Properties and Microstructure of an Interfacial Transition Zone Enhanced by Silica Fume in Concrete Prepared with Coal Gangue as an Aggregate

The wide application of concrete prepared with coal gangue (CG) as an aggregate (CPCGA) is limited because of the low mechanical strength and strong water absorption capacity of CG. This paper used silica fume (SF) to improve the performance of the interfacial transition zone (ITZ) in CPCGA and revealed the enhancement mechanism. The results showed that the compressive strength of CPCGA prepared with cement replaced by a suitable amount of SF at the age of 28 days increased by more than 30%, and the flexural strength increased by over 20%. The SF could effectively reduce the porosity and micropore size in the ITZ of CPCGA, and the porosity of the ITZ in CPCGA added with 7.50% SF decreased by 44.22, 46.16, and 24.46% at distances from the aggregate surface of 10, 50, and 100 μm, respectively, compared with CPCGA without SF. Further research showed that Ca­(OH)2 (CH) generated in the cement hydration reaction reacted with a large amount of active SiO2 in SF to restrain the formation of coarse CH in the ITZ of CPCGA, and the calcium silicate hydrate (C–S–H) gel generated filled the ITZ micropores to reduce the ITZ porosity further. Moreover, the reaction of SF and CH helped to promote the hydration reaction of cement to proceed thoroughly in CPCGA, thus improving CPCGA performance.