Application of AC impedance technique in study of lightweight aggregate-paste interface

• The resistance of the continuous conduction paths in LWA cannot be ignored.
• The resistivity of the ITZ around LWA is higher than that of the bulk paste.
• The strong water absorbability of LWA leads to a high resistivity of the ITZ.

Find a simple and effective method to study the aggregate-paste interface is important to enrich our understanding of the role of aggregate, which occupy about 70% of concrete volume, on the microstructure of the interfacial transition zone (ITZ). In this paper, the alternating current (AC) impedance technique, together with SEM, EDX, XRD and microhardness test, were used to examine the ITZ between paste and lightweight aggregate (LWA) with different types, particle sizes and prewetting degrees. The limestone aggregate was also included for comparison. The results show that, for the same concrete sample, the diameter of high-frequency semicircle has obvious growth with the increase of the curing time, which indicates the microstructure of the concrete becomes denser. However, to study the microstructure of the interface, it is also need to adopt Song’s equivalent circuit model. By measuring the resistance of the continuous conduction paths (CCPs) in LWA, it is found that the LWA cannot be entirely seen as an insulator like rock aggregate, thus the resistance of the CCPs in lightweight concrete should be seen as a parallel circuit of the bulk paste, ITZ and opening pores in LWA. Impedance results show that the resistivity of the ITZ around rock aggregate is lower whereas that around LWA is higher, than that of the bulk paste. Moreover, the stronger the LWA absorbs the water, the higher the resistivity of the ITZ will be. From the results of SEM and microhardness it can also be seen that, the “wall effect” does not seem to occur on the surface of LWA, many cluster hydrates closely packed together in the ITZ, and its microhardness is significant higher than that of the bulk cement paste. Although the saturated LWA has weak water absorbing capacity in the early time, it can significantly improve the long term microstructure of the ITZ. All the results obtained are consistent with the impedance findings.