Understanding early stages of crystallization of calcium carbonate Using Electron Microscopy.

Session

Poster Session 4

Authors

Maryam Afzali Haji Dela (1)

Affiliations

1. Leeds University

Abstract text

Calcium carbonate (CaCO₃) crystallisation occurs in a variety of processes such as bio-mineralisation in shellfish and production of personal health products, and construction materials. Equally important is the prevention of unwanted deposition of CaCO₃ in the form of mineral scales in the oil and water industries causing major operational difficulties from clogging pipes to increasing the amount of energy needed to remove them. Despite numerous studies having been carried out on CaCO3 crystallization, the mechanism via which crystallisation occurs is still not fully understood. This is because crystallisation occurs at the nano-scale and significant changes can occur over extremely short time scales which are very difficult to study experimentally. Understanding the early stages of CaCO₃ crystallisation would not only allow the ability to control undesirable scale formation but also enables us to mimic biocompatible structures for bio-applications such as bone replacement implants. 

The purpose of this study was to understand the early stages of crystallisation of CaCO₃. Conventional scanning electron microscopy (SEM), transmission electron microscopy (TEM),  plunge-freezing vacuum dry techniques, and Cryo-EM were used to investigate the presence of polymorphs including the amorphous state of CaCO₃ which were formed at different degrees of supersaturation. The ultimate aim of this research s to use liquid cell (LC) TEM to observe the formation of CaCO₃ in real-time_.

The crystallisation of CaCO₃ was carried out in bulk solution at 25±1 ºC. Supersaturated solutions of calcium carbonate were initially prepared by mixing identical molar ratios and volumes of CaCl₂ and Na₂CO₃ leading to final CaCO₃ concentrations of 0.5, 1.0, 2.5, and 4.5 mM. In addition, the morphological properties of the particles as a function of changes in a molar ratio of CaCl₂/Na₂CO₃ were investigated.

During crystallisation of CaCO₃ solutions (0.5, 1.0,  2.5  and 4.5 mM) containing equal molar ratios of CaCl₂ and Na₂CO₃,  a variety of CaCO₃ particles including amorphous CaCO₃ (ACC) particles, hexagonal vaterite, and rhombohedral calcite were observed, but no evidence was found for the presence of aragonite. However, a considerably different molar ratio of CaCl₂ and Na₂CO₃ was found to give rise to the formation of aragonite in the solution at 25º C, despite the fact that aragonite is an abundant polymorph at temperatures between 60°C to 80 °C.