Regulation

Developing high performance construction materials with coal combustion products

In Australia, we are fortunate to have high quality black coal deposits readily available. It is from these deposits that we source steaming coal, which is capable of liberating heat when burnt in dedicated coal-fired power stations to generate electricity. During this burning process, coal combustion products (CCPs) are formed.

Heavier particles fall to the bottom of the furnace and cool to form furnace bottom ash (FBA), while lighter particles are carried by exhaust gases through the station and are later captured as a fly ash stream.

FBA can make up 10 to 20 per cent of the ash produced from a power station, from fine sand-sized to coarse particles up to 5mm. It has a chemical composition similar to fly ash but may contain higher levels of loss on ignition (LOI ? unburnt carbon). However, because it is coarser and more highly fused than fly ash, the higher LOI has no real significance and it tends to be inert, acting more like a sand particle than a cementitious particle. For this reason, these coarse FBA particles make an excellent free-draining sand material and have been used for some time as a partial sand replacement in some concretes, roadbases and blended materials.

The whole fly ash stream is often referred to as run-of-station (ROS) ash, as it represents the ash produced from a burning process run.
 
{{image2-a:r-w:250}}This stream constitutes the bulk of the power station ash and is made up of coarse, medium and fine particles. It is usually the finer materials, within this stream, that are sought for use as a supplementary cementitious material (SCM), to AS3582.1, for use in concrete and other application, etc.

The unwanted coarse or medium fractions of the fly ash stream and the unwanted FBA are normally stored within the station grounds in reject silos, ash dams or repositories, which hold this material for extended periods. While these coarser CCPs may not be suitable for use as an SCM in concrete, they can become very successful additions to other materials used in other applications, such as roadbases, fills and, in some cases, sand replacements in concrete.

As coal-fired power stations are used extensively in New South Wales and other states to provide base load electricity to the grid, for all domestic, commercial and industrial uses, large quantities of coal are pulverised and burnt and subsequently large quantities of CCPs are produced daily. Despite this regular and ongoing supply of CCPs, only a fraction of these materials are being re-used as SCMs in concrete or in other applications.

A massive opportunity therefore exists for the re-use of these CCPs (coarse fly ash and bottom ash), especially in applications as a fine aggregate. With the introduction of loading facilities at some base load stations, CCPs are now more available than ever before.

Before we use CCPs, we need to understand their properties and in what applications they are best suited.

PROPERTIES AND APPLICATIONS OF CCPs
The properties of the CCPs formed within a power station depend on the properties of the coal used to feed the station, the burning processes and the efficiency of the capturing mechanisms.

As discussed, due to the coarse sand size of FBA and its essentially inert nature, it is commonly used in applications where grading improvement is required or its free-draining nature is preferred.

Fly ash is currently designated in grades of fine, medium and coarse in AS3582.1 and material with less than 55 per cent passing the 45-micron (?m) sieve is not covered in this standard (Supplementary Cementitious Materials for Use with Portland Cement ? Part 1: Fly Ash ? as shown in Table 1).

Due to the large quantity of fly ash created within the power station and the limited facilities able to capture it, ROS ash may include proportions of complying fine, medium and coarse-grade fly ash as well as coarser materials considered reject ash. In this way, ROS ash is a valuable resource.

One of the most important uses to date of ROS ash has been when it is incorporated into natural or crushed rock roadbases to improve their mechanical and physical properties.

MODIFICATION AND STABILISATION OF PAVEMENTS
When the strength or stiffness of a natural or crushed rock material is insufficient for its intended purpose, a small addition of binder may be incorporated to improve these properties. This binder can be made up of Portland cement, slag, lime, fly ash or combinations of these.
 
The quantity of binder added usually dictates the increase in strength and stiffness of the combined material and this is usually measured by the unconfined compressive strength (UCS) at 28 days (or seven days accelerated). A small addition of binder would normally be described as modification and is defined as a UCS >0.7MPa but <1.5MPa, while a larger addition may lead to a stabilised material where the UCS >1.5MPa.

{{image3-a:c-w:600}}When it comes to binders, it may seem that the finer the binder, the more efficient it is in modifying or stabilising construction materials. This is not always the case and the coarser ROS ash can be very beneficial in improving the overall grading of the crushed rock material while also improving its workability, handling and ease of compaction.

In the presence of added lime and moisture, the latent pozzolanic nature of the ROS ash also reacts to form a weak cementing glue to further bind the crushed rock together. It is this binding ability that helps develop and improve the material properties of the ROS ash roadbase blends.

Depending on the level of ROS ash/lime binder addition, crushed rock may have improved compressive strength, stiffness, moisture sensitivity and reduced permeability. This may lead to reduced pavement thickness and/or better pavement life, depending on the extent and duration of site loading.

In addition, ROS ash can assist in the control of plasticity in clay bearing materials. The plasticity index test (PI) is conducted on the <425mm fraction and an addition of ROS material can be used to reduce the PI of the final blend by diluting the quantity of clay bearing material <425mm in the blend and reducing the ability of these materials to absorb water and swell.

This may mean materials that were too high in plasticity to be previously used in roadbases, etc, such as scalpings, may now be incorporated in controlled percentages to produce high quality construction materials that will be workable, bind easily and not be water-sensitive to rises in the water table or inundation due to flooding etc. This can mean tremendous benefit to operating quarries, as it allows them to produce high quality materials while better managing their stock balance on the ground.

{{image4-a:r-w:250}}In considering the specifics of roadbase design, we need to develop a rock skeleton within the material to provide shear strength, a continuous combined grading to encourage aggregate interlock and fines to encourage ease of compaction without promoting over-compaction and potential breakdown, to achieve in-situ density in the field.

The addition of ROS CCPs assists in improving roadbase workability, which makes it easier to place, potentially requiring less compaction at the same moisture content, to achieve in-situ density, therefore minimising the risk of potential aggregate breakdown, with the void-filling and binding nature also being significant.

Fly ash has been used in the supply of concrete since 1958 in NSW as an SCM. It has also been used extensively as a binder supplement in modification and stabilising applications in the construction of pavements. Given the abundance of CCPs and their tremendous benefits when used in construction materials, now is the time for industry to fully utilise these products.

Existing power station production far exceeds our current usage and valuable CCPs are being stockpiled in large landfills when they could be used right now to benefit our economy and provide a better environmental outcome for everyone.

Michael van Koeverden is a director of Engineered Material Solutions (EMS), a specialist consultancy dedicated to providing professional advice in quarry, concrete and mining materials. For more information, visit www.quarryconsultants.com.au

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