Mechanical properties of green structural concrete with ultrahigh-volume fly ash

Using a high dosage of fly ash in concrete is an effective approach to control the heat release rate, reduce the material cost and enhance the sustainability. However, ultrahigh-volume fly ash (UHVFA) concrete, with fly ash replacing over 60% of the binder by weight, often exhibits low compressive strength at an early stage, which limits the material to non-structural or semi-structural applications. Though different approaches have been proposed to increase the strength, the efficacy of some of the methods is debatable, because of the high energy consumption and/or low cost-benefit ratio. This study aims to increase the compressive strength of UHVFA concrete by the simple and practical method of reducing the water/binder ratio while adding super-plasticizers to maintain workability. Mortar samples were used to explore the influence of silica fume, and Portland cement was replaced with fly ash at five different percentages (20%, 40%, 60%, 80% and 98%). Mechanical properties up to 360-day age were recorded, and the cementing efficiency factor of the fly ash was studied. With a suitable mix proportion, even with 80% of the binder replaced by fly ash, the compressive strength of the mortar and concrete can reach over 40 MPa at 7-day age, and over 60 MPa at 28-day age. Compared to commercial Grade 45 concrete, the proposed green structural concrete shows a reduction in CO2 emission of around 70%, a reduction in embodied energy of more than 60%, and a reduction in material cost of 15%.