IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 https://doi.org/10.15623/ijret.2018.0708008 Received: 29-05-2018, Accepted: 10-07-2018, Published: 31-07-2018 _______________________________________________________________________________________ Volume: 07 Issue: 08 | Aug-2018, Available @ www.ijret.org 68 THE EFFECT OF RANDOM AGGREGATE FIELD ON SUPERCRITICAL CARBONATION OF CONCRETE Xiaoxiong Zha 1 , Changchun Ren 2 , Jiaqian Ning 3 1 Department of Civil and Environmental Engineering, Harbin Institute of Technology, Shenzhen 518055, PR China 2 College of Urban Construction, Wuhan University of Science and Technology, Wuhan, 430065, PR China 3 Department of Civil and Environmental Engineering, Harbin Institute of Technology, Shenzhen 518055, PR China Abstract In order to study the supercritical carbonation process of concrete from the mesoscopic level, this paper is based on the theory of aggregate grading, by introducing a randomly distributed two-dimensional circular aggregate to simulate the supercritical carbonation process of concrete, the uneven carbonation boundary of concrete is obtained, and the carbonation degree of concrete is described by the maximum carbonation depth. The aggregate distribution of concrete is obtained by discussing the selection principle of the maximum particle size, the minimum particle size and the volume fraction of the aggregate; after the obtained aggregate distribution model is added to the supercritical carbonation model, the irregular boundary of concrete carbonation can be well simulated. The results show that the uneven distribution of aggregate in the cement mortar will lead to the non-uniformity of the carbonation depth of the concrete, which can be used to evaluate the effect of aggregate on the carbonation depth of the concrete. Keywords: - Supercritical carbonation, Random aggregate, Carbonation depth --------------------------------------------------------------------***---------------------------------------------------------------------- 1. INTRODUCTION The carbonation process of concrete consumes alkaline substances in concrete and destroys the alkaline environment in concrete, which leads to corrosion of steel bars and affects the durability of concrete[1]. Ashraf[2] reviewed the carbonation process of cement-based materials, indicating that carbonation may have a positive and negative impact on concrete performance. The carbonation reaction will change the porosity and pore size distribution in the concrete, affect the permeability, increase the compactness, and then affect the performance of the concrete[3][4]. In recent years, due to the rapid development of accelerated carbonation technology, the beneficial aspects of carbonation can be used to improve the performance of cement-based materials, M. FernándezBertos et al.[5] summarized the research and application of accelerated carbonation technology, and concluded that accelerated carbonation technology has great application prospects in the direction of waste solidification, and it is also a way to effectively absorb carbon dioxide, a greenhouse gas. The research[6] of Zha and Wang proved that the carbon storage capacity of different types of cement materials is different. Feng[7] used accelerated carbonation technology to transform the performance of cement bricks. In summary, further research is needed on the carbonation process of cement-based materials to maximize the positive effects of carbonation and reduce the negative effects of carbonation. Carbonation of cement-based materials is a complex multi- physics process. The scholars of the former Soviet Union derived the theoretical model of natural carbonation of classical concrete by Fick's first law of diffusion. Anna V. Saetta et al. established differential equations between water vapor diffusion, thermal diffusion in concrete materials, and carbon dioxide diffusion, and applied finite element methods to solve this coupled nonlinear equations[8][9]. When the temperature and pressure exceed the critical values of carbon dioxide of 304.12 K and 7.38 MPa, the carbon dioxide is in a supercritical fluid state. At this time, carbon dioxide has a gas-like diffusion property and a liquid-like density, which is an excellent solvent. The carbon dioxide in this state has a low surface tension and can be easily diffused into a porous medium to accelerate the carbonation of the cement-based material. For the supercritical carbonation reaction, Zha et al. [10] established a mathematical model of supercritical carbonation with a pressure gradient as the main driving force under supercritical conditions. The model considers the chemical reaction, gas-liquid two-phase mass transfer and heat transfer, and compares the experimental data to verify the credibility of the model, which provides a basis for predicting the carbonation degree of cement-based materials in supercritical conditions under actual conditions. Carbonation depth is one of the most important characteristics used to define the degree of carbonation reaction. In natural carbonation [11] and supercritical carbonation [10], the resulting carbonized regions are in an irregular shape and can be described by the maximum and minimum carbonation depths. However, in the current theoretical and numerical models, most of the concrete is considered to be an isotropic homogenous material, and the average carbonation depth is obtained instead of the