title: Time-Dependent Influence of Calcium Hydroxide, Alkali Hydroxides, and Sulfates on Pozzolanic Metakaolin Reactions: Experimental Investigations and Stoichiometric Modeling
Creators: name: Weise, Kira
Publisher: name: UNSPECIFIED
Year: 2024

description: The use of mortar and concrete is indispensable for civil engineering construction, but the cement production is highly energy intensive and causes vast CO₂ emissions. In this context, supplementary cementitious materials (SCMs) are one promising way of reducing the clinker content in cements leading to a more environmentally friendly binder material. In the last years, calcined clays have shown a great potential for the use as SCMs due to their large worldwide availability and lower CO₂ emissions compared to cement clinker. Their beneficial performance in binder systems is mainly driven by the pozzolanic reaction of metakaolin (MK), meaning the ability to react with calcium hydroxide (CH). 

The pozzolanic MK reactions are influenced by several factors arising from the specific binder system. This research thesis mainly focusses on the CH availability, as well as the presence of alkali hydroxides and sulfates. The main aim is to understand in detail these influences und model pozzolanic MK reactions encompassing both short-term kinetics and long-term transformation processes. 

The overall research concept comprises an extensive literature review, constituting the foundation of this thesis, two methodology studies addressing major challenges related to the analysis of the specific paste samples together with one article, discussing the major experimental results. Additionally, this work is complemented by a simplified stoichiometric and kinetic modeling of MK-CH systems. 

For the experimental program, paste samples consisting of MK, CH, water, alkali hydroxides (KOH and NaOH) and/or sulfates (K₂SO₄ and Na₂SO₄) were prepared with two different MK/CH weight ratios of 0.33 and 1.0. 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 selected paste samples. Long term investigations up to 245 days with a reaction temperature of 40 °C include (quantitative) XRD, thermogravimetric (TGA) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDX). Especially for the high MK/CH ratio of 1.0, novel findings were generated, i.e. the potential formation of alkaline aluminosilicates and hindered pozzolanic MK reactions for the highest OH-/CH weight ratio of 0.0307 as well as the deceleration of pozzolanic reactions with the incorporation of sulfates. Additionally, sulfates were detected in Si-rich hydrogarnet phases. 

Experimental findings together with results from literature were used to characterize pozzolanic reaction products and stoichiometric equations, that were used for the development of a fundamental kinetic and stoichiometric reaction model. For the pozzolanic MK reaction, two separate reaction processes are identified forming C₄AH₁₃ and C₂ASH₈. The reaction kinetics are determined by a reaction peak fitting of heat flow curves serving as input parameters in the developed model. The model allows to predict the phase assemblage of MK-CH systems over time validated with experimental results from thermogravimetric analysis (TGA), XRD and helium pycnometry (solid volume). With the help of the proposed model, the temperature dependency of the pozzolanic metakaolin reactions is analyzed, revealing an activation energy for the main (C₂ASH₈) pozzolanic reaction of 84 kJ/mol in the temperature range of 20 to 40 °C. The main model limitations are the fixed C-A-S-H and hydrogarnet compositions, that are discussed based on scanning electron microscopy along with energy-dispersive X-ray spectroscopy (SEM/EDX).

The experimental study provides a deeper understanding regarding the influences of CH availability, alkali hydroxides and sulfates on pozzolanic MK reactions, whereas the developed modeling approach covers the main reaction processes in these systems. This research thesis serves as a fundamental basis for further model extensions, e.g. the incorporation of sulfates and/or carbonates, and is supposed to find its use in the design of novel “low-carbon” binders in the future.