Hydration and mechanical properties of UHPC matrix containing limestone and different levels of metakaolin

Abstract

In this paper, the hydration process and mechanical properties of UHPC matrix incorporated with limestone (LS) and different substitution rates of metakaolin (MK) were investigated. MK was added with five levels of substitution (0, 5%, 10%, 15% and 20%) and LS was kept at 30% of the total binder mass. Mechanical properties, hydration heat, hydration products and pore structure measurements were carried out for every mixture. The results show that the hydration rate as well as the hydration heat peak of UHPC pastes decrease with MK level in the first 24 h possibly due to the higher superplasticizer usage, but the hydration process is accelerated with the incorporation of MK after 24 h. The pozzolanic reaction of MK and synergistic effect with LS consumes portlandite and induces the formation of more CSH gels with the improved microstructure of UHPC matrix. The trace of calcium monocarboaluminate (Mc) phase can be observed in UHPC paste samples with a low MK substitution (5%), and becomes weaker with the higher MK dosage. The pozzolanic reaction of MK with replacement level of 15% results in the best long-term compressive strength of UHPC mortars. Therefore, the incorporation of both MK and LS at an optimal level is favorable to manufacture UHPC with improved mechanical properties.