DOI: 10.7764/RDLC.18.3.476The interface between aggregate and mortar or paste is a weak phase. This has a serious effect on the properties of concretes. Because of the high porosity of lightweight aggregate, the interface of lightweight concrete is different from that of normal weight concrete. In this study, the effect of interfacial interactions between lightweight aggregate and mortar on the mechanical and chloride ion transport properties of concretes was investigated. The test variables were the volume fraction of the lightweight aggregate and the water–cement (W/C) ratio. The elastic modulus, the electrical charge passed from the rapid chloride penetration test (RCPT), and chloride ion penetration depth were determined. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used to observe the microstructures of the interfaces. SEM observation of the lightweight concrete revealed that the interface bonding between mortar and aggregate is considerably firmer than that of normal weight concrete. However, test results indicate that the ability of the lightweight concrete to resist chloride ion intrusion is worse than that of normal weight concrete. Comparison with theoretical models reveals that a negative factor affects the chloride ion transmission in lightweight concrete. As the volume fraction of lightweight aggregate increased, this negative influence is also increased. Chloride ions were detected both in the mortar and the lightweight aggregate in the EDX test. This indicates that chloride ions passed through the lightweight aggregate during the RCPT and can thus influence chloride ion transmission throughout lightweight concrete.