Improving the strength of metakaolin-lime based binder

¹ Department of Architecture, Istanbul Technical University, Istanbul, Türkiye
² Department of Civil Engineering, School of Engineering and Natural Sciences, Istanbul Medipol University, Istanbul, Türkiye
³ Department of Architecture and Design, Kocaeli University, Kocaeli, Türkiye

^* Corresponding author: ozturkmer@itu.edu.tr, mertozturk@medipol.edu.tr

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.