Secondary curing effect on the hydration of ultra-high performance concrete

Abstract

Once the dense microstructure has been formulated, the external water supply has been generally regarded as ineffective for enhancing the material properties of ultra-high performance concrete (UHPC). Two days of high temperature curing is typically required after one day curing at ambient conditions to facilitate UHPC production. In this study, the curing process was divided into 1 day ambient curing, 1 day curing at 90–180 °C, 1 day water curing, and a subsequent 1 day curing at 90 °C. The UHPC hydration process, including consumption of silica fume and production of C-S-H, was quantitatively analyzed by the X-ray diffraction-based partial- or no-known crystal structure (PONKCS) method. The obtained strength was almost proportional to the initially applied temperature, and the secondary curing program (i.e., water curing and subsequent curing at 90 °C) showed an advantage for the later strength development of UHPC by promoting the reformation of C-S-H and the pozzolanic reaction. On the other hand, the partial decomposition of C-S-H at temperatures of 120–180 °C was observed from the diffraction and thermal analyses, but had a negligible effect on the development of material properties at that moment of the modification. Meanwhile, the decomposed C-S-H was efficiently reformed once the secondary curing was applied, which has a positive effect on structural properties. Although UHPC has an early formation of a dense microstructure, this study confirmed that secondary supplied water could sufficiently penetrate the matrix of UHPC, which can be used to enhance the long-term properties by reformulating C-S-H and enhancing pozzolanic reaction.