Civil Engineering Reference
In-Depth Information
Fig. 2.2 Pore size
distribution of bottom ash
(Kim and Lee
2011
)
2.3.1.2 Boiler Slag or Wet Bottom Ash
Boiler slag is a vitrified material, which is a very durable and environmentally
stable form that permanently immobilises its chemical constituents into the glassy
amorphous structure. Boiler slag is made of porous, glassy, angular, uniform sized
smooth granular particles. The quenched slag becomes somewhat vesicular or
porous if gases are trapped in the slag. Boiler slag generated from burning of
lignite or sub-bituminous coal tends to be more porous than that of bituminous
coals (Lovell and Te-Chih
1992
).
The boiler slag primarily comprises particles, which can be regarded as single-
sized coarse to fine sand with 90-100 % passing a 4.75 mm mesh sieve, 40-60 %
passing a 2.0 mm mesh, 10 % or less passing a 0.42 mm mesh and 5 % or less
passing a 0.075 mm mesh (Majizadeh et al.
1979
). Boiler slag is black in colour,
hard, and durable with a resistance to surface wear.
Boiler slag typically contains 40-60 % SiO
2
, 18-38 % Al
2
O
3
, 2-7 % Fe
2
O
3
,
1-4 % CaO, 0.5-3.0 % MgO and 0.5-2.0 % TiO
2
. The chemical composition of
boiler slag is also governed by the coal source. Boiler slag exhibits less abrasion and
soundness loss than bottom ash as a result of its glassy surface texture and lower
porosity. The predominate minerals present in boiler slags are mullite, quartz, cal-
cium silicate and quicklime. The specific gravity of boiler slag usually ranges from
2.3 to 2.9. The dry unit weight of boiler slag usually ranges from 960 to 1,440 kg/m
3
.
Occasionally, the dry unit weight of boiler slag may reach 1,760 kg/m
3
.
Deleterious materials, such as soluble sulphates or coal pyrites, should be
removed from the bottom ash and boiler slag before attempting to use these
materials as an aggregate. Pyrites can be removed from the coal before it is burned
using sink-float techniques, or from the bottom ash or boiler slag using magnetic
separation. Due to salt content (soluble chlorides and sulphates) and low pH and
electrical resistivity, bottom ash and boiler slag may be potentially corrosive and
therefore evaluation of the corrosive nature of the bottom ash being used should be
investigated. Corrosivity indicator tests normally used to evaluate bottom ash are
pH, electrical resistivity, soluble chloride content and soluble sulphate content.
