In-Situ Kinetic Investigation of Calcium Aluminate Formation

John Salasin,Claudia Rawn

doi:10.3390/ceramics1010016

John Salasin, Claudia Rawn

Open Access

https://doi.org/10.3390/ceramics1010016

Copy DOI

Abstract

Rapid in-situ non-ambient X-ray diffraction represents a powerful tool for characterizing the evolution of crystalline materials in real time. The calcium aluminate system and formation of Ca12Al14O33 (C12A7) is particularly sensitive to processing conditions. This report characterizes the kinetic pathways to thermodynamic equilibrium as a function of atmosphere (ambient, dry, and vacuum) and reactant heterogeneity (as-received, milled, and sol-gel reactants). When reactants are heterogenous (as-received and milled), intermediary phases of Ca3Al2O6 (C3A) and CaAl2O4 (CA) are observed as the route to C12A7 formation and Ca5Al6O14 (C5A3) is only observed as a decomposition product of C12A7. When reactants are heterogenous, C12A7 is only thermodynamically favorable under ambient conditions due to the stability provided by hydration. When reactants are homogenous (sol-gel), direct crystallization of C12A7 from an amorphous precursor is observed at low temperature regardless of atmosphere defining C12A7 as the kinetic equilibrium. These findings accurately define the heterogenous formation pathways and report for the first time the formation of C12A7 under a carbon-free vacuum environment.

Highlights

Ca12 Al14 O33, the mineral mayenite or C12A7, crystallizes in a clathrate structure where a positively-charged framework is balanced by occluded anionic species
With the the tool tool of of in in situ this study study characterizes characterizesthe thekinetic kineticpathways pathwaysofofa aC12A7
In situ rapid XRD data provide insight into the kinetics of solid-state phase transformations and represents a valuable tool to further understand the evolution of materials toward thermodynamic equilibrium

Summary

Introduction

Ca12 Al14 O33 , the mineral mayenite or C12A7, crystallizes in a clathrate structure where a positively-charged framework is balanced by occluded anionic species. The high interconnectivity of the cages in the clathrate structure leads to high occluded anion mobility. The functionality of the structure is derived from the mobility and diversity of occluded chemical species leading to a wide range of applications [1,2]. The C12A7 structure, occluded anion speciation, lattice parameter, and kinetic formation are all heavily correlated to processing conditions [3,4,5]. With the thermodynamic phase equilibria defined, an in situ kinetic structural investigation of C12A7 is warranted to better understand the proposed kinetic pathways to thermodynamic equilibrium. With this knowledge, careful control of synthesis can lead to optimization of material properties for a wide variety of applications

Methods

Results

Discussion

Conclusion