Thermal conductivity of portlandite

A cement is any of various substances used for bonding or setting to a hard material. A popular one, Portland cement, is a mixture of calcium silicates and aluminates made by heating limestone (CaCO₃) with clay (that contains aluminosilicates) in a kiln. The product is then ground to a fine powder. When Portland cement is mixed with water it sets in a few hours and then hardens over a longer period of time due to the formation of hydrated aluminates and silicates.

The hydration process is, thus, so important that some processes have been developed in order to activate cement, i.e., to enhance the early hydration rate. The hydration phase produces C-S-H gel. Cement properties (strength, durability…) will depend on C-S-H gel structure and composition.

Cement is one of the most consumed artificial materials, primarily used for the construction of concrete structures. The cement paste acts as a glue to the natural aggregates in concrete, primarily responsible for sustaining externally applied mechanical loading. Being a key part of hydrated cement paste, the importance of portlandite is undeniable from an engineering point of view.

Portlandite’s chemical composition is Ca(OH)₂. Its share to the paste is almost 20 % by volume, formed by the hydration of alite (Ca₃SiO₄ or C₃S) and belite (Ca₂SiO₅ or C₂S). Portlandite is a crystalline component with a hexagonal prism shape, responsible for the development of early age strength of fresh cement paste along with C-S-H gel.

Portlandite has an indirect role in protecting the corrosion of embedded reinforcing steel in concrete by creating a high pH environment with calcium ions. This high pH environment also helps in mitigating the sulfate attack. And it even has a potential application in capturing carbon dioxide from air.

Thermal properties of concrete can potentially be utilized in making thermal energy storage (TES) devices for renewable energy plants, especially solar power plants. TES, also known as heat or cold storage, allows the storage of heat and cold to be used later. TES can aid in the efficient use and provision of thermal energy whenever there is a mismatch between energy generation and use. This mismatch can be in terms of time, temperature, power, or site. In this context, and given the considerable prevalence of portlandite in hardened cement paste, comprehensive investigations into the thermal characteristics of this constituent material are needed.

Harden cement paste is a heterogeneous composite material, and its degree of heterogeneity increases with lowering the visual scale level. Portlandite is typically distinguishable below a length scale of 10⁻⁴m where a direct experiment on this particular component is difficult. An alternative way to study the material is using simulation approaches. Of these, molecular dynamics (MD) is computationally less expensive than density functional approaches.

Now, a team of researchers used MD to investigate ¹ the properties of portlandite. Besides, an experimental investigation has been conducted with modulated differential scanning calorimetry (MDSC) to get the bulk thermal conductivity of portlandite.

The researchers used three different force fields – computational methods used to estimate the forces between molecules or coarse-grained particles – to study the structural, mechanical, vibrational, and thermal properties of portlandite. They found that all three force fields (ClayFF, IFF, and ReaxFF) are capable to capture equilibrium cell parameters in order with the experimental value.

The authors provide the full thermal conductivity tensor of portlandite along with the possible mechanism associated with thermal transport. The frequency components of thermal conductivity demonstrate that the contribution of lower frequency is more significant to thermal conduction. Even though the volumetric conductivity or bulk conductivity of all three force fields is in line with the experimental value, the results indicate that the calculation of thermal conductivity is very sensitive to the choice of force field.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.