SUPER ASH®

Answers to some common questions on fly-ash

What exactly is fly-ash?

The term fly-ash or pulverized fuel ash (pfa) is used to describe the material extracted from the flu gases of coal-fired power stations via the use of electrostatic precipitators. Fly-ash can come in various size fractions (different collection "zones"), ranging from relatively coarse through to ultra fine material. The fraction most often employed for the purpose of concrete production is sized similarly to cement.

What are the main chemical/mineralogical characteristics of fly-ash?

Chemical/mineralogical aspects can vary according to the type of coal and the exact manner of its burning. However, in general terms, fly-ash consists predominantly of silicate glass containing calcium, aluminium, magnesium and/or iron, together with a small quantity of crystalline material and some unburned carbon. The presence of silicate glass accounts for the so-called "pozzolanic" properties displayed by fly-ash materials: i.e. in combination with lime and water, fly-ash will react from calcium silicate hydrates (CSH) akin to the primary hydration products of Portland cement. Since the hydration of Portland cement also gives rise to significant quantities of lime, this means that fly-ash can be used as a supplementary cementitious material, serving as a partial substitute for Portland cement.

What is meant by a Type C or a Type F fly-ash?

The technical literature on fly-ash often distinguishes between what we have come to be known as Type C and Type F materials. The main difference has had to do with the prevailing lime (CaO) content of the ash, a feature tied to the constituent of the original coal source. So-called high calcium fly-ashes (having CaO contents higher than 10% are classed as Type F. Both types find widespread use in concrete production overseas. By virtue of its high lime content, Type C fly-ash is both pozzolanic and cementitious which fact means that it is also well-suited to use in soil stabilization work. Note that Fern Super-Ash is of the Type C variety.

How does the inclusion of fly-ash affect the strength of hardened concrete?

Under normal conditions the pozzolanic reaction tends to proceed somewhat more slowly than the hydration of ordinary Portland cement. Accordingly, all other things being equal, the use of fly-ash as a partial substitute for cement will generally result in a lesser rate of strength gain at early ages. Depending on the quantity of fly-ash involved and the details of the mix design, fly-ash concrete may still "lag" a little with respect to the standard 28-day strength measure but will usually catch up in the fullness of time. Note that is strength-lag feature can be offset in a large extent through appropriate adjustments to the concrete mix design (taking full advantage of the scope for a reduction in water demand brought by the use of fly-ash).

How much of the Portland cement in concrete can be replaced by fly-ash?

For routine concrete applications, cement replacement rates of 10-30% are typical. Generally speaking, the higher is the total cementitious content (cement + fly-ash), the greater is the relative amount of fly-ash that can be included without incurring undue strength penalties. For certain special concrete applications (mass concrete, self compacting concrete, flow able fill, etc), overseas experience would indicate that fly-ash contents can be taken to beyond 50% of the total cementitious material as required. Note that relatively high levels of fly-ash addition are often accompanied by the use of high-range water-reducers.

What about the durability characteristics of fly-ash concrete?

There are various improvements in concrete durability to be had from the inclusion of supplementary cementitious materials such as fly-ash. Concretes containing fly-ash tend to be somewhat less permeable (tighter paste structure) and show superior resistance to corrosion under mild-acid conditions. Fly-ash concretes are especially suitable for use in structures exposed to marine environments. Thus, for example, overseas research indicates that a concrete with 30% fly-ash is likely to provide for at least at 10-fold improvement in resistance to the ingress of chlorides as compared to that of a conventional concrete of the same compressive strength. This is far greater than can be achieved through the use of high-reactivity (ultra fine) pozzolans such as silica fume. In addition, fly-ash concretes are a good deal more user-friendly than concretes containing silica fume ( and show none of the sticky characteristics commonly associated with the latter). Technical advise on the design of fly-ash concretes to meet specific durability requirements relating to reinforced structures exposed to marine (Class C) or near marine (Class B2) environments may be obtained by contacting Fern Cement.

Summary of Benefits of "SUPER-ASH" in Concrete

• Easier to work
• More cohesive, easier to pump,
• Easier to place and compact,
• Reduction in water demand for any given level of slump, and
• Superior durability characteristics.

Excellent for use in the following areas

• All domestic applications, including house floors and driveways,
• Agricultural applications, including cowshed floors and races,
• Processing plants, Wineries, Service Stations and Garages,
• Reinforced concrete structures in coastal and marine environments, and
• Road and soil stabilization.

With the appropriate mix design using SUPER-ASH, current code requirements for a 50-year design life can normally be achieved at lesser concrete strength and/or with reduced amounts of cover compared to conventional solutions.