高温后超高韧性水泥基复合材料的微观结构特征研究

Microstructure characteristics of ultra-high toughness cementitious composites after exposure to high temperature

利用背散射电子图像分析法(BSE-IA)研究了高温对混杂钢纤维超高韧性水泥基复合材料(UHTCC)基体及界面过渡区微观结构的影响,揭示了钢纤维高温锈蚀机理。研究发现:常温下UHTCC基体结构较疏松,界面过渡区内孔隙率较大;而后,温度升高促使粉煤灰二次水化,基体与界面过渡区微观结构趋于致密,在400 ℃达到最密实状态;400 ℃以后,氢氧化钙等水化产物逐渐分解,导致基体内因缺陷增多而变疏松,大量裂缝沿着骨料界面产生并逐渐扩展成网络。常温下钢纤维与基体间存在一结构疏松过渡区,随着温度升高,该过渡区消失。钢纤维在400 ℃时局部发生点蚀,而后随着温度升高锈蚀区域逐渐扩大,直至完全锈蚀。以上结果表明,400 ℃以下高温对UHTCC微观结构有益;而500 ℃及以上,在水化产物高温分解与钢纤维锈蚀的共同作用下,UHTCC微观结构迅速劣化。

The microstructure characteristics of hybrid steel fiber ultra-high toughness cementitious composites (UHTCC)after exposure to high temperature were investigated by backscattered electron image analysis(BSE-IA). Results show that fly ash (FA) was un-reacted at room temperature, which leads to the high porosity of UHTCC pastes and interfacial transition zone. Before 400 ℃, the compactness of UHTCC pastes improved as the temperature rising, owing to the pozzolanic reaction of FA;Thereafter, the compactness of UHTCC pastes becomes worse and worse as the temperature gradually arising, owing to the decomposition of hydration products and the formation of micro-cracks. Interfacial transition zones between steel fiber and pastes are relatively porous at room temperature and becomes denser as temperature rise to 400 ℃. Steel fiber corrosion first sporadically observed at 400 ℃, and it gradually increases as temperature arising. Finally at 800 ℃, most parts of steel fiber corrode. Based on the findings above, we can conclude that temperature lower than 400 ℃ is favorable for the microstructure of UHTCC, and temperature higher than 500 ℃ induces defects within UHTCC.