Distribution and factors of interfacial transition zone between metakaolin-slag-based geopolymer and granite aggregate

The alkali-activated metakaolin ground granulated blast fumace slag(GGBFS) and granite were used as cementitious material and aggregate, respectively, to prepare geopolymer concrete. And SEM-EDS and microhardness test were conducted to investigate the microstructure, distribution around the aggregate surface, and mechanical properties of the bond zone between geopolymer paste and aggregate. Test results indicate that an interfacial transition zone (ITZ) exists in the bond zone between geopolymer gel and aggregates. The ITZ includes crack and N-A-S-H gel, whose chemical composition is different from geopolymer gel. The distribution of ITZ around aggregate surface is not uniform, since the ITZ near soffit of aggregate has a weaker microstructure and a lower hardness than those on upper and side surfaces of aggregate. With the increase in the liquid/solid ratio and the dimension of aggregate, the width of crack in ITZ at aggregate soffit increases and the thickness of N-A-S-H gel decreases, meanwhile changes of mixture and aggregate dimension have no obvious effect on the microstructure and the strength of ITZ at upper and side interface around aggregate, which indicates a significant inhomogeneity. It can be concluded from test results that the interface between geopolymer gel and aggregate at aggregate soffit will be the weak zone of geopolymer concrete.