Ultra-high performance concrete (UHPC) is a promising material which exhibits extrodinary mechanical properties and durability.A UHPC paste containing fly ash and silica fume was prepared and its microstructure evolution during hydration was investigated with backscattered electron (BSE) images,thermogravimetry (TG) and nitrogen adsorption method.The results showed that the hydration of cement was fast at early ages,and it slowed down after 7 days.Due to its low activity,fly ash reacted slowly in the paste.The reaction degree of fly ash was only 7％ by 28 days.The content of Ca(OH)2 increased quickly at first as the cement hydrated.After 3 days it started to decline due to the consumption by pozzolanic reactions of silica fume and fly ash.There was still some Ca(OH)2 existing in the paste by 28 days.Moreover,the specific surface area and the porosity of UHPC paste de creased as the curing progressed,resulting in a denser microstructure.%超高性能混凝土(UHPC)具有卓越的力学性能和耐久性能,应用前景广阔.采用扫描电镜背散射电子图像、热重法和氮气吸附法系统研究了水泥-粉煤灰-硅灰基UHPC浆体水化过程中微观结构的演变过程.结果表明:UHPC浆体在早期水泥水化较快,但7d后水化变得较为缓慢,粉煤灰在UHPC浆体中反应较为缓慢,28 d时反应程度仅为7％;UHPC浆体中Ca(OH)2含量早期上升快速,由于硅灰和粉煤灰的火山灰反应逐渐消耗,3d后含量开始下降,但28 d时浆体中仍存在部分Ca(OH)2;此外,在水化过程中,UHPC浆体的比表面积不断降低,孔隙率逐渐下降,水化产物变得更为致密.
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