Phase constitution at interfacial between lightweight aggregate/concrete cement paste composite

Previous microscopic investigation has suggested the interfacial zone of normal weight concrete is ∼50 μm outside the aggregate surface. For lightweight concrete, the interface will be different due to the porous nature of lightweight aggregate surface. In this study, the interfacial zone of lightweight concrete was examined using scanning electronic microscopy. The composition changes of calcium (Ca), silicon (Si), aluminium (Al) and iron (Fe) contents, across the aggregate/cement interface, were analysed with energy dispersive X-ray analysis. The test results indicated that the interfacial zone is characterised by a decrease in Si composition percentage and an increase in the Ca composition percentage from the lightweight aggregate across the interfacial zone towards the hardened cement paste. The aggregate shell forms a transitional zone inside the aggregate for different elements. For lightweight concrete, the interfacial zone is composed of a contact zone of 100 μm outside the porous lightweight aggregate and 100 μm inside the aggregate. There is an overall transitional shell region of ∼300 μm at the aggregate surface which limits further absorption of cement paste into the aggregate and formed the diffusion bonding between the aggregate and cement paste. © 2008 W. S. Maney & Son Ltd.