Study on the effect of mineral admixture on the water stability of UHPC under long-term immersion

Abstract

Ultra-high performance concrete (UHPC) is prone to re-hydration as well as generating new hydration products in high humidity environments, which causes internal expansion and surface cracking. The addition of mineral admixture has a positive effect on long-term water stability of UHPC. In this study, four groups of UHPC specimens with different mineral admixture contents were prepared. Long-term immersion experiments of up to 360 days were conducted. The physical and mechanical properties of those specimens were investigated, and the re-hydration mechanism was revealed by NMR, XRD, and SEM tests. The specimens blended with silica fume and fly ash showed the best water stability, which exhibited the best physical and mechanical properties, and the less water absorption under long-term room temperature immersion. Compared with reference specimen of pure cement, the flexural strength, compressive strength of blended specimens was increased by 12%, 3.6%, and the mass change rate was reduced by 0.95%, respectively. In the early stage of immersion, the gel pore water and capillary pore water in the interior of the specimens were consumed and converted into free water. After 180 d, the percentage of water molecules with strong free degree was increased and the free water molecules were gradually reduced. The mechanism is that the small particle size of the silica fume and fly ash makes the UHPC stacking more compact. In addition, those mineral admixtures have higher volcanic ash effect, which can react with calcium hydroxide in hydration products to generate C-S-H and resulting in denser of UHPC internal structural. The double effect of filling effect and volcanic ash effect prevents more water from entering, which makes UHPC blended with silica fume and fly ash have better water stability.