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[Display omitted] • Stoichiometric model combines kinetics of pozzolanic metakaolin reactions with long-term transformation processes. • Primary pozzolanic metakaolin reactions are divided into C 4 AH 13 and C 2 ASH 8 contributions. • Activation energy for the main (C 2 ASH 8) pozzolanic metakaolin reaction is determined as 84 kJ/mol. • KOH accelerates reaction processes and reduces the activation energy of the pozzolanic metakaolin reaction to 69 kJ/mol. In the pursue of environmentally-friendly binders for the construction industry, metakaolin (MK) has emerged as promising material, with its hardening performance primarily driven by pozzolanic reactions. However, in systems containing MK and calcium hydroxide (CH), transformation reactions from metastable to stable phases, particularly in excessive CH conditions, can adversely affect material properties. To anticipate these processes, this study introduces a stoichiometry-based reaction modeling approach for pozzolanic MK reactions, encompassing both short-term kinetics and long-term transformation processes. The primary pozzolanic reactions of MK are briefly outlined, highlighting two sequential, partially overlapping reactions forming C 4 AH 13 and C 2 ASH 8. Short-term reaction kinetics are modeled using isothermal calorimetry measurements and deconvoluting the two reaction peaks. The model is in good agreement with experimental quantitative X–ray diffraction (qXRD), thermogravimetric analysis (TGA) and helium pycnometer (density, i.e. solid volume) results. Model limitations are discussed based on qualitative scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) analysis. Leveraging the proposed model, the temperature dependency of pozzolanic MK reactions is analyzed, revealing an activation energy for the primary reaction of 84 kJ/mol. The model is intended to lay a foundation for designing innovative binder systems based on metakaolin, while paving the way for sustainable construction in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • The oxide composition designated as metakaolin to calcium hydroxide ratio determines C-A-S-H composition and time dependent (trans)formation reactions. • Stratlingite destabilizes in the presence of calcium hydroxide and transforms to more stable hydrogarnet phases. • Sulfate addition leads to rapid ettringite formation, that may transform to monosulfoaluminate and later to hydrogarnet. • Carbonates stabilize ettringite and favor formation of carboaluminates, which may decompose at later curing ages. • Enhanced pozzolanic metakaolin reactivity modeling for low-carbon binder design requires improved understanding of structure-related dissolution kinetics. Metakaolin (MK) has emerged as a highly promising supplementary material in low carbon binders for the construction industry. In wide range of applications, from lime to cement–based materials, its hardening performance relies on the pozzolanic reactivity between MK and calcium hydroxide (CH), resulting in the formation of diverse calcium-aluminate/silicate hydrates. The reaction sequence is affected by specific conditions dictated by the binder system employed in various applications. To advance the design of binders that reduce the carbon emissions, a systematical review on MK-based reactions is crucial. This review encompasses a broad range of MK/CH ratios and examines the effect of alkali hydroxides, carbonates and sulfates. The focus is on the formation and stability of pozzolanic hydrate phases over time and under different curing temperatures. Additionally, the review addresses the characteristics that directly affect MK reactivity, such as the (calcined) clay structure and the dissolution of the reactants. The systematic findings shed light on the hydrate phase assemblage, enabling a better understanding of the reaction mechanisms in complex systems, like MK cementitious blends. The results of this review serve as a valuable foundation for the development of novel "low-carbon" binder designs and compositions for both cementitious and lime-based binders. [ABSTRACT FROM AUTHOR]

In the quest for environmentally sustainable binders within the construction industry, metakaolin (MK) has emerged as a highly promising material. Its reactivity and hardening performance encompass a wide range of applications, spanning from lime to cement-based materials. This performance is primarily underpinned by the pozzolanic reaction with calcium hydroxide (CH), leading to the formation of various calcium aluminate silicate hydrates. The intricacies of the hydration kinetics and resultant reaction products hinge on several factors woven into the specifics of the binder type. This study investigates the influence of CH availability, explored through initially mixed MK/CH weight ratios of 0.33 and 1.0. Furthermore, the study examines the impact of introducing alkali hydroxides (KOH and NaOH) and/or sulfates (K 2 SO 4 and Na 2 SO 4) on the pozzolanic reactions. Short-term analysis employed inductively coupled plasma optical emission spectrometry (ICP-OES), and pH measurements for pore solution as well as isothermal calorimetry, and in-situ X-ray diffraction (XRD) on paste samples. Long-term investigations extended to 245 days at 40 °C, incorporating XRD, thermogravimetric (TGA) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDX). Results provide insights into the kinetics of phase assemblage and compositions of C-A-S-H gels, highlighting the transformation of metastable C 2 ASH 8 and C 4 AH 13 to stable Si-rich hydrogarnet phases, incorporating sulfate, under excess CH conditions. Higher MK/CH enriches Si/Ca, Al/Ca, enhancing C-A-S-H gels, while KOH addition further boosts Al/Ca ratios. [Display omitted] • High OH-/CH weight ratio (0.0307) promotes formation of alkaline aluminosilicates. • For higher MK/CH (1.0), sulfate addition slows pozzolanic reactions. • Excess CH transforms C 4 AH 13 and C 2 ASH 8 into hydrogarnet phases. • Sulfate localizes in aluminum-rich phases: AFt, AFm and hydrogarnet. • Higher MK/CH boosts Si/Ca, Al/Ca, enhancing CASH phases, while KOH increases Al/Ca. [ABSTRACT FROM AUTHOR]