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Metakaolin (MK) is a high-quality, reactive nanomaterial that holds promising potential for large-scale use in improving the sustainability of cement and concrete production. It can replace cement due to its pozzolanic reaction with calcium hydroxide and water to form cementitious compounds. Therefore, understanding the dissolution mechanism is crucial to fully comprehending its pozzolanic reactivity. In this study, we present an approach for computing the activation energies required for the dissolution of metakaolin (MK) silicate units at far-from-equilibrium conditions using the improved dimer method (IDM) and the transition-state theory (TST) within density functional theory (DFT). Four different models were prepared to calculate the activation energies required for breaking oxo-bridging bonds between silicate or aluminate units. Our results showed that the activation energy for breaking the oxo-bridging bond to a silicate neighbor is higher than that to an aluminate neighbor due to the ionic interaction. However, for complete silicate tetrahedra dissolution, a higher activation energy is required for breaking the oxo-bridging bond to the aluminate neighbor compared to the silicate neighbor. The findings provide methodology for missing input data to predict the mesoscopic dissolution rate, e.g., by the atomistic kinetic Monte Carlo (KMC) upscaling approach. [ABSTRACT FROM AUTHOR]

[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]

Concrete structures are increasingly becoming exposed to organic acid attack conditions, such as those found in agriculture and food-related industries. This paper aims to experimentally verify the thermodynamic modeling of cement pastes under acetic acid attack. For this, a modeling approach implemented in IPHREEQC via Matlab is described, and results are compared with measured pH and compositions of equilibrated solutions (MP-AES) as well as unreacted/precipitated solids (XRF, XRD and STA) for a wide range of acid concentrations. The 11% replacement of cement by silica fume (SF) led to a 60 or 70% reduction (measured or modeled, respectively) of Portlandite content in the hardened cement paste due to the pozzolanic reaction resulting in higher content of CSH phases, which has effects on the progression of dissolution processes and a resulting pH with increased acid concentrations. Considering that no fitting parameter was used, the model predictions showed good agreement with measured values of pH, dissolved ion concentrations and composition of the remaining (degraded) solids overall. The discrepancies here were more pronounced at very high acid concentrations (equilibrium pH < ~4), i.e., after the full dissolution of hydrate phases due to limitations in the model used to describe Al-, Si- and Fe-gel phases and/or identified experimental challenges in precipitation of calcium and aluminum acetate hydrates. [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]

The reactivity of supplementary cementitious materials (SCMs) is a key issue in the sustainability of cement-based materials. In this study, the effect of drying with isopropanol and acetone as well as the interpretation of thermogravimetric data on the results of an R3 test for evaluation of the SCM pozzolanic reaction were investigated. R3 samples consisting of calcium hydroxide, potassium hydroxide, potassium sulphate, water, and SCM were prepared. Besides silica fume, three different types of calcined clays were investigated as SCMs. These were a relatively pure metakaolin, a quartz-rich metakaolin, and a mixed calcined clay, where the amount of other types of clays was two times higher than the kaolinite content. Thermogravimetric analysis (TGA) was carried out on seven-day-old samples dried with isopropanol and acetone to stop the reaction processes. Additional calorimetric measurement of the R3 samples was carried out for evaluation of the reaction kinetics. Results show that drying with isopropanol is more suitable for analysis of R3 samples compared to acetone. The use of acetone results in increased carbonation and TGA mass losses until 40 (isothermal drying for 30 min) and 105 °C (ramp heating), indicating that parts of the acetone remain in the sample, causing problems in the interpretation of TGA data. A mass balance approach was proposed to calculate calcium hydroxide consumption from TGA data, while also considering the amount of carbonates in the sample and TGA data corrections of original SCMs. With this approach, an improvement of the linear correlation of TGA results and heat release from calorimetric measurement was achieved. [ABSTRACT FROM AUTHOR]

This study aims to increase the pozzolanic reactivity of metakaolin (MK) in Portland cement (PC) blends by adding additional calcium hydroxide (CH_add) to the initial mixture. Cement paste samples were prepared with PC, MK and water with a water-to-binder ratio of 0.6. Cement replacement ratios were chosen from 5 to 40 wt.% MK. For higher replacement ratios, i.e., 20, 30 and 40 wt.% MK, CH_add was included in the mixture. CH_add-to-MK ratios of 0.1, 0.25 and 0.5 were investigated. Thermogravimetric analysis (TGA) was carried out to study the pozzolanic reactivity after 1, 7, 28 and 56 days of hydration. A modified mass balance approach was used to normalize thermogravimetric data and to calculate the calcium hydroxide (CH) consumption of samples with CH_add. Results showed that, without CH_add, a replacement ratio of 30 wt.% or higher results in the complete consumption of CH after 28 days at the latest. In these samples, the pozzolanic reaction of MK turned out to be restricted by the amount of CH available from the cement hydration. The increased amount of CH in the samples with CH_add resulted in an enhanced pozzolanic reaction of MK as confirmed by CH consumption measurements from TGA. [ABSTRACT FROM AUTHOR]