Industrial Waste Aggregates - Recycled Aggregate in Concrete - page 42

to spray the surface of the slag to assist in the cooling process. The air-cooled blast
furnace slag is normally processed in a crushing and screening plant to manu-
facture products of particular maximum sizes and gradings. Crushed air-cooled
slag is angular, roughly cubical, and has textures ranging from rough, vesicular
surfaces to smooth glassy surfaces with conchoidal fractures. Processed air-cooled
slag exhibits good abrasion resistance, good soundness characteristics and high
bearing strength. The cementitious property of air-cooled slag is poorer than in
other types of slag prepared by rapid quenching. Slag sand is very angular and
coarse and therefore mixes containing this product require a high fine sand content
or the use of a mix containing three sand types. In general, blast furnace slag
processed for use as a concrete aggregate complies with the same requirements for
naturally occurring dense aggregate. While complying with these requirements,
air-cooled blast furnace slag aggregate differs from the range of naturally occur-
ring dense aggregates in certain properties. The particle and bulk densities of air-
cooled slag are slightly lower than those of natural aggregates. It has higher water
absorption and Los Angeles value. Some typical properties of air-cooled slag are
presented in Table 2.10 .
If the molten slag is cooled and solidified by adding controlled quantities of
water, air-, or steam, the resultant slag becomes a lightweight expanded or foamed
type of product. Foamed blast furnace slag is distinguishable from air-cooled slag
by its relatively high porosity and low bulk density. Crushed expanded slag is
angular, roughly cubical in shape and has a texture that is rougher than that of air-
cooled slag. The porosity of expanded blast furnace slag aggregates is higher than
that of air-cooled slag. The bulk relative density of expanded slag is difficult to
determine accurately, but it is approximately 70 % of that of air-cooled slag.
Typical compacted unit weights for expanded blast furnace slag aggregates range
from 800 to 1,040 kg/m 3 .
The molten slag can be pelletised during cooling and solidification process. The
produced pellets can be made more crystalline or more vitrified (glassy). Crys-
talline pellet can be used as aggregate. The pelletised blast furnace slag has smooth
texture and round shape. Consequently, the porosity and water absorption are
much lower than those of air-cooled or expanded slag. Pellet sizes range from 13
to 0.1 mm, with the bulk of the product in the 1.0-9.5 mm range. Pelletised blast
furnace slag has a unit weight of about 840 kg/m 3 .
Collins and Sanjayan ( 1999 ) reported the use of an air-cooled porous slag with
maximum diameter of 14 mm as a coarse aggregate in preparation of alkali
activated slag concrete. The specific gravity of the slag aggregate was 2.71, which
was slightly lower than used basalt aggregate (2.95). The water absorption
capacity of slag was 4.4 % and considerably higher than basalt aggregate (1.2 %).
Etxeberria et al. ( 2010 ) reported the use of blast furnace slag of size range of
4.75-25 mm as a complete or partial replacement of coarse aggregate in prepa-
ration of concrete. The physical properties of the aggregate were determined
according to British standard, BS EN 1097-6:2000. The oven dry and saturated
surface dry densities of the aggregate were 2.27 and 2.37 g/cm 3 respectively ,
which are lower than the used natural aggregate (about 2.7 g/cm 3 ). On the other