Effect and mechanism of coal gangue concrete modification by fly ash.

• Coal gangue concrete (CGC) with 30% fly ash (FA) content has the best workability and strength. • The modification effect of FA on the working properties and strength of CGC is determined. • FA modifies CGC via physical and chemical changes in pores and ITZ structure. • Appropriate FA content enhances the effect of average gel pore size on the properties. Fly ash (FA) and gangue are common solid wastes. In this study, coarse aggregates were replaced with coal gangue at a constant substitution rate of 40% to promote the application of coal gangue in concrete production. The modification effect of FA content on the working and mechanical properties of coal gangue concrete (CGC) and its modification mechanism were revealed. At the macro level, the compression, splitting tensile, and flexural strengths were tested by using a pressure tester. From the microscopic aspect, the pore structure of the CGC was analyzed by nuclear magnetic resonance technique. The chemical products of the mortar and the micromorphology of the interfacial transition zone (ITZ) were characterized via scanning electron microscopy and energy dispersive spectroscopy. Results showed that the working and mechanical properties of the CGC can be significantly improved by adding an appropriate FA content. From a physical aspect, the modification mechanism is that fine FA particles fill the pores of the CGC during early cement hydration and contribute to the strength as microaggregates. Chemically, the secondary hydration of the FA particles changes the microstructure of the mortar and the ITZ. On the basis of grey relational analysis, the gel pore volume has the highest correlation with the concrete strength and the average gel pore size presents a linear relationship with the mechanical strength. Furthermore, appropriate addition of FA can significantly improve the average gel pore size. [ABSTRACT FROM AUTHOR]