There is increasing demand and interest in aggregates from non-traditional sources such as from industrial by-products and recycled construction and demolition (C&D) wastes. The American Concrete Institute (ACI)1 focuses on the removal and reuse of hardened concrete whereas the Department of the Environment and Water Resources in Australia and CSIRO2 have developed a guide on the use of recycled concrete and masonry materials.
The Waste & Resources Action Programme (WRAP) in the UK classified aggregates from primary, recycled and secondary material resources. Recycled aggregates encompass industrial by-products and reused construction products, all of which were once considered wastes and dumped in landfill. The recently introduced European Standards for aggregates do not discriminate between different sources, and are for ‘aggregates from natural, recycled and manufactured materials’. The focus is on fitness for purpose rather than origin of the resource.
The purpose of this report is to review the various sources of aggregate and examine their potential use in concrete and/or road construction materials.
Classification of Aggregates
For the purpose of this report, the following classifications are adopted.
Natural aggregate Construction aggregates produced from natural sources such as gravel and sand, and extractive products such as crushed rock.
Manufactured aggregate Aggregates manufactured from selected naturally occurring materials, by-products of industrial processes or a combination of these.
Recycled aggregate Aggregates derived from the processing of materials previously used in a product and/or in construction.
Reused by-product Aggregates produced from by-products of industrial processes.
Fly Ash Aggregates
Description Several lightweight concrete aggregates can be produced from fly ash. In addition to the use of furnace bottom ash in concrete masonry, pellets of fly ash can be bound by thermal fusion or chemically, using cement or lime. Such materials have many desirable properties.
In the mid 1990s, Pacific Power conducted a feasibility study of the production of sintered fly ash aggregates (Powerlyte) and examined the use of such aggregates in concrete production. Fly ash was pelletised and fired at controlled temperature to produce synthetic coarse and fine aggregates. These fly ash aggregates have a specific gravity range of 1.20–1.47, a bulk density range of 650–790 kg/m3 and very high absorption from 16–24.8%.
There has been production of chemically-bound fly ash aggregate in Australia but there has been no commercial development of sintered fly ash aggregate.
Applications and limitations Concrete containing sintered fly ash aggregates were experimentally investigated5. It was found that the density of the hardened concrete ranged between 1740 and 1840 kg/m3 compared with 2400 kg/m3 for the control concrete made from natural aggregates. Structural grade 25 concrete could be produced but with significantly higher cement contents compared to the control. The elastic modulus was found to be around 14–17 GPa compared to 28–30 GPa in the control. The drying shrinkage was found to be higher than the control.
Description Manufactured sand is a purpose-made crushed fine aggregate produced from a suitable source material and designed for use in concrete or road construction. Only source materials with suitable strength, durability and shape characteristics are considered. Production generally involves crushing, screening and possibly washing. Separation into discrete fractions, recombining and blending may be necessary.
Extensive research is being conducted in Australia by Cement Concrete & Aggregates Australia (CCAA) to support the specification of manufactured sands in Australian Standards and to provide guidance on its use as fine aggregate in concrete. The RTA has also carrying out an R&D project which supports the revision of its specification for natural and manufactured sands for asphalt and concrete mixes.
CCAA research6 recommended specification of manufactured sand in terms of density, grading, material finer than 75 um, particle shape, quantity and quality limits on deleterious fines, durability and impurities.
Manufactured sand is widely available in Australia.
Recycled aggregates are aggregates derived from the processing of materials previously used in a product and/or in construction. Examples include recycled concrete from construction and demolition waste material (C&D), reclaimed aggregate from asphalt pavement and scrap tyres.
Coal Washery Reject (CWR)
Description Coal reject material or colliery spoil is a by-product of the coal preparation process and is mostly produced in conjunction with the beneficiation of coking coal. Raw coal is washed to remove the high ash materials contained within it when it is mined. The material producing ash includes: plies of non-carbonaceous or low carbonaceous material within the coal seam; roof and floor material inadvertently mined with the coal; and dyke rocks that have intruded into the coal seam.
The generated quantity of CWR in Australia divides evenly between New South Wales and Queensland – the two principal coal producing states. About 20% of the total CWR is produced as tailings and 80% as coarse coal reject48.
Applications and limitations CWR has been used successfully within unsealed road pavements and for haul roads at many collieries. CWR has been used sparingly by the NSW RTA in the Lithgow and Wollongong areas as a subbase material. Subbase CWR is typically stabilised with cement in Lithgow and lime in Wollongong.