Nano-CSH modified high volume fly ash concrete: Early-age properties and environmental impact analysis

Abstract

To reduce the carbon footprint in concrete industry, fly ash (FA) as the solid waste has been widely used to replace cement in concrete material. Nano-CSH crystals is used in this research to improve the early age properties of concrete with high volume fly ash (HVFA). The characteristic analysis of the HVFA concrete incorporating nano-CSH crystals was first conducted including early age strength development and setting time. The changes in hydration and microstructure due to the adding of nano-CSH crystals were explored by the isothermal calorimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM) testings. The environmental impact of concrete mixes was assessed using CMLCA software. The FA dosage in cement was up to 70% and the investigated nano-CSH crystals were 0.5%, 1% and 1.5%. The results showed that addition of 1% nano-CSH crystals in HVFA60% mixes and incorporation of 1.5% nano-CSH crystals in HVFA70% mixes achieved the highest strength and the shortest setting time. The adding of nano-CSH crystals accelerated the hydration of C3A and C3S to form more C–S–H and ettringite responsible for concrete strength and densified the overall microstructure. Ca(OH)2 content is reduced at the age of 28 days due to its reaction with FA. Adding 1.5% nano-CSH crystals in HVFA70% mix and incorporating 1% nano-CSH crystals in HVFA60% mix decreased the total environmental impact by 6.39% and 4.36% compared to pure HVFA70 and pure HVFA60 mixes respectively.