Contents lists available at ScienceDirect Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp Microstructure development and mechanism of hardened cement paste incorporating graphene oxide during carbonation Wu-Jian Long a , Yu-cun Gu a , Feng Xing a , Kamal H. Khayat b,* a Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, Shenzhen Durability Center for Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen, 518060, Guangdong, PR China b Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, 65401, USA ARTICLE INFO Keywords: Carbonation Graphene oxide (GO) Cement Microstructure Electrochemistry ABSTRACT In this work, the carbonation mechanism of cement paste containing graphene oxide (GO) was examined by evaluating its electrochemical, kinetic, and microstructural characteristics under accelerated carbonation, cor- responding to a 20% concentration of CO 2 , a temperature of (30 ± 1)°C, and a relative humidity of 65%–70%. Transport properties of the composites were studied using a non-destructive electrochemical impedance spec- troscopy (EIS) technique, while their carbonation kinetics was investigated via thermogravimetric analysis (TGA). The obtained EIS results indicated that the ion diffusion and transport resistance increased after the incorporation of GO, while TGA results revealed that the carbonation of portlandite (CH) and calcium-silicate- hydrate (C-S-H) was significantly inhibited during early ages of carbonation due to the increased degree of hydration. In addition, the hybrid GO/hydration products from the carbonation process were characterized, and the formation of a hydrated phase coated with a carbonated layer was observed via scanning electron micro- scopy and energy dispersive spectroscopy. Porosity variations of the studied materials during carbonation were also evaluated using a mercury intrusion porosimetry method. The porosity of the OPC decreased more sig- nificantly during the initial carbonation period as compared to the effect observed for the GO cement-based material. 1. Introduction Cement-based materials are extensively used worldwide in civil infrastructure, which is often exposed to harsh environments and severe loading conditions. The durability of structures in aggressive environ- ments is a key parameter that strongly affects their service lives and maintenance costs. In particular, carbonation of cement-based mate- rials represents a serious problem that reduces their durability, and it is commonly considered a neutralization phenomenon inside hardened cement-based materials. Carbonation involves the reaction between the carbon dioxide and calcium-containing phases, which reduces the pH of the pore solution [1–3], thus leading to de-passivation of reinforcing bars and corrosion in the presence of moisture and oxygen [4–7]. The hydration products interacting with dissolved CO 2 mainly consist of portlandite (CH) and calcium-silicate-hydrate (C-S-H). Although CH is expected to be more susceptible to carbonation from a thermodynamic point of view, the carbonation processes of CH and C-S- H occur simultaneously [8–10]. In previous studies, the morphology of calcium carbonate products (including calcite, vaterite, and aragonite minerals [11–13]) and amorphous calcium carbonate formed under various conditions has been investigated [14–17]. The obtained results demonstrate that vaterite and aragonite are usually formed at larger CO 2 concentrations, low pH of the cement matrix, or low C/S ratio of the C-S-H exposed to CO 2 gas. It has also been found that carbonation produces significant changes in the microstructure of cement-based materials. According to the results of a number of studies [18–20], the capillary pores with sizes ranging between 10 and 300 nm can be re- duced by carbonation, causing a systematic reduction in total porosity. It has been found [15] that the observed decrease in total porosity of cement-based materials results not only from the CH carbonation, but also from the decalcification and polymerization of C-S-H and forma- tion of amorphous silica gel. Recently, several researchers have attempted to apply nano- technology to cement-based materials to improve their mechanical performance and durability [21,22]. It is known that nanoparticles can fill the voids in the cement matrix and enhance the suppression of the ion transport, and thus improve its durability, including the resistance of carbonation [23–29]. Among various nanomaterials, graphene oxide https://doi.org/10.1016/j.cemconcomp.2018.08.016 Received 9 January 2018; Received in revised form 2 June 2018; Accepted 27 August 2018 * Corresponding author. Faculty of Civil, Architectural and Environmental Engineering, Missouri, University of Science and Technology, Rolla, MO, 65409, USA. E-mail address: khayatk@mst.edu (K.H. Khayat). Cement and Concrete Composites 94 (2018) 72–84 Available online 29 August 2018 0958-9465/ © 2018 Elsevier Ltd. All rights reserved. T