2014) in general as a substitute for small
amounts of cement.
In most studies, several researchers limit the cement substitution content with
BA to 5% 10% (Da Luz Garcia and Sousa-Coutinho, 2013). Maschio et al. (2011)
used 5% 30% of ashes coming from spruce chip, crushed to a maximum size of
0.30 mm and with 5% of BA. Table 2.4 shows that similar physical and mechanical
properties were obtained in mortar with BBA in comparison with the control mix-
ture. This premise justifies the importance of the particle size distribution to obtain
appropriate mechanical behaviour.
On the other hand, it should be noted that ashes with a high potassium content
reduce the properties at advanced ages due to the development of ASR.
Once the viability of its use in mortar has been verified, investigations were car-
ried out using BBA as a partial substitution for sand. Substitutions of less than 20%
sand by BBA do not produce significant decreases in the mechanical properties of
concrete, especially at advanced ages.