Redox reaction models for carbonation of hardened cement under elevated temperature up to 1000°C

Abstract

Concrete structures are exposed to various temperature environments and often come into contact with CO2 under these conditions. With the recent surge in interest in carbonation, there is a growing demand for a numerical model that can more comprehensively and flexibly account for the reactions of hydration, carbonation, decomposition, and recovery of cement hydrates. A comprehensive framework for concrete carbonation is proposed from a wide perspective of temperature with thermo-chemical modeling including de-carbonation and re-carbonation. By overlaying rates of bidirectional redox reactions, the dominant reaction at any temperature can be determined. The model framework, encompassing cement hydration, carbonation, de-hydration, de-carbonation, re-hydration, and re-carbonation, was experimentally validated by previous measurements of changes in weight and porosity under different CO2 concentrations and repeated exposure to high temperatures followed by post-fire-curing. These validations demonstrate a strong connection between the micro- and macroscopic properties of concrete in the proposed model. With this scheme, fire resistance of concrete composite under unexpected cases was explored computationally. Significant weight loss and strength reduction of cement paste are predicted after the first heating cycle, followed by gradual decline in subsequent repetitions of temperature. In the cases with water post-fire-curing, it is observed that the strength tends to restore even with repeated heating. In the cases with RH-60 % vapor post-fire-curing, the strength recovery is significantly increased by re-carbonation, but is still smaller than that of the cases with water post-fire-cuing.