Design of novel high volume fly ash composites considering early age properties

© 2021 Zhiyuan ZhouAs the environmental problems have attracted more and more attention over the recent years, the cement production in concrete industry has become a global concern. In the concrete construction industry, ordinary Portland cement (OPC), as an important binder in concrete, is identified to be the major cause of energy cost and carbon emissions Thus to reduce the environmental impact caused by the cement production, sustainable concrete with the use of less cement has become a necessity. Supplementary cementitious materials (SCMs) have been developed to be used as admixtures in concrete. Fly ash (FA) which is one of the most commonly used SCMs is a waste material from the combustion of coal in electricity stations. It has a large storage over the world and continues to be produced over time. However, the most significant problem of concrete with FA, especially large amount of FA, is the slow early age strength development. A comprehensive literature review on different properties of FA concrete are first provided including mechanical properties, setting time, heat of hydration, workability, self-compacting concrete, shrinkage and creep, several major durability properties (chloride ingress, sulfate attack, carbonation, alkali-silica reaction) and microstructure. Thus the general properties of FA concrete could be better understood. The hydration and strength properties of FA concrete are further investigated for concrete with the incorporation of FA from different regions. The FA were sourced from both Indonesia and Australia including Gladstone, Port Augusta and Bayswater. It was found that the main difference of the FA in different regions is the particle size distribution and the differences in chemical composition. Higher fineness of FA particles leads to higher hydration rate and strength. The degree of hydration and chemically bound water (WB) is linearly correlated with the compressive strength. To improve the early age strength of the overall high volume fly ash (HVFA) concrete mixes, The HVFA concrete mixes were optimized by improving the aggregate grading, reducing the water to binder (w/b) ratio and adjusting the paste to void volume ratio (Vp/Vv). It was found that the concrete mainly fail in pastes rather than aggregate. Thus, the early age strength of HVFA pastes by the adding of admixtures was especially determined. The admixtures used to improve the compressive strength of HVFA pastes were nano-CSH crystals, calcium formate (CF), and hydrated lime (HL). The underlying mechanism of how theses admixtures work in the pastes was investigated by hydration and microstructural testing approaches. It was found that the adding of single nano-CSH crystals, CF or HL could improve the compressive strength of HVFA pastes due to different mechanisms. However, the addition of combined CF and HL decreased the strength mainly due to cracks and pores caused by rapid hydration. As the adiabatic temperature rise is an important parameter that could significantly affect the properties of concrete, the adiabatic temperature rises of OPC and FA concrete were modelled from the heat of hydration curves. The accuracy of the modelling was successfully improved by the adjustment of hydration parameters. Finally, machine learning (ML), as a statistical tool, was used in this research to predict the compressive strength of HVFA composites with the addition of different admixtures. The accuracy of the model could be further improved by increasing the data sets