Backscattered electron imaging studies on the interfacial zone between high strength lightweight aggregate and cement paste

Much attention has been given to the influence of the interfacial zone between aggregate and cement paste on the various properties of concrete. Previous research has shown that for high strength lightweight aggregates with a very dense outer shell. such as Liapor 8, the nature of the interfacial zone is similar to that in normal weight concrete, where a more porous interfacial zone with massive layers of Ca(OH)₂ may form. In the work reported here the effects of silica fume, water/cement ratio, and the varying moisture conditions of the aggregate on the interfacial zone between cement paste and Liapor 8 were investigated. The incorporation of silica fume reduced the porosity and the thickness of the interfacial zone, but in some areas a concentration of Ca(0H)₂ adjacent to lightweight aggregate particles was still visible. The interfacial zone was also improved hy reducing the water/cement ratio due to reduced bleeding. It was the moisture conditions on the aggregate surface, rather than the total moisture content of the aggregate, which was the important factor concerning the microstructure of the interfacial zone.