Microstructure and strength development of quaternary blend high-volume fly ash concrete

This study investigates low cement quaternary blend HVFA concrete mixes utilizing up to 80% cement replacement using fly ash, hydrated lime and nano-silica. The optimized concrete mixes achieved a compressive strength of 55 MPa and 48 MPa, for HVFA-65 and HVFA-80 concretes, respectively. Additional fly ash and hydrated lime dosage in HVFA concrete increased the rate of hydration of the C3A and C4AF phases but decreased the hydration of the C3S phase. This resulted in lower early age strength development in the HVFA concrete than occurs in PC concrete but significantly higher than for fly ash and hydrated lime alone. The addition of the nano-silica resulted in an increase in C–S–H gel incorporation of tetrahedrally coordinated aluminium (AlIV) into the HVFA concrete and the substitution of Si by Al in the C–S–H gel, leading to an increase in compressive strength in the HVFA concrete. Early age carbonation was increased with a higher of fly ash percentage. However, the reaction products dissolved in the pore water to form calcium bicarbonate with time.