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Rheological investigation of building materials

The construction industry is confronted with a wide range of public and industrial projects in which different kinds of materials, from natural to synthetic, are in use. These materials are predominantly composites, such as bricks, cement, and concrete, and modified materials, such as fabrics, foams, glass, metals, plastics, and ceramics. All these materials contribute to the long-term stability, functionality, isolative properties, moisture control, and fire resistance of a building. 

Rheological behavior of building materials

Rheological measurements can be applied for these materials to control the quality, to improve the processing behavior, and to optimize the long-term behavior of the end products.  

Typically measured building materials

Ceramic slurries

Ceramic materials exist in a plurality of diverse textures for a variety of applications. Ceramic slurries consist of several components. One of these components is clay such as kaolin, which is a potassium silicate of the mineral kaolinite. Kaolin is also called china clay for historical reasons and became famous all over the world, for example, in the form of tea cups. It is created by the decomposition and transformation of silicate rocks. Pure kaolin is snow-white; when mixed with quartz or feldspath it changes to a gray-yellow color (crude kaolin, kaolin sand). Kaolin which has been created by the weathering of granite and feldspath can be found in many locations all over the world.

In the building industry ceramic materials are commonly used for tiles (for floors, countertops, and fireplaces), bricks, insulators, and sanitary ware. Whatever form it has in the end, at the beginning of the processing chain ceramic is processed and transported in the form of a slurry.

Rheological tests on ceramic slurries

The processing and transport characteristics of slurries are strongly dependent on their rheological properties. Knowledge of the rheological parameters (like yield point and viscosity) is therefore essential, particularly when transporting a large amount of slurry. With a rotational rheometer, flow and viscosity curves can be measured and the yield point can also be calculated. In rheology the force limit that has to be exceeded to overcome the network of forces of the internal structure is described with the yield point, e.g. when pumping the slurry.Measuring the yield point and the viscosity function gives important information for a better understanding of the flow behavior of slurries in pipes. It also helps to solve problems with slurries which are difficult to pump. The rheological properties of ceramic slurries can be influenced by varying the slurry composition, e.g. the volume concentration (amount of water), additives (solids, polymers, liquids), particle size, and by the process conditions such as pumping temperature or flow velocity.

This test requires a rheometer.

Composite floors

Before flooring (parquet, laminate, floor tiles, vinyl flooring, etc.) can be finished, a screed or composite floor has to be laid as a basis. This acts as a filling and clearing material between the cement and the flooring and serves as a load-spreading layer. Below the screed heating equipment, noise or temperature damping material can also be installed.

A composite floor is a special kind of floor that is put directly on the cement and is therefore completely bound to it. This composition is prepared to bear all kinds of forces due to deformation, thermal stress, and traffic load, but lacks insulation against noise or heat. Therefore, these kinds of floors can mainly be found in cellars, storage rooms and as a base for drives, where high dynamic loads can be expected. 

Rheological tests on composite floors

In order to avoid damage due to the high loads which may be applied, the deformation properties of the composite floor have to be investigated and adjusted. The setting behavior after it is applied as a free-flowing material is also very important for the handling of the floor. With a rheometer, for example, the flow behavior of a material over a certain time period can be measured in order to determine the setting time of the mixture. The amount and type of setting retarder used affects the setting time. By measuring samples of free-flowing composite floors with different proportions of retarder added, the suitable ratio of components can be identified to optimize the time-dependent behavior.

This test requires a rheometer with a ball measuring system.

Glass

This transparent material in the solid state can be found in the form of packaging material (jars, bottles, flacons) and is also used in the building industry (windows, facades, insulation), the furniture industry (mirrors, tables, shelves), and automotive industry (windscreens, backlights) – to name just a few of the fields of application. Whereas glass behaves like a solid at room temperature, it can only be formed as a melt. In the liquid state it can be poured, blown, pressed and molded into a variety of shapes.

The properties of glass strongly depend on its composition. Generally the basic material glass consists of silicon dioxide, but the properties can be modified by adding other components such as metals. Glass wool, for example, is a special kind of glass that is used as an isolation material because of its low thermal conductivity (among other properties). Whereas glass often has a melting temperature of around 500 °C, special glasses can also have a melting point above or around 1000 °C. At temperatures beyond this melting point glass can be processed as melt. 

Rheological tests on glass

Rheological measurements of glass melts and especially the characterization of the temperature-dependent behavior are performed, for example, to optimize the fairly energy-intensive production process. Besides this, torsional tests with solid glass bars at room temperature are possible.

Furthermore, newly developed glass materials need to be tested in small-scale laboratory experiments before going to mass production. Whether the chemical composition of glass is changed or a new treatment technique is involved, this will affect the physical properties of glass. Knowledge of the viscosity values and also the softening point and melting point is crucial in this case. Such measurements are carried out at manufacturing companies but also at universities and other research institutes, as the application and modification of special glasses are still in the field of basic research. For special glasses in the molten state not only the temperature-dependent behavior but also the shear-dependent behavior is of great interest. 

In fact, measurements of glass melts are time-consuming because it takes some time until the sample reaches the required temperature. With a furnace or high-temperature rheometer system it is possible to perform automated rotational tests and oscillatory tests using a software program presetting and controlling, for example, a predefined measuring temperature in a temperature range of up to 1600 °C.

This test requires a furnace rheometer system combined with a rheometer head.

Plaster

Plaster is a frequently used building material. It is a dry powder like cement and mortar that can be processed when mixed with water and afterwards hardens under dry conditions. Unlike mortar and cement, plaster remains quite soft after setting and can be easily manipulated with metal tools or even sandpaper. These characteristics make plaster a finishing rather than a load-bearing material. Applications are, for example, the smoothening of interior or exterior surfaces such as walls, ceilings, and the exterior rendering of buildings.

There are different kinds of plaster, such as lime plasters, cement plasters, or resin-based plasters. The latter differ from strictly mineral plasters in the addition of organic binders.

Rheological tests on plaster

Information about the rheological behavior is essential for the production (processing properties), development (influence of additives on the properties) as well as for the quality control. This behavior can be described in terms of viscosity and flow characteristics such as the structural regeneration after shearing. 

Viscosity of plaster can, for example, be determined with a rotational rheometer in combination with a ball measuring system developed for tests on semi-solid dispersions containing particles up to 5 mm in diameter. During the measurement a ball moves on a circular path through the sample. Only during the first rotation, the ball shears unsheared material which is not yet cleared of particles. Therefore, a rheometer that is able to control a flow curve over several decades of the rotational speed within only one rotation is necessary. The knowledge of the flow behavior and viscosity of plaster under different conditions like flow velocity and temperature allows for conclusions about the workability.

This test requires a rheometer with a ball measuring system.