Rehydration of ultra-high performance concrete matrix incorporating metakaolin under long-term water curing

Abstract

This study investigated the late age rehydration of UHPC matrix containing metakaolin (MK) under long-term water curing (90–365 d). The UHPC matrixes were divided into two series including binary mixtures incorporating MK and ternary mixtures incorporating MK and limestone powder (LS). Steam curing (variable curing temperature and duration) and standard curing were performed on the UHPC matrix before long-term water curing. Compressive and flexural strengths, chemically bound water, microstructures, hydration degree were measured to evaluate the late-age rehydration of UHPC matrix. The results indicated that the strength of UHPC matrix generally increased from 90 to 270 d and then decreased from 270 to 365 d. The ultimate strength of UHPC matrix is closely related to the early age curing regime. The rehydration of long-term water cured UHPC matrix is attributable to unhydrated cement clinker as well as pozzolanic reaction of MK. The higher steam curing temperature degraded the rehydration degree of binary matrix while has little effects on that of ternary matrix samples. It was demonstrated that small pores below 20 nm increased with the long-term water curing in UHPC matrix. Additionally, a little increase of large pores above 200 μm was detected for MK added samples, contributing to strength degradation of UHPC matrix under long-term water immersion. Therefore, the rehydration of UHPC was influenced by both cementitious composition and early age curing regime.