再水化作用对超高性能混凝土基体显微结构和水稳定性的影响

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

通过开展超高性能混凝土(UHPC)基体的再水化试验,测试了不同再水化时间水泥石的化学结合水量、试件膨胀率和抗压强度,基于Krstulovi?–Dabi?的水泥水化动力学和水泥水化微观信息,建立了再水化模型,同时结合微观形貌变化和孔结构变化,基于建立的模型,分析了再水化作用对其水稳定性的影响机理。结果表明:在低水灰比(0.15~0.30)范围内,水灰比越低,再水化结合水量在前期越大,后期反而越小;再水化过程中,试件膨胀率随水灰比的降低呈增大趋势;不同水灰比水泥石的抗压强度随再水化时间的增长呈增大和减小交替出现的趋势。根据再水化模型计算的水泥水化度模型预测结果和试验结果吻合良好,表明所建立的模型可以较准确地模拟UHPC基体的再水化过程。水灰比0.30水泥石较大的水泥水化速率导致其具有较高的抗压强度增长幅度。再水化前期,水泥水化速率快,再水化产物不断填补水泥石内部初始孔隙,后期水泥水化速率缓慢,再水化产物体积膨胀导致水泥石出现微裂缝,UHPC基体性能劣化。

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 Krstulovi?–Dabi? 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.