Hielscher Ultrasonics: Ultrasonic Mixing of Cement Paste for Concrete

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The ultrasonic mixing of cement paste offers great benefits for precast molding, drycast and concrete plants. The advantages are shorter initial and final set time, 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 cementious 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

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, silicas, fly ash, pigments or carbon nanotubes ultrasonic dispersing improves the performance of these materials, 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 5hrs 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, only.

The ultrasonic cavitational mixing leads to faster growth of C‑S‑H‑phases.

The growth of C‑S‑H-phases correlates to the temperature in the cement paste during the hydration period. 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.

 

Reference

  Diff.

Power Ultrasonics

Initial Set      

5 hr 15 min       

-29%        

3 hr 45 min

Final Set

6 hr 45 min

-33%

4 hr 30 min

Slump

122 mm (4.8')

+30%

158 mm (6.2')

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 can allows for reduced dosage of superplasticizers.

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 in order to form a high concentration - yet pumpable paste. The Hielscher ultrasonic mixer, deagglomerates and disperses the particles using cavitational shear. In result, the entire surface of each particle is fully exposed to water.

In the case of cement paste, hydration starts after ultrasonic processing. Therefore, the Hielscher ultrasonic mixer should be used inline, as the cement past cannot be stored for long periods. The schematic drawing below illustrates the process. In a 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.

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