In this research both the microstructure and thickness of the interfacial transition zone (ITZ) in concretes of Portland cement and lightweight aggregates (LWA) are studied. It has been established that the microstructure in the ITZ strongly depends on the nature of the aggregate, specifically its porosity and water absorption. This study aims at researching the influence of physical properties such as density, porosity and morphology of lightweight aggregates such as pumice and expanded clays, on the microstructure and thickness of ITZ, and determine the effect that these factors have in turn on the mechanical properties as compressive strength of lightweight concretes (LWC). Lightweight aggregates were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and X-ray Fluorescence (XRF), to determine their mineralogical, morphological and chemical characteristics. The characterization of ITZ by SEM-EDS, and conventional optical microscopy, was carried out on specimens of concrete manufactured with LWA and with a conventional aggregate, in order to evaluate its thickness; furthermore, to determine the porosity, digital image processing (DIP) was performed. Lightweight aggregates contributed to the formation of a dense and thinner ITZ, when compare to the ITZ of a conventional concrete. The lower porosity and greater amount Of hydrated cement phases in the ITZ of lightweight aggregates are attributed to their physical, morphological properties and chemical and mineralogical composition; which contributed to the decrease of the wall effect, gestating from its surface the formation of C-S-H, achieving interlacing of hydrated phases on the surface of these aggregates with the cementitious matrix. (C) 2017 Elsevier Ltd. All rights reserved.