13.3 SKILLS

13.3 SKILLS

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SKILLS
Rahmat Dirgantara
PhD Candidate

Development of Brown Coal Geopolymer Concrete

by Rahmat Dirgantara, PhD Candidate, RMIT University; Supervised by Dr David Law and Assoc. Prof. Tom Molyneaux at School of Civil, Environmental and Chemical Engineering, RMIT University

World-wide, the production of cement releases almost 2 billion tonnes of CO2 into the atmosphere annually. Fly ash from coal-fired power generation contains silicate materials that can be used as a binder material, and could replace the use of ordinary cement in some applications.
Rahmat Dirgantara’s has commenced a BCIA-supported PhD investigating the specific chemical make-up of brown coal fly ash from Victorian power stations, how brown coal derived fly ash can be used to make a novel geopolymer concrete, and the factors that contribute to optimal mechanical strength.

The purpose of the research is to investigate the use of brown coal (BC) fly ash (FA) as a binder to produce geopolymer concrete and to assess its mechanical properties. The potential use of BC FA as a binder in geopolymer concrete could result in utilisation of an industrial by-product from BC burning power stations.

The use of Ordinary Portland (OP) cement as the main binder material in concrete raises a number of environmental concerns due to the energy consumption and the emission of CO
2. It is estimated that one tonne of cement releases between 0.7 and 1.0 tonnes of CO2. Other concerns have also highlighted the use of coal as a primary energy source and the release of FA as a by-product, some of which becomes environmental waste.

Annual production of BC worldwide in 2008 was estimated to be 938 million tonnes. In Australia, the use of BC in Victoria alone annually produces more than 500Kt of combined BC FA and bottom ash. So far little research has been undertaken on the feasibility of using BC FA as a waste product and there is no commercial use of the material in the construction industry with the majority of the material being sent to landfill at present.

FA can be categorised as either class F or class C. Class F FA is produced from burning anthracite and bituminous coals, while class C FA is produced from sub-bituminous coal and lignite. FA as an industrial by-product contains silicate materials that have been used as an alternative binder material to OP cement.

The activation process to create alkali-activated concrete, known as geopolymer concrete, uses 100% FA as a binder and is due to the activation of the aluminosilicates by high concentration alkali. Its reaction is distinctly different to the activation of the FA by the Ca(OH)
2 produced by hydration of the OP cement when FA uses as cement replacement. To date research has focussed on class F FA with high aluminosilicate content, which is required for the activation process. Therefore, if the composition of the aluminosilicates in BC FA is sufficient it may be feasible to use them to produce geopolymer concrete.
The FA used in this study comes from three separate sources located in the Latrobe Valley; Loy Yang, Yallourn and Hazelwood; and was recovered directly from the electrostatic precipitators with no pre-treatment prior to being supplied. Large variation in the chemical composition of the FA has been observed over time, despite being from the same source and this is attributed to the natural variations in the coal.

Initial trials were undertaken on mortar specimens using BC FA from all three Latrobe Valley power stations. The maximum strength of the geopolymer mortar obtained from the Loy Yang BC FA was 56 MPa, while Yallourn and Hazelwood BC FA gave compressive strengths of approximately 10 MPa. The results showed that the critical factor determining the compressive strength was the aluminosilicate content of the FA. The mineralogical composition, morphology, liquid to solid ratio and sulphate content were also all identified as influencing the overall performance.

Further trials have been undertaken using Loy Yang BC FA to manufacture concrete specimens. The concretes produced had a maximum compressive strength of 60 MPa, more than sufficient for a construction grade concrete. However, the results did show considerable variability between 20–60 MPa, which is attributed to the variability in the unrefined FA. Overall, the research demonstrates that the manufacture of concrete is feasible using BC FA as a geopolymeric material. At present further investigation is being undertaken to determine the optimum mix design, the mechanical properties, durability characteristics and the treatment of the raw BC FA.


Above: Brown Coal Geopolymer Mortar BC FA from
A. Loy Yang; B. Yallourn; and C. Hazelwood.


Above: Brown Coal Geopolymer Concrete BC FA
from Loy Yang.




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