Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 63

lighter, than a natural coarse aggregate (although it can be deliberately
foamed, specifically to make a lightweight aggregate). The vesicularity
means that care is needed to use the aggregate in a saturated condition if
rapid slump loss and lack of pumpability are to be avoided. It also tends
to cause a distinct difference in SG (particle density) between different size
fractions. Excellent bond tends to be developed owing to both the vesicu-
larity and the chemical composition of the aggregate and particle shape
tends to be better than natural aggregates.
Some sources of slag may have a tendency to cause popouts as a result
of remnants of crushed limestone deliberately added to provide the desired
conditions in the blast furnace. However, this can be avoided if the lime-
stone is added in smaller particle sizes and combustion is very thorough
and even. Slag processing companies undertake measures to oxidise any
sulfides present to prevent blue spotting. With these possible exceptions,
the material tends to be a stable and satisfactory aggregate, even under
fire conditions. Drying shrinkage is usually relatively low, perhaps because
some chemical reaction takes place at the aggregate surface, causing a slight
expansion that partially offsets drying shrinkage.
The authors have found that crusher fines produced from a particular
slag source when combined with a local dune sand make a very satisfactory
fine aggregate in terms of strength at a given cement content and work-
ability, even compared to a good, long-graded, natural sand. However, it
should be noted that the granulated slag, which can be ground to produce
GGBS, although it may look like sand, may not perform well when so used.
This is because it is in a puffed state like rice bubble cereals and so the
grains are weak.
3.5 CONCRETE AGGREGATE FROM STEEL SLAG
Alex Leshchinsky
Steel furnace slag is a nonmetallic product consisting of calcium silicates
and ferrites combined with fused oxides of iron (15%-25%), aluminium,
calcium, magnesium, and manganese. The material, a by-product of steel
manufacturing, is produced in a molten condition simultaneously with steel
in a basic oxygen furnace. After the air-cooling, the material has a predom-
inantly crystalline structure. Air-cooled steel slag is crushed and screened
for the aggregate.
Steel slag aggregate is being used in asphalt and road base. In asphalt,
replacing natural aggregate with steel slag aggregate brings some advan-
tages, such as improvement in skid resistance and enhancement in durabil-
ity. However, the demand for steel aggregate is much lower than its output
from steel operations. Therefore, steel slag aggregate is usually very cheap.
The average world market price for steel slag aggregate is of the order of
US$0.50/t.