Precast concrete materials, manufacture, properties and usage - Chapter 4 pot

4

FLY ASH

This material, also known as pulverised fuel ash or PFA, is a by-product

of electricity generation from pulverised coal firing. It is mainly of

interest to those countries having this form of power production, but

even in some of those countries it is not necessarily used everywhere

because of transport costs.

It has a beneficial action in many applications in in situ concrete

where its pozzolanic (long-term cementitious effect in the presence of

lime and water) and exotherm control properties, as well as its ability to

give ordinary Portland mixes an improved sulphate resistance, have been

used to advantage. As far as precast concrete product properties are

concerned these benefits are of little value because of early strength

requirements, generally small sections being cast, and good compaction,

respectively. What is of interest to the precaster are the following

questions:

(a) Does the addition improve the early (0–10 minute old) handling

properties?

(b) Does the addition improve the early strength (6–18 hours old)?

(c) Has the product better surface appearance and arrisses?

(d) How are other relevant properties affected?

(e) Does one get less wear and tear on machinery and plant?

This chapter divides into several parts, the first part dealing with a

description of fly ash, and the remaining parts dealing with specific

process studies of applications researched by the author. There is one

matter to note before proceeding, however, and that is a criticism

(constructive) of the terminology ‘cement replacement’. Depending upon

how one defines the control mix (the mix not containing fly ash) any

addition of ash to the mix is a replacement of the cement and/or the

aggregate. The only factor that is of interest is that of the concrete being

Copyright Applied Science Publishers Ltd 1982

economical to produce as a function of materials price, the total cost of

production and the number of rejects.

4.1 PROPERTIES OF FLY ASH

Fly ash is a light slate grey to dark grey or brown powder extracted from

the flue gases of a power station, usually by means of electrostatic

precipitators. Its colour is governed mainly by the amount and particle

size of the residual unburnt carbon, and secondly by the iron oxide.

Table 4.1 gives the reader an idea of the ranges of chemicals in fly

ashes internationally, bearing in mind that sources, other than those

specifically selected, can be modern, old or standby power stations.

TABLE 4.1

RANGES OF CHEMICAL MAKE-UPS OF FLY ASHES

The large ranges shown arise not only from the varying efficiencies of

the boilers but also from the fact that a single power station may well

rely upon supplies from more than one colliery and that there could be

several seams being worked in each colliery. Apart from the sulphate and

carbon contents, precast concrete product performance is luckily quite

insensitive to the chemical make-up of the ash.

The first four chemicals, with the fluxing alkalis, form very small

hollow glass balls, resulting in a low bulk density material. The presence

of lime at high levels can result in cementitious properties and it is

advisable to ensure that high-lime fly ashes are dry-stored otherwise they

will slowly harden. The magnesia could cause expansive properties in the

concrete if it is in the form of periclase. Although it is generally not in this

form, Standards assume that it could cause trouble and specify limits.

The sulphate is one of the troublesome ingredients because concretes

Copyright Applied Science Publishers Ltd 1982