Correlating hydration mechanism and microstructure development to LC3 synergistic effect improvement in UHPC under hot-water curing condition

Limestone-calcined clay (LC3) can be used to prepare ultra-high performance concrete (UHPC) to reduce its environmental impact. However, the application is restricted because the synergistic effect of LC3 could not be fully activated under a high LC3 replacement ratio due to the low CH content (2.73%~5.74% for LC3–45% vs. 11.12~15.32% for LC3–15%), leading to low strength. This study unveils the correlation between synergistic effect improvement and hydration process and microstructure development based on a series of microanalyses with different LC3 replacement ratios (15%, 30%, and 45%) under hot-water curing. By promoting the production of C-(A)-S-H at early age, more portlandite is formed to react with limestone and calcined clay, and more carboaluminate is precipitated (an increase of 19.22~258.03%, 66.22~289.49%, and 7.99~129.28% for LC3–15%, LC3–30%, and LC3–45%, respectively). In the UHPLC3 system, sulfate content affects the synergistic effect of LC3, Ett/AFm phase equilibrium, and the hydration process, which is influenced by the curing temperature. A higher LC3 replacement ratio results in higher sulfate content to promote the formation of Ett at early age and provides a more stable environment for the growth of Ett by reducing the transformation to monosulphate form of AFm. The precipitation of carbonaluminate is found to be correlated to the pore size threshold ( D t ) and the threshold temperatures controlling the limited D t are 40°C and 50°C for LC3–45% and LC3–15%/30%, respectively. The increase in compressive strength is about 42.87~79.41% for LC3–45% under hot-water curing, higher than that of LC3–15% (18.17%~42.74%), which is consistent with the hydration products evolution analysis. Finally, a comprehensive mechanism is unveiled to understand the hydration process and microstructure development under hot-water curing condition.