Mechanical performances and microstructures of metakaolin contained UHPC matrix under steam curing conditions

This study focusing on learning about the influence of steam curing on mechanical as well as microscopic performances of metakaolin (MK) contained UHPC matrix. Different levels of MK were introduced into UHPC matrix owning a water to binder ratio (w/b) of 0.20. Consequently, the hydration, mechanical property and microstructures of UHPC matrix were investigated at various steam curing regimes with variable curing temperatures and durations. The compressive and flexural strength tests, chemically bound water, reaction of metakaolin, DTA, XRD analysis, as well as MIP and SEM measurements were performed for investigating relevant properties of UHPC matrix. The test results show that steam curing evidently improves the early age compressive strength of UHPC matrix while restrains the late age strength growth. The higher curing temperature facilitates a higher MK reaction degree in the MK incorporated mixture during early ages but decreases the cement hydration degree. However, increasing the steam curing duration from 24 h to 48 h at both 55 and 90 degrees C does not significantly affect the facilitation of the pozzolanic reaction of MK. Pore structure of MK contained mixture with steam curing is similar to the reference with 20 degrees C water curing at 28 d. The steam curing at 90 degrees C results in a higher porosity for the mixture blended with MK than that at 55 degrees C. However, the enhanced porosity does not obviously impair the compressive strength of UHPC matrix. Therefore, a suitable steam curing regime is required for UHPC matrix containing MK to enhance the strength and long-term durability. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

This study found that steam curing significantly enhances the early compressive strength of UHPC matrix, but restrains late strength growth. Higher curing temperature facilitates a higher reaction degree of MK with cement, but decreases cement hydration degree. In terms of pore structure, the influence of steam curing on MK mixture is not significant.