Mechanical properties and microstructure of nano-reinforced concrete containing etched fly ash cenosphere

Utilizing fly ash cenosphere, which is a coal-based waste, as a substitute for cement to prepare concrete can effectively mitigate its negative environmental effect. A surface etching process for fly ash cenosphere was proposed in this paper, and nano-silica was also adopted to coordinatively reinforce concrete performance. The microstructure morphology and phase composition of the cenosphere before and after surface etching were compared using scanning electron microscopy and X-ray diffraction and the influence of the surface etched cenosphere on the hydration properties was characterized by thermogravimetric testing to determine the effectiveness of the proposed etching procedure. The effects of surface etched cenosphere and nano silica were studied using compression and split-tensile tests and scanning electron microscopy-energy dispersive spectroscopy. Results show that the etching procedure proposed in this paper accelerates the release of Si and Al elements inside fly ash cenosphere. The etched micro-pores provide effective paths for water migration, enabling fly ash cenosphere to have internal curing effect and increased pozzolanic activity. Thus, cement hydration and mechanical properties of concrete are significantly improved. However, when the dosage of fly ash cenosphere increases to 40%, the negative impact of the hollow structure of fly ash cenosphere is more significant, resulting in decreased concrete strengths. Furthermore, the synergistic enhancing effect of surface etched cenosphere and nano silica is significant with increased Si/Ca and Al/Ca ratios at the interface, which improves the composition of hydration products and optimizes the interfacial transition zone and thus benefits the microstructure and strength development of concrete.