Ultrasonic Mixing of Cement Paste For Concrete

The ultrasonic mixing of cement paste offers great benefits for precast molding, drycast, and concrete plants. These benefits include shorter initial and final set times, lower dosage of superplasticizer, faster and more complete hydration, as well as higher compressive strength.

Traditional concrete mixing technologies, such as “on-road-mixing” or rotary mixers, provide insufficient mixing action to disperse agglomerates of cement particles and other cementitious materials, such as fly ash or silica. While the outer particles of such agglomerates are exposed to water, the inner particle surfaces remain dry. This results in slow and incomplete hydration.

Benefits of Ultrasonic Mixing Technology for Concrete

Ultrasonic dispersing is the most advanced technology to deagglomerate and disperse micron-size and nano-size materials in liquids. Ultrasonic mixing uses cavitational shear forces that are more effective in the mixing of fine size materials than conventional rotary mixers and rotor‑stator mixers. For cement, silica, fly ash, pigments, or CNTs, the performance of these materials is significantly increased by ultrasonic dispersing, as it improves the particle distribution and contact with water.

During the hydration—the reaction of cement with water—C‑S‑H‑phases grow needle-like structures. The pictures below show the microstructure in cement paste after 5 hours of hydration. In the ultrasonicated cement paste, the C‑S‑H‑phases are almost 500nm long, while in the unsonicated paste, C‑S‑H‑phases are around 100nm.

Microstructure of Cement Paste After 5hrs Hydration Time

with ultrasonic processing	without ultrasonic processing
Portland Cement Paste (CEM I42.5R), C. Rössler (2009) – Bauhaus University Weimar

 
Mixing by ultrasound-induced cavitation leads to faster growth of C‑S‑H‑phases.

 
Hydration Temperature

Influence of Power Ultrasonication (PUS) on Time-Temperature Curves of Cement Pastes

 
Compressional Strength

Influence of Power Ultrasonication (PUS) on Compressional Strength of Mortar Prisms

 
Ultrasound Pulse Velocity

Influence of Power Ultrasonication (PUS) on Ultrasonic Pulse Velocity of Hydrating Cement Pastes

The growth of C‑S‑H-phases correlates to the temperature in the cement paste during the hydration period (click at right graphic). In the ultrasonically mixed cement paste, the hydration starts approx. one hour earlier. Earlier hydration correlates with earlier increase in compressional strength. The increased hydration speed can be measured by the ultrasound pulse velocity, too.

In particular for precast and drycast concrete, this leads to significantly shorter time until the cast concrete can be taken from the mold. Studies by the Bauhaus University (Germany) showed the following reduction of set times.

Another interesting benefit of ultrasonic mixing is the influence on the fluidity. As shown in the table above, the slump increases by approx. 30%. This allows for reduced dosage of superplasticizers.

Process Integration of Ultrasonic Mixers into Cement Production

Hielscher offers ultrasonic mixers for the effective dispersing of cement, silica, fly ash, pigments, or CNTs. First, any dry material should be premixed with water to form a high concentration yet pumpable paste. The Hielscher ultrasonic mixer deagglomerates and disperses the particles using cavitational shear. As a result, the entire surface of each particle is fully exposed to water.

Ultrasonic Processing of Cement Paste

In the case of cement paste, hydration starts after ultrasonic processing. Therefore, the Hielscher ultrasonic mixer should be used inline, as the cement paste cannot be stored for long periods. The schematic drawing below illustrates the process. In the next step, the aggregate, such as sand or gravel, is added and mixed with the cement paste. As the cement particles are already well dispersed at that stage, the cement paste blends well with the aggregate. The concrete is then ready to be filled into precast molds or for transportation. A break-up tank next to the ultrasonic mixer may be used to process more continuously in case of unsteady concrete demand.
Read more about ultrasonic deagglomeration of cement particles!

Ultrasonic Dispersing of Silica, Fly Ash, and Nanomaterials

The dispersing of silica, fly ash, pigments, or other nanomaterials, such as carbon nanotubes, requires other processing intensities and energy levels. For this reason, we recommend a separate ultrasonic mixer to produce a well-dispersed slurry/paste that is then added to the concrete mix. Please click on the graphic above for a schematic drawing of this process.

The ultrasonic mixing equipment needed for scale-up can be determined exactly based on pilot scale tests using the UIP1000hdT, which is a 1,000-watt powerful pilot-scale sonicator. The table below shows general device recommendations depending on the batch volume or flow rate of cement paste to be processed.

With up to 16kW of ultrasonic mixing power per single ultrasonic probe, Hielscher offers the processing power required for high-volume applications. This technology is easy to test and scales up linearly.