PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 227

3. curing at higher temperatures
4. using non-calcium chloride accelerators such as triethanolamine, sodium
thiocyanate, calcium formate, calcium nitrite, or calcium nitrate

■

Supplementary Cementitious Admixtures

6.11.6

Several byproducts of other industries have been used in concrete as sup-
plementary cementitious admixtures since the 1970s, especially in North
America. These materials have been used to improve some properties of
concrete and to reduce the problem of discarding them. Since these materi-
als are cementitious, they can be used in addition to or as a partial replace-
ment for portland cement. In fact, two or more of these supplementary
cementitious additives have been used together to enhance concrete proper-
ties. These supplementary cementitious materials include fly ash, ground
granulated blast furnace slag, silica fume, and natural pozzolans.

Fly Ash Fly ash is the most commonly used pozzolan in civil engineering
structures. Fly ash is a by-product of the coal industry. Combusting pulver-
ized coal in an electric power plant burns off the carbon and most volatile
materials. However, depending on the source and type of coal, a significant
amount of impurities passes through the combustion chamber. The carbon
contents of common coals ranges from 70 to 100 percent. The noncarbon
percentages are impurities (e.g., clay, feldspar, quartz, and shale), which fuse
as they pass through the combustion chamber. Exhaust gas carries the fused
material, fly ash, out of the combustion chamber. The fly ash cools into
spheres, which may be solid, hollow (cenospheres), or hollow and filled
with other spheres (plerospheres). Particle diameters range from to
more than 0.1 mm, with an average of 0.015 mm to 0.020 mm, and are 70%
to 90% smaller than 0.045 mm. Fly ash is primarily a silica glass composed
of silica

1 mm

SiO 2 , alumina Al 2 O 3 , iron oxide Fe 2 O 3 , and lime (CaO). Fly

1 2 1 2 1 2

ash is classified (ASTM C618) as follows:

Class N —Raw or calcined natural pozzolans, including diatomaceous
earths, opaline cherts and shales, ruffs and volcanic ashes or
pumicites, and some calcined clays and shales
Class F —Fly ash with pozzolan properties
Class C —Fly ash with pozzolan and cementitious properties

Class F fly ash usually has less than 5% CaO but may contain up to 10%.
Class C fly ash has 15% to 30% CaO.
The spherical shape of fly ash increases the workability of the fresh con-
crete. In addition, fly ash extends the hydration process, allowing a greater
strength development and reduced porosity. Studies have shown that con-
crete containing more than 20% fly ash by weight of cement has a much
smaller pore size distribution than portland cement concrete without fly
ash. The lower heat of hydration reduces the early strength of the concrete.
The extended reaction permits a continuous gaining of strength beyond
what can be accomplished with plain portland cement.