The microstructure and physical properties of heavy oil fuel ash replaced Portland cement for use in flowable fill concrete and the production of concrete masonry units

Abstract

This study aims at investigating the reutilization of heavy oil fuel ash (HOFA) as a cement replacement material in low grade concrete applications. Preliminary investigation and material characterisation of HOFA using scanning electron microscopy and X-ray energy-dispersive analysis, X-ray diffraction analysis, BET gas adsorption method and losson- ignition was performed. Various percentage replacements of Portland cement with HOFA were investigated by mortar testing and assessment of two low grade concretes to be used as a flowable backfilling material and concrete masonry units. The ash contained a high amount of unburned carbon, had a high specific surface area, low density and was X-ray amorphous. Incorporation in a cement-based mixture led to an extended setting time, increased water demand and high levels of Portlandite were observed. A suitable flowable fill concrete was achieved when limiting the cement replacement to 30%. Strength reductions were avoided with the concrete masonry units, and units of adequate strength were produced with replacement levels of 20%. Inclusion of HOFA in a concrete effected the hydration reaction of Portland cement. A 30% and at least 20% cement replacement for flowable fill concrete and hollow masonry units respectively is recommended.