Civil Engineering Reference
In-Depth Information
Fig. 2.1 Particle size distribution of a CBA and of natural sand a Bai et al. (
2005
); b Ghafoori
and Bucholc (
1996
)
The particle size of CBA also depends on the factors indicated earlier. In
literature, the use of CBA as fine or coarse aggregate is reported (Table
2.1
). The
CBA used in Ghafoori and Bucholc (
1996
) study was an well-graded fine
aggregate with a fineness modulus (FM) of 2.80 (ASTM's recommendation of FM
range for fine aggregate is 2.3-3.1). Figure
2.1
shows the particle size distribution
of two typical CBA used as partial substitution of fine aggregate in concrete.
Compared to natural sand aggregate, which is dense, normally smooth in texture,
and round in shape. The particles of CBA aggregate are porous and angular in shape.
It has rough surface texture, large numbers of micro-pores (circular holes with a
diameter in a range of 0.5-5 lm) and internal pores. Some spherical shaped fly ash
particles having a few micrometer diameters can be deposited on the surfaces of CBA
(Kim and Lee
2011
). The shape and porous structure of CBA makes it necessary to
use a high amount of water during preparation of the concrete mix. A typical pore-
size distribution curve of bottom ash (presented in Fig.
2.2
) indicates that the
nanostructure of bottom ash is quite dense (Kim and Lee
2011
).
The chemical properties of CBA are normally controlled by the properties of
coal (its origin). Table
2.2
shows oxide composition of some CBA samples. CBA
is composed primarily of silica (SiO
2
), ferric oxide (Fe
2
O
3
) and alumina (Al
2
O
3
),
with smaller quantities of calcium oxide (CaO), potassium oxide (K
2
O), sodium
oxide (Na
2
O), magnesium oxide (MgO), titanium oxide (TiO
2
), phosphorous
pentoxide (P
2
O
5
) and sulphur trioxide (SO
3
).
The minerals identified in different CBA used as aggregate in concrete are
presented in Table
2.2
. Figure
2.3
shows the X-ray diffraction (XRD) pattern of
typical low calcium CBA. The major minerals found in CBA are quartz, mullite
and a non-crystalline glassy phase. In some CBA, iron containing minerals like
hematite and magnetite may also be present. The fused and glassy texture of CBA
normally would make an ideal substitute for the aggregate fraction of concrete.
Unlike its companion—pulverised fuel ash (PFA) or coal fly ash, CBA usually
has low pozzolanicity, which makes it unsuitable to be used as a mineral addition
in cement. It may contain higher concentrations of unburned carbon. Some power
plants recover coal mill rejects with bottom ash and therefore CBA may contain
pyrites that come from mill rejects, which can cause expansion in concrete.
