Microstructural analysis of interfacial transition zone (ITZ) and its impact on the compressive strength of lightweight concretes P. Vargas, Oscar Restrepo-Baena, Jorge I. Tobón ⇑ Department of Materials and Minerals, Cement and Building Materials Group, Universidad Nacional de Colombia, Colombia highlights  Microstructure in the ITZ of lightweight concretes was studied.  Lightweight aggregates (LWA) contributed to the formation of a dense and thinner ITZ.  Lightweight structural concretes were developed and explained. article info Article history: Received 9 September 2016 Received in revised form 23 December 2016 Accepted 25 January 2017 Keywords: Lightweight aggregates Lightweight concrete ITZ microstructure Lightweight concrete compressive strength abstract 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 aggre- gates 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. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The ITZ is a layer formed between an aggregate and the matrix of cement paste, composed of a double layer ‘‘duplex film” of cal- cium hydroxide crystals (Ca(OH) 2 ) oriented to the aggregate’s side and hydrated calcium silicate, C-S-H gel, oriented to the side of the paste, with a thickness of about 1 lm. Farthest from the aggregates is the main interface zone of about 40–50 lm thick, containing larger crystals of calcium hydroxide [1]. In this zone the cement particles are unable to bind intimately with the relatively large particles of the aggregate, ‘‘the wall effect”; consequently, the ITZ has a much higher porosity (2 to 3 times) than that of the hardened cement paste farthest from the aggregate particles [2]. The ITZ properties are affected by the characteristics of the aggregate and the cementitious matrix, as well. Several researchers have found that the grain size distribution of the cement, the water cement ratio, the aggregate size, and the type of aggregate have important effects on the ITZ properties [1–4]. Significant efforts have been devoted to optimizing the dosage and to the study of mechanical properties of lightweight concrete (LWC), to make them competent with normal weight concretes [5–7], which have led to the study of ITZ microstructure [3,4,8–11]. Finding that the mechanisms responsible for the ITZ formation http://dx.doi.org/10.1016/j.conbuildmat.2017.01.101 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: jitobon@unal.edu.co (J.I. Tobón). Construction and Building Materials 137 (2017) 381–389 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat