The use of brown coal fly ash as a replacement of cement in concrete masonry bricks
Creators: name: KHODR, Muhamed
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400505
subject: Construction materials

description: The focus of this study was to produce 100% brown coal fly ash geopolymer concrete by replacing Portland Cement. Portland Cement production is not regarded as environmentally friendly due to high carbon emissions, being responsible for 5% of global emissions. An alternative is to substitute fly ash for Portland Cement. Australia has an abundance of brown coal fly ash as it is the main source of primary energy in the country. Currently, the majority of the material is being stored in landfills. However, there is no commercial use for the material in the industry, as the brown coal fly ash cannot be used as a direct replacement material for Portland Cement due to the high sulphur and calcium content and low aluminosilicate content. Although, the potential exists to use brown coal fly ash as a geopolymeric material, yet there remains a significant amount of research needed to be conducted. One possible application is the production of geopolymer concrete bricks. The current study aims to develop a greater understanding of the chemical and physical properties of the material, leading to an optimised mix design for a geopolymer concrete brick. A comprehensive laboratory study will be conducted to examine the properties of brown coal geopolymer bricks, which will then be interpreted using state of the art material analysis techniques.

The proposed project aimed at investigating the use of brown coal fly ash from Latrobe Valley power stations for manufacturing geopolymer masonry bricks. The research work allowed for a comprehensive understanding of the fundamental chemistry behind the activation of the brown coal fly ash and the reaction mechanisms involved to enable the development of brown coal fly ash geopolymer concrete bricks. That research allowed for the identification of manufacturing techniques to investigate relationships between compressive strength and processing parameters and to understand the reaction kinetics and microstructure developments. A literature review is carried out to identify the key parameters that affect the production of fly ash geopolymers. Then a comprehensive outline of experimental methods was designed and executed with state of the art techniques. This allowed for a detailed analysis to understand the factors pertaining to manufacturing a suitable mix design for brown coal fly ash geopolymers.

The first phase of research was developed to gain a principal understanding of the physical, chemical and mineralogical properties of the Loy Yang and Yallourn fly ash to produce a 100% fly ash based geopolymer mortar. The optimisation of the Loy Yang geopolymer mortar was initially conducted to identify the chemical properties that were influential in the production of a satisfactory geopolymer strength. This was then followed by the optimisation of the Yallourn geopolymer mortar. The Loy Yang mortars were able to produce characteristic compressive strengths acceptable in load-bearing bricks, whereas Yallourn mortars produced characteristic compressive strengths only acceptable as non-load-bearing bricks. The second phase of the research was transposing the optimal geopolymer mortar mix designs into optimal geopolymer concrete mix designs while merging the mix design with the optimal Adbri Masonry (commercial partner) concrete brick mix design. The reference mix designs allowed for optimisation of both Loy Yang and Yallourn geopolymer concrete mix designs, with the Loy Yang requiring increased water content as the original mix design was deemed to be too dry. 

The key factors that influenced the compressive strength of the geopolymer mortars and concrete have been identified and reported. The amorphous content is considered a vital aspect during the initial reaction process of fly ash geopolymers. The amount of unburned carbon content contained in the fly ash can hinder the reactive process, and ultimately, compressive strength as the unburned carbon has the ability to absorb the activating solution thus reducing the particle to liquid interaction ratio in conjunction with lowering workability. Also, fly ash with higher surface areas showed lower flowability then fly ash with smaller surface areas. It was identified that the higher number of fly ash with 45 microns particles allowed for increased reactivity whereas primarily angular shaped fly ash suffered from reduced workability. The optimal range of workability lies between the 110 -150 mm slump, which corresponded with higher strength displayed for each respective precursor fly ash. Higher quantities of aluminium incorporation into the silicate matrix during the reaction process led to improved compressive strengths. This is illustrated by the formation of reactive aluminosilicate bonds in the range of 800-1000 cm-1 after geopolymerization, which is evidence of a high degree of reaction. In addition, a more negative fly ash zeta potential of the ash was identified as improving the initial deprotonation and overall reactivity of the geopolymer. Whereas, a less negative zeta potential of the mortar leads to increased agglomeration and improved gel development. Following geopolymerization, increases in the level of Quartz and decreases in the level of Moganite correlated with improved compressive strength of the geopolymers. Overall, Loy Yang geopolymers performed better, primarily due to higher aluminosilicate content than its fly ash counterpart.

The final step was to transition the optimal geopolymer concrete mix designs to producing commercially acceptable bricks. The results showed that the structural integrity of the specimens was reduced in larger batches, indicating that reactivity  and compressive strength was reduced. This was followed by a detailed analysis which identified a relationship between heat transfer, curing regime and structural integrity in large volume geopolymer brick application. Further research is required in understanding the relationship between the properties of the fly ash, mixing parameters, curing procedures and the overall process of brown coal geopolymer concrete brick application.