Ultrasonic mixing of cement paste

The advantages are shorter initial and final set time, lower dosage of superplasticizer, faster and more complete hydration as well as higher compressive strength.

Conventional concrete mixing technologies provide mostly deficient mixing action to disperse the agglomerates of cement particles and other cementious materials, such as fly ash orsilica. The problem resulting from this, is that the outer particles of such agglomerates are exposed to water, but the inner particle surfaces remain dry. The consequence is a 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. For cement, fly ash, silicas, pigments and carbon nanotubes ultrasonic dispersing advances the particle distribution and the contact of the particles with water. Especially micro- and nanosilica as well as nanotubes are used to improve the compressive strength of high-performance concrete. Based on the cavitational shearforces, ultrasonic mixing of fine size materials is more effective than conventional rotary mixers and rotor-stator mixers.

During the hydration - the reaction of cement with water - C‑S‑H‑phases (calciumsilicahydrate) grow crystalline needle structures. The pictures below show the microstructure in cement paste after 5hrs of hydration. In the sonicated 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. There is a direct correlation between the growth of C‑S‑H-phases and the temperature in the cement paste during the hydration period. In the ultrasonically mixed cement paste, the hydration starts approx. one hour earlier. In turn, the earlier hydration correlates with the earlier increase in compressional strength.

In particular for precast and drycast concrete, this leads to significantly shorter time until the cast concrete can be taken from the mold. 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.