Cementitious materials - Concrete Mix Design, Quality Control and Specification - page 14

2.2.2 Composition of fly ash
There are two types of fly ash, according to the classification in ASTM 618,
Class F and Class C. Class F ash is the true pozzolanic material, silica (as SiO 2 )
being the most important constituent, and alumina and iron oxide are also
active (see Table 2.1). Class C ash also contains appreciable amounts of cal-
cium compounds and may have some minor hydraulic cementing value in the
absence of cement (a very few sources may produce usable concrete without
any cement at all). Certainly it is possible to use it in larger proportion than
Class F ash in a similar manner to, but not to the same extent as, GGBS. Class
C ash may be less effective than Class F ash in providing sulfate resistance
and reducing alkali silica reaction. It is possible to use high replacement levels
of Type F fly ash as in the case of roller compacted concrete and massive ele-
ments. Depending on temperature, it will take a long time to develop strength.
Most of our experience is with Class F ash. Class C ash may in gen-
eral produce similar effects but (as noted in the section on mix design
competitions) substantial differences are possible. There is also a significant
difference in the durability properties.
Carbon is the most important impurity as it can inhibit the action of
admixtures, particularly air entraining admixtures. It is measured by loss
on ignition, which should not exceed 6% 1 and should preferably be very
much less. However, the really important requirement is that it should be
as consistent as possible since otherwise it may be very difficult to control
air content. However, there has been a report of rice husk ash containing
up to 23% of carbon being successfully used in particular circumstances
(see Section 2.5), so possibly higher percentages in fly ash would not neces-
sarily render it useless in all circumstances.
Other impurities are alkalies and magnesium, which need to be limited
in cement but are not often a problem.

Table 2.1 Typical chemical composition of cementitious and pozzolanic materials
Portland cement

Fly ash Fly ash (Type C) Slag Silica fume

SiO 2 20 50 40 35 93

Al 2 O 3 5 30 15 15 2

Fe 2 O 3 4 10 6 1.5 <1

CaO 65 2.5 25 40 <1

MgO <2 <2 5 7 <1

Na 2 O <2 <2 <2 <1 <1

K 2 O <2 <2 <2 <1 <1

SO 3 <4 <3 3 <1 <1

LOl <2 <2 <2 — <2

1 ASTM 618, AS 3582.