Influence of Rehydration Effect on Microstructure and Water Stability of Ultra-high Performance Concrete Matrix

A rehydration test of ultra-high performance concrete (UHPC) matrix was conducted to measure the chemically bound water contents, specimen expansion ratios, and compressive strengths of hardened cement pastes at different rehydration time, and the rehydration model was proposed based on the Krstulović-Dabić hydration dynamics of cement and microstructure information of cement hydration. The mechanism regarding the rehydration effect on the water stability was discussed, and the microscopic appearance change with pore structure change was analyzed based on the proposed model. The results show that the bound water content of rehydration increases during the early period but decreases during the late period as the water-cement ratio decreases from 0.30 to 0.15. The specimen expansion ratio increases as the water-cement ratio decreases during the rehydration process. The compressive strength of hardened cement pastes at different water-cement ratios has an alternating tendency of increasing and decreasing with the increase of rehydration time. The simulated data by the model are in good agreement with the experimental results, indicating that the proposed model can accurately simulate the rehydration process of UHPC matrix. The increased scope of compressive strength of hardened cement paste with the water-cement ratio of 0.30 is greater due to its higher hydration rate of cement. During the early period of rehydration, a hydration rate of cement becomes greater, and the rehydration products gradually fill in the initial pores of hardened cement paste. However, during the late period of rehydration, a hydration rate of cement is lower, and the rehydration products cause some microcracks of hardened cement paste due to its volume expansion, resulting in the degradation of properties of UHPC matrix. © 2020, Editorial Department of Journal of the Chinese Ceramic Society. All right reserved.