Development of brown coal fly ash geopolymer concrete

Substitution of Portland cement by pozzolanic material (PM) or supplementary cementing materials (SCM) is potentially the most effective way of decreasing both energy consumption and the production of greenhouse gases from Ordinary Portland (OP) cement production. One of the most common waste materials used at present is Pulverised Fly Ash (PFA) from coal fired power stations. This by-product can be interground, blended with the cement or substituted for cement. The levels of substitution can be from 20 to 70% of the OP cement or even a 100% replacement in geopolymer concretes.<br><br>ASTM C618 defines fly ash into 2 classes, class F is produced from burning anthracite and bituminous coals and class C fly ash from lignite and sub-bituminous coals. In Australia only, lignite is referred to as brown coal and categorized as neither class F nor class C due to the high sulphur content. In geopolymer concrete, the silicate materials in the fly ash binder react with the alkaline activator and become soluble reactants which form geopolymer paste that binds the aggregates and other unreacted materials together. Geopolymer concretes based on class F and class C fly ash have been found to give similar strength to both OP and blended cements concretes. However, to date little research has been undertaken on the feasibility of using brown coal fly ash as a binder. At present there is also no commercial use of the material in the construction industry with the majority of the material being sent to landfill.<br><br>This research investigated the possibility of using brown coal fly ash as a binder to produce geopolymer concrete, specifically Australia - Victoria - La Trobe Valley brown coal fly ash. The research comprised: a review of brown coal, class C and F fly ashes and their geopolymerization mechanisms; production of brown coal fly ash geopolymer mortar and optimization of the mix design to produce geopolymer concrete; evaluation of the reaction kinetics, microstructural development of geopolymeric formulations, and mechanical durability characteristics of geopolymer concrete.<br><br>The initial research investigated brown coal fly ash geopolymer mortar using fly ash from three power plants: Loy Yang, Yallourn and Hazelwood. The Loy Yang brown coal fly ash geopolymer mortar displayed compressive strengths compatible with the production of geopolymer concrete with acceptable strength for use in the construction industry. However, the geopolymer mortar specimens from Hazelwood and Yallourn brown coal fly ash gave results that indicated they were are not feasible to use for geopolymer concrete due to their inherent chemical composition.<br><br>Based on the optimal mortar composition the production and properties of geopolymer concrete produced from Loy Yang brown coal fly ash was investigated. The results showed that concrete with a compressive strength over 40 MPa could be produced, though the alkali modulus and SiO2/Al2O3 ratio were significantly more restrained that those previously reported for class F based geopolymer concrete. In addition to the compressive strengths, the FTIR and zeta potential data showed that the Loy Yang brown coal fly ash has properties feasible for the manufacture of a geopolymer concrete that could be used as a construction material. However, variability in the performance between batches of the Loy Yang brown coal fly ash and incomplete geopolymerization of large scale specimens under elevated temperature curing conditions were observed. The microscopy and porosity data showed that the geopolymer produced had a large number of pores in the macropore region together with a large number of interconnected pores and an inhomogeneous structure. This was reflected in the durability analysis, resistivity, UPV, carbonation, chloride diffusion, air permeability and sorptivity data which all gave results indicative of poor quality concrete. These results coupled with the variability in the chemical composition within the ash and the high sulphur content raise concerns over the consistency of the concrete produced and the long term performance. Overall, the research provides a fundamental understanding of the geopolymerization mechanism for Loy Yang brown coal fly ash geopolymer mortar and concrete and presents an opportunity for potentially diverting a waste stream into a useful material.