Industry News

Coal power station waste as a fine aggregate replacement in concrete

The theme for the Construction Materials Industry Conference 2012 (CMIC12) – Essential industry for Australia?s future – not only emphasises the importance of the construction material industry in developing Australia?s infrastructure and economy but also highlights the importance of advancing sustainable practices.

{{image3-a:r-w:250}}Concrete is recognised as the world?s most widely used construction material for various reasons, including its reputation for durability, efficiency, cost-effectiveness and architectural flexibility.

With millions of cubic metres of concrete consumed in New South Wales alone and the industry contributing $1.2 billion to the NSW economy, it is the base for our economy and modern society (CCAA 2011). In effect, it is important that concrete is produced and consumed in a sustainable manner for a better future.

{{image2-a:l-w:250}}The main constituents of concrete include cement, water, coarse aggregates, fine aggregates and admixtures, which are becoming increasingly popular to enhance the properties of concrete to better suit its application. Due to concrete?s high demand in the construction industry, Earth?s finite resources are becoming a prominent issue.

{{image4-a:c-w:500}}Due to the high demand for concrete in NSW, there has been considerable concern about the depleting natural sand sources in the NSW region. Fine aggregate makes up about 25 to 30 per cent of the total concrete volume. Currently, most of the natural fine sand sources for the Sydney metropolitan region are located in the Blue Mountains, Central Coast and Kurnell.

In the near future, most of the natural sand sources will either be depleted or exhausted, which will induce sand to be transported from longer distances, increasing the cost of concrete in NSW and also its environmental impact.

Bottom ash is a by-product of the combustion of coal in a coal-powered power plant. Currently there is an abundant amount of bottom ash available in various coal power stations around NSW. There has been research performed on using bottom ash as a fine aggregate replacement in other countries but bottom ashes from different sources have been proven to have different properties depending on their sources. In effect, it is an imperative for Australia?s concrete industry to invest in researching the potential for the use of bottom ash as an aggregate in concrete and to convert the so-called ?waste? into a ?resource?.

{{image5-a:c-w:600}}Therefore, this research study explored the feasibility of using bottom ash (BA) as a partial fine aggregate replacement in concrete from the following power plants: Bayswater (BBA), Mt Piper (MTP) and Wallerawang (WNG) (refer to Figure 1).

The bottom ash was trialled in combination with natural sands as well as with manufactured sand.

The study explored the inclusion of the BA on the engineering properties of concrete, including: fresh properties (slump and mass per unit volume) and hardened properties (compressive strength and drying shrinkage).
BBA, MTP and WNG were selected as partial fine aggregate replacements (by weight of the total fine aggregate content) at 20 per cent, 30 per cent, 40 per cent and 50 per cent. A field trial was also performed to assess the practicality of utilising bottom ash as a fine aggregate. The following raw materials were utilised in the concrete mixes:

a) General purpose cement.
b) Fly ash.
c) 20mm concrete agg.
d) 10mm concrete agg.
e) Fine sand.
f) Coarse sand – natural/manufactured.

A normal class 32MPa mix design from one of Hanson?s concrete plants was utilised as a control mix and the fine aggregate content was altered, as shown in Figure 2 (page 29), to form the experimental procedure. There were three groups of mixes, including Bayswater, Mt Piper and Wallerawang bottom ash as the fine aggregate replacement. The field trial was also conducted using local materials. The chemical composition as determined by X-ray florescence (XRF) analysis of the BAs is given in Table 1.
As can be seen, the bottom ashes have a similar chemical composition to the fly ash; therefore it can be suspected that the bottom ash has the potential to improve the later age strength of concrete.

Fresh concrete: Workability and mass per unit volume (MPV)
The slump was chosen to be fixed and it was controlled within 80 ? 15mm (the Australian Industry Standard for normal concrete). The MPV results obtained from the experimental procedure are depicted in the figures below. A plausible reason for the observed decrease in MPV could be the lower specific gravity of the BA mixes as outlined in Table 2.

Hardened concrete: Compressive strength
The hardened concrete properties of the various groups followed a similar trend; however, the performance of the bottom ashes varied, with the Bayswater outperforming the Mt Piper and Wallerawang bottom ashes respectively. Figure 5 depicts the development of compressive strength, up to 91 days? curing, for the Bayswater BA mixes. It can be seen that the control mix reached higher compressive strength than the BA mixes with the exception of the CFS80-BBA20 mix after 28 days.
Hardened concrete: Drying shrinkage
In the mixes with the BAs, the general trend was observed to be that the drying shrinkage increases with the increase in BA content of the mix; however, as depicted in Figure 6, it is observed that the drying shrinkage of the mix CFS80-BBA20 was lower than the drying shrinkage of the control mix. At week 13, the drying shrinkage of the CFS80-BBA20 was 70 ?s lower than the control mix.

Although the drying shrinkage of the concrete mixes increased with the increase in bottom ash content, the drying shrinkage was observed to be less than 750 ?s for all the concrete mixes, which is acceptable in the general application of concrete.

A field trial was conducted using Bayswater bottom ash and manufactured sand in the Hanson Brandy Hill quarry. The field trial consisted of using the concrete mix to pour a 10m3 slab.
The aim of the field trial was to assess the practicality of utilising Bayswater bottom ash in concrete plants and agitator trucks as well as other on-site issues that cannot be tested in the lab. The field trial was successfully performed and the concrete mix met all the fresh and hardened concrete properties that were practically required. The photos taken on the day depict the progress of the field trial as well as the end product.

From this investigation, it can be concluded that BA can be satisfactorily used as a partial fine aggregate replacement in concrete up to 20 per cent of the total fine aggregate content with minimal effect on the fresh and hardened concrete properties. It was also concluded that there is potential for certain sources of BA to produce lower MPV, higher later age compressive strength and lower drying shrinkage, therefore offering   an improved performing concrete.
The experimental investigation also shows bottom ash provides a viable fine aggregate replacement in concrete to be used in combination with manufactured sand that virtually eliminates the need for natural sand in concrete. However, further research and development is required to optimise the use of such mixes.

The successful application of such concrete in the field trial provides good support that concrete produced using bottom ash and manufactured sand can be utilised in practical applications.

Rishabh Satsangi is the technical manager at Hanson Construction Materials Pty Ltd.

Thanks to Hanson Construction Materials Pty Ltd for its support during the conduct of this research investigation.

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