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 for a long time, which cause internal expansion and local cracking of the surface. The addition of mineral admixture has a positive effect on long-term water stability. 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 the specimens were investigated, and the re-hydration mechanism was revealed by NMR, XRD, and SEM tests. The results show that the mineral admixture specimens exhibited better physical and mechanical properties, and were more bound to water molecules under long-term room temperature immersion. The specimens blended with silica fume and fly ash showed the best stability. The flexural strength, compressive strength, and percentage of bound water were increased (12%, 3.6%, 29%), and the mass change rate was reduced by 0.95%. The free water percentage was measured by NMR showing a decrease-increase-decrease trend, which corresponded to the rehydration strengthening-destruction-repair process of the specimen structure. The mechanism is that the addition of mineral admixture made the interior denser, making it difficult for external moisture to enter.

Keywords: UHPC, re-hydration, mineral admixtures, water stability, immersion