The influence of porous aggregate on microstructure of the interfacial transition zone in lightweight concrete

The results of research of interfacial transition zone (ITZ) between sintered fly ash porous aggregate and cement paste are presented in this paper. The tests were carried out on concrete made of initially dry and pre-saturated with water lightweight aggregate, but at the same nominal water-cement ratio. The analysis of concrete specimens under scanning electron microscope revealed the lack of ITZ around sintered fly ash aggregate, considered as a layer of different microstructure in comparison to bulk cement matrix. Generally, the layer of higher portlandite content was not found in the analysed areas around the aggregate grains. However, in the case of concrete made of pre-saturated with water aggregate some areas rich in ettringite was observed. This ettringite formed the agglomerations or the layers, relatively long. Although the initial saturation of aggregate or its lack considerably influenced the microstructure of the paste in concrete, it was out of importance with regard to the depth of cement paste penetration into the open pores of lightweight aggregate. The formation of C-S-H phase in the internal aggregate pores was also observed in the case of both examined concrete.