Influence of properties of fly ash from different sources on the mix design and performance of geopolymer concrete

Geopolymer concrete produced using 100% fly ash as the main binder replacing Portland cement (PC) has been the focus of the research study. A major challenge in the specification of geopolymer mix designs is the variability in the fly ash used and the impact of that variability on the performance of the geopolymer produced. The research to date has concentrated on the properties of these materials, with distinct variations in performance noted. Little research has been undertaken on understanding the chemistry behind these variations and in characterizing the components of the fly ash and the activators and how their interaction and relative concentrations determine the performance and properties of the geopolymer concrete produced in long term. This research study aimed at developing a fundamental understanding of the physical, mineralogical and chemical properties of fly ash on the performance of 100% fly ash based geopolymer concrete. The broader literature review was conducted initially to identify properties of fly ash affecting compressive strength of geopolymer. Then a comprehensive experimental programme was designed and executed with a wide range of state of the art techniques to understand the specific influence of individual properties of fly ash and combined effect which affects the compressive strength of geopolymer. The key factors affecting the performance of geopolymers made from a total of five chemically and physically distinct fly ash is reported. The key factor identified as influencing the strength was the workability, with a flow in the range between 110 ± 5% and 140 ± 5% required for optimal performance. In this flow range, the strength of geopolymer is governed by the specific surface area of precursor fly ash coupled with the quantity of amorphous phase up to 20mm in particle size. In addition a negative zeta potential of the fly ash was identified as assisting gel formation with the smaller the negative zeta potential of the geopolymer product the larger the quantity of gel formation and higher the compressive strength observed. The uniformity of the distribution of SiO2 and Al2O3 in the fly ash is observed to directly influence the dissolution of the amorphous surface layer in the initial geopolymerization process and control the aluminosilicate gel precipitation and gel-phase creation. This study shows that the higher the uniformity of distribution, and more stable the conversion of aluminium from octahedral to tetrahedral coordination the higher the aluminium amalgamation with silicates leading to production of a three dimensional polysialate-siloxo polymeric gel network with high rigidity and stability, which in turn results in higher compressive strength. A high CaO content in fly ash further leads to high compressive strength. The second phase of this study was dealing with the performance of geopolymer concrete up to one year using a range of fly ash with same mixing process, providing a systematic long term study of the mechanical and durability properties of a range of geopolymer concrete. Hence, the research data presented here will be extremely useful to understand the long term behaviour of geopolymer concrete made with the wide range of fly ash that are available across the world. The results show a considerable increase in performance observed between 90 and 365 days for all concrete depending on the fly ash properties. This is attributed to an on-going geopolymerization which results in continuing gel formation leading to a more densely packed microstructure, with an associated reduction in meso-pores and macro-pores. The nature of the gel matrix formed, in terms of uniformity and compactness, was observed to determine the mechanical properties. The presence of a high quantity of CaO leads to a densely packed microstructure at an early age, giving high early compressive strength. The nature of the interfacial transition zone formed between coarse aggregate and mortar and its density was observed to govern the tensile strength. An increase in porosity and micro cracks was seen to negatively affect the compactness of the gel matrix, which in turn affects the elastic modulus. The packing density coupled, with the pore size distribution, were observed to determine the permeation and diffusion characteristics of the concrete. A high quantity of meso-pores in the gel paste was observed to increase the water absorption while a high quantity of macro-pores leads to an increase in the water and air permeability of geopolymer concrete. Notably the initial chloride diffusion coefficients are analogous to those observed in Portland and blended cement concretes and also observed to decrease with the age in a similar manner. At last the applicability of current relationships of Portland cement concrete as specified in Australian Standards (AS) and American Concrete Institute (ACI) for geopolymer concrete have been critically examined. The results indicated that the flexural strength of geopolymer concrete is higher than those predicted using current design equations for Portland cement concrete of similar compressive strengths. However, splitting tensile strength of geopolymer concrete is comparable to those predicted using current design equation for Portland cement concrete for similar compressive strength. It was also observed that ACI stated equation significantly overestimates the splitting tensile strength of geopolymer concrete. Similarly, Australian Standards overvalues the elastic modulus of geopolymer concrete