Improving the strength of metakaolin-lime based binder

Abstract

Pozzolanic reaction of low-calcium metakaolin (MK) with calcium hydroxide (CH) at ambient temperature in the presence of water forms a series of hydrated phases such as tetracalcium aluminate hydrate (C4AH13), calcium silicate hydrate (C-S-H) and calcium aluminum silicate hydrate (stratlingite - C2ASH8). Stratlingite is the main crystalline phase and is responsible for the strength of the binder. Tetracalcium aluminate hydrate (C4AH13) is carbonated upon contact with air or converted to hemicarboaluminate and/or monocarboaluminate if the system contains excess carbonate phases (calcite or calcareous aggregates). However, the calcium aluminate hydrates formed after the reaction of MK with lime lose their strength over time due to their instability. Especially in a high humidity environment where there is no carbonation, the presence of both stratlingite and (CH) in the binder creates weak phases called katoite (Ca3Al2(SiO4)(OH)8) and can reduce the mechanical strength and durability. On the other hand, it has been determined that using chemical activators to increase the pozzolanic reactivity is the most feasible method, although it increases the cost of the material. In this study, it is aimed to eliminate the phases that cause katoite formation and improve the performance of the binder by using alkaline hydroxide solution in a mixture of metakaolin and slaked lime, similar to the pore solution caused by hydrated cement. For this aim, the effect of sodium carbonate (N-Na2CO3), quicklime (C-CaO) and calcite (CC-CaCO3) on the (CH-Ca(OH)2) activated metakaolin system is investigated through the experimental campaign. The preparation of the mixtures is done by the novel one-part mixing method.