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Rheology is a branch of physics that focusses on the distortion and flow of matter. In particular, rheology is concerned with the non-Newtonian motion and flow of liquids, as well as the plastic movement of solids. Rheology is most commonly applied to subjects such as engineering, physiology, and biology. As for materials science, rheology has applications in confectionery, paint, and cement paste, among other sources. Materials science is known for intricate flow characteristics, making them an interesting point of focus in rheological study.
Cement paste is the porous mixture of water and cement, commonly used as a construction material in buildings. However, cement paste is considered to have both viscoplastic and viscoelastic characteristics - viscoplastic meaning the material acts as a solid below a crucial stress point but continues to flow like a viscous liquid at other levels of stress, and viscoelastic is the property of materials that reveals viscous and elastic qualities during deformation.
When viscous substance faces temperature or pressure shifts, it will combat shear flow, whereas when elastic materials undergo stress, they exhibit a form of flexibility in which they revert to their original state once the stress has been removed. Due to cement paste’s dual nature as both viscous and elastic, its rheology is studied when it is in both forms - solid or liquid.
Rheology of Cement Paste
Concrete rheology is a specific branch of rheology that relates to the concrete and mortar workability properties of cement paste. Less water used in a concrete mix can stimulate an increase in mechanical properties of the cement paste. Despite this increase, lessening the water application can result in difficult mixing of the product and, ultimately, a trickier application.
To minimize the downfalls of less water to concrete mix, superplasticizers are applied to the mixture. Superplasticizers are additives in the form of water reducers - ordinary plasticizers can reduce water content in concrete by about 15 percent, whereas superplasticizers can lower water content by more than 30 percent. These additives allow the mixture’s mortar properties to improve and in some cases work at optimal potential.
When measuring the rheology of cement pastes, there are a series of complications. Rheological properties can change just from a chemical reaction experienced over time - cement paste typically has a very large number of particles, making them susceptible to many-particle interactions, thus influencing their initial properties. Also, additives or admixtures used in cement paste can change the properties of the original substance. Due to the presented difficulties, cement paste is reviewed with varying rheological properties that notably change with time - the substances can also be observed as non-linear rheological behavior.
Rheology science is often implemented to understand, at a more detailed level, the properties of freshly made cement paste. Unlike workability tests, rheological properties can be the starting point of numerical explanation and provide a substantial description of flow properties.
Rheometers typically analyze and measure the following features of cement paste: geometries (parallel plate geometry and concentric cylinders’ geometry), thixotropic behavior, visco‐elasticity, steady flow properties, equilibrium conditions, gap width, particle migration/movement, and plug flow. However, there is no known way to complete all of these tests with a single source or procedure - many applications and studies require multiple resources, as well as thorough and expansive data.
Conclusion
The intricate relationship between chemical additives or admixtures and cement paste can yield an unforeseeable concrete performance. This unpredictability paves the way for concrete incompatibilities. Cement paste rheology can measure, as well as detect, these incompatibilities early on, therefore producing a paste that is guaranteed to work as best as possible. For example, by being able to measure all features of cement paste and its future workability, construction can prevent early-age cracking and other cement dysfunctions resulting from applied pressure.
The workability of concrete is understandably its major concern in most construction applications, but concrete rheology advances on workability tests, as well as bridges the gap between the knowledge of how cement paste will perform now and how it will perform in years from now. Rheology is capable of testing how cement paste will react under stress, the viscosity levels, and shear rate.
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