Calcite crystallization in the cement system: morphological diversity, growth mechanism and shape evolution

Area of Science:

• Materials Science
• Geochemistry
• Civil Engineering

Background:

• Carbonation is crucial in construction, impacting mineralization, CO2 sequestration, and corrosion.
• The nanoscale mechanism of carbonation in calcium silicate hydrate and portlandite remains poorly understood.
• Observing carbonation at the nanoscale is critical for understanding its dynamics.

Purpose of the Study:

• To reveal a new view on carbonation in cement systems at the nanoscale.
• To investigate the morphological diversity, growth mechanism, and shape evolution of calcium carbonate.
• To elucidate the factors controlling calcite crystal morphology.

Main Methods:

• Utilized electron microscopy for nanoscale observation.
• Employed first-principles calculations for theoretical analysis.
• Applied aqueous chemistry principles to understand reaction mechanisms.

Main Results:

• Identified two crystalline forms of calcium carbonate (cubic and spindle), both determined to be calcite.
• Established that the ratio of calcium ([Ca2+]) to carbonate ([CO3]) ions, not reactant type, dictates crystal morphology.
• Demonstrated that excess calcium ions inhibit specific crystal face growth, influencing shape.
• Developed a predictive relationship between relative ionic concentration and length-to-diameter ratio for shape transformation.

Conclusions:

• The study provides a nanoscale perspective on cement carbonation, moving beyond macroscopic observations.
• Insights into calcite crystal morphology control can inform strategies for enhancing cement performance and CO2 utilization.
• This research deepens the understanding of the chemical nature of carbonation in cementitious materials.